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Peroxidase activity of yeast iso-1-cytochrome c Villegas, Jose 2006

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P E R O X I D A S E A C T I V I T Y O F Y E A S T I S O - 1 - C Y T O C H R O M E C by Jose Villegas M.Sc , National Autonomous University of Mexico, 1999 A THESIS SUBMITTED IN PARTIAL FULFILLMENT OF THE REQUIREMENTS FOR THE DEGREE OF DOCTOR OF PHILOSOPHY In The Faculty of Graduate Studies Biochemistry and Molecular Biology UNIVERSITY OF BRITISH C O L U M B I A April 2006 © Jose Villegas, 2006 A B S T R A C T Understanding the peroxidase activity of cytochrome c is essential for the development of potential applications of this useful property in diagnostics, chemical synthesis and bioremediation. Such knowledge may also be relevant for understanding the chemical basis of the physiological consequences of this reaction under conditions of oxidative stress induced by toxic species generated by various cellular processes. In the present work, the thermodynamic, spectroscopic and catalytic properties of two classes of cytochrome c variants have been studied in an effort to gain greater insight into the structural basis for the peroxidase activity o f the cytochrome. The ultimate goal of this was to identify a strategy for the design or identification of new cytochrome c variants with significantly enhanced peroxidase activity. Development of one class of these variants was based on structural insight provided by X-ray crystallography, and the resulting variants were produced by site-directed mutagenesis. The other class of variants were obtained by a directed evolution method in which randomly-generated variants with elevated peroxidase activity were identified by the use of an activity screen. The results obtained suggest that the changes in the peroxidase activity observed for each variant are related not only to an overall destabilization of the protein structure but also to localized variations around the prosthetic group that function in combination to regulate the kinetic behavior of this protein. Such structural factors are discussed here in the context of the thermodynamic, spectroscopic and catalytic properties of each variant. 11 T A B L E OF CONTENTS A B S T R A C T .ii T A B L E OF CONTENTS , i i i LIST OF TABLES viii LIST OF FIGURES xi DEDICATIONS xii 1. INTRODUCTION 1 I. Cytochrome c ; 1 A. History 1 B. Classification 2 C. Structural characteristics of c-type cytochromes 6 D. Function 10 II. Catalytic properties of cytochrome c 13 A . Overview and applications 13 B. Peroxidase activity of cytochrome c 15 C. Influence of cytochrome structure on peroxidase activity 19 D. Spectroscopic methods for characterization of cytochrome c 19 E. Practical advantages of a cytochrome c-based catalyst 21 •ILL Design of cytochrome c variants 22 A. Mutation sites 23 TV. Directed evolution • .26 A. Experimental strategies to identify functionally useful structural variants 27 B. Activity screening strategies . 30 C. Examples of activity enhancement through directed evolution 32 V. Limitations of oxidative catalysis 35 A. Catalytic inactivation and degradation products . 35 B. Principal challenges to the conversion of cytochrome c into an efficient catalyst 38 2. METHODS 40 I. Computer simulation methods 40 II. Molecular biology methods 40 A. Expression of yeast iso-1-cytochrome c 40 B. Production and purification of recombinant cytochrome c 41 C. Site-directed mutagenesis of cytochrome c 42 D. Random mutagenesis 43 E. Screening assay 44 HI. Spectroscopic methods 45 A. Electronic absorption spectroscopy 45 B. Spectropolarimetry 45 C. Magnetic circular dichroism 45 D. NMR spectroscopy 46 E. Electron spin resonance spectroscopy 46 IV. Kinetic methods 47 A. Steady-state kinetics 47 1. Determination of k i and k3 47 iv 2. Inactivation constant 47 3. Partition ratio ...48 4. Heme decay 49 V . Physical methods 49 A . Thermal denaturation curves 49 B. p K a value 50 C. Direct electrochemistry 50 V I . Protein analytical methods 51 A . Electrophoresis 51 B. Exogenous ligand binding to the heme 51 C. Trypsin digestion ; 52 D . Mass spectrometry 52 3. R E S U L T S : 54 I. Kinetic parameters o f cytochrome c variants 54 A . Peroxidase substrate and progress curve 54 B. Steady state kinetics analysis 57 C. k i n a c t and the partition ratio 59 II. Stability of variants towards temperature, p H and H 2 O 2 62 III. Reduction potential and thermodynamic parameters 65 IV. Spectroscopic characteristics of cytochrome c variants 69 A . Electronic absorption spectroscopy '. 69 B. Spectropolarimetry 71 C. Magnetic circular dichroism 74 v D. N M R spectroscopy 76 1. 1H N M R spectra.. 76 2. 2D N O E S Y spectra 78 3. HM8 80 4. HM3 81 5. HP7-lb 82 6. HP7-la 82 7. HM5 83 8..HM1 83 V. Random mutagenesis 84 A. Mutagenic reaction 85 B. Screening methods 88 C. G84E variant 92 1. Kinetic parameters 92 2. Stability parameters 94 3. Reduction potential and thermodynamic parameters '. 95 4. Spectroscopic characteristics of G84E 96 VI. The mechanism of cytochrome c inactivation by hydrogen peroxide 97 A. Heme bleaching 98 B. Exogenous ligand binding to the heme 100 C. Peroxide-induced oligomerization of fenicytochrome c 101 D. Identification of inactivation reaction products. 105 4. DISCUSSION 109 v i I. Site-directed variants UO II. Random mutagenesis : U 8 A. The G84E variant 120 TJJ. Inactivation reaction 125 IV. Future directions 129 5. REFERENCES 1 3 1 vii LIST OF T A B L E S Table I. Variety of catalytic activities displayed by cytochrome c 14 Table II. Catalytic parameters derived from the peroxidase ping pong analysis for cytochrome c variants ; 59 Table HI. Inactivation constant for cytochrome c variants at three concentrations of H2O2 60 Table TV. Partition ratio for cytochrome c variants calculated from the coefficient of product oxidized versus the amount of catalyst at three concentrations of H2O2 61 Table V . Parameters related to stability of ferricytochrome c variants 64 Table V I . Electrochemical properties of wild-type iso-1 -cytochrome c and variants 68 Table VII. Results from random mutagenesis of the cytochrome c gene 87 Table VIII. Rate constants k i and k 3 constants obtained from steady-state kinetics analysis of the peroxidase activity of wild-type ferricytochrome c and the G84E variant 93 Table IX. Inactivation and partition ratio comparison of the G84E variant versus the W T protein 94 Table X . Parameters related to stability of the randomly generated ferricytoclirome c 95 Table X I . Reduction potential and thermodynamic parameters for the G84E variant 96 Table XII . Electronic absorption spectroscopy of the G84E variant 97 v m LIST OF F I G U R E S Figure 1. Examples of various heme groups distributed among the cytochrome c family ; 4 Figure 2. Ribbon view of iso-1-cytochrome c 7 Figure 3. Role of cytochrome c in mitochondrial electron transport 11 Figure 4. Peroxidase cycle showing the path for compound I (CI) and compound II (CII) starting from the ferric form of the enzyme 16 Figure 5. Intermediates considered for the kinetic analysis of the peroxidase cycle 17 Figure 6. Mutation region in (A) W T , (B) N52I/Y67F, (C) A59 , and (D) N 5 2 I W 5 9 A / Y F 6 F 7 variants of yeast iso-1-cytochrome c in the oxidized state 24 Figure 7. Two different strategies for the in vitro evolution of enzymes 28 Figure 8. Substrates used to test the peroxidase activity of cytochrome c 55 Figure 9. Typical reaction curves for A B T S and dicarboxidine substrates 56 Figure 10. Calculation of ki and k3 kinetic constants 58 Figure 11. Thermal denaturation curves of cytochrome c variants 63 Figure 12. Cycl ic voltammograms of cytochrome c variants using a three electrode system with a modified gold electrode as working electrode 66 Figure 13. Temperature-dependent variations of the midpoint redox potentital for cytochrome c variants 67 Figure 14. Electronic spectra of cytochrome c variants 70 Figure 15. Far U V circular dichroism spectra of ferricytochrome c variants :.. .72 Figure 16. Visible circular dichroism spectra of cytochrome c variants 73 ix Figure 17. Near IR magnetic circular dichroism spectra of the proteins studied in this work 75 Figure 18. Nomenclature conventions for the heme group '... .76 Figure 19. 1H N M R spectra of ferricytochrome c variants 77 Figure 20a. Up field region of the NOESY spectra of wild-type and variant ferricytochrome c 79 Figure 20b. Expanded view of Figure 20a showing the area around HM8 and HM3 for each variant 80 Figure 21. Agarose gels (2%) to estimate the amount of D N A produced per cycle of mutagenic PCR (A) and the yield at different concentrations of MnCb (B) 86 Figure 22. Minimum amount of cytochrome c for which peroxidase activity was detectable with peroxidase substrates ABTS and H2O2 ; 88 Figure 23. Principal steps in the initial peroxidase activity screening strategy 89 Figure 24. Modified screening protocol that permitted detection of peroxidase activity in E coli cells expressing cytochrome c variants 90 Figure 25. PCR plate showing the positive reaction of variant G84E after repeated dilutions of the substrate solution, indicated by a dark green color absent in all the other tubes 92 Figure 26. Decrease in absorbance at the Soret maximum ("bleaching") following addition of H2O2 to ferricytochrome c :.99 Figure 27. Change in Soret spectrum following incubation wild-type ferricytochrome or the double variant with K C N 100 Figure 28. Oligorimezation of cytochrome c in the presence of H 2 0 2 observed by SDS P A G E electrophoresis 102 Figure 29. SDS P A G E electrophoresis of the products resulting from reaction of wild-type ferricytochrome c with various ratios of [H202]/[ferricytochrome c] 103 x Figure 30. Absorbance change versus time for the heme absorbance of WT cytochrome c (2uM) in the presence of IX , 10X and 100X excess (2 uM, 0.2 and 20 uM, respectively) of H 2 0 2 104 Figure 31. General strategy to identify protein modifications resulting from the reaction of ferricytochrome c with H2O2 in the presence of a spin trap by HPLC/MS peptide mapping 106 Figure 32. Reaction of cytochrome c with H2C>2 in the presence of P O B N analyzed by LC/MS 107 Figure 33a. Stereo drawing showing the region around HM8 in the WT and the variant N52I/W59A/Y67F 117 Figure 33b. Stereo drawing of the region around HM3 and HP7 in the WT and N52I/W59A/Y67F proteins 117 Figure 34. Stereo-drawing from the region around Argl3 in the WT protein 123 Figure 35. Structural changes produced by the G84E as estimated by molecular simulation 125 x i To my famity: To my invafua6Ce parents, Juan Jose and Luz Maria To my belovedgrandmother, Ccrrmen To my dear brother, filejandro To my Coving wife, j\driana To my littte treasure, C^teste xii 1. INTRODUCTION I. Cytochrome c A. History The first report describing cytochrome c was published in the 1880s by Charles A . MacMunn, a practicing physician. MacMunn identified what he referred to as "myohaematin," which he studied initially as a "pigment" distributed in several animal tissues that he had analyzed by microspectroscopy, one of his hobbies (Scott 1996). MacMunn ' s work was the target of intense criticism by his contemporaries and was disregarded for many years. The primary limitation of MacMunn ' s work was in considering myohaematin to be a single substance rather than a combination of compounds. The "rediscovery" of cytochrome c and the acknowledgment of MacMunn 's results in the mid 1920s by David Ke i l in , an Oxford parasitologist who was initially interested in the fate of hemoglobin during the life cycle of the fly Gasterophilus, is a good example of serendipity (Bendall 2004). In his initial investigations of fly tissues with a low-dispersion spectroscope, K e i l i n identified four characteristic bands in the spectrum at 604, 566, 550 and 520 nm. B y analogy with other heme proteins already known, he assigned the first three to the a-bands of three different components a, b and c, while the last one was ascribed as the combination of the a-bands of all of them (Keil in 1966). In addition, by observing the presence or absence of such bands in the spectra of fly muscle exposed to different oxygenation conditions, K e i l i n demonstrated that this protein undergoes reduction and oxidation in vivo. K e i l i n not only introduced the term cytochrome (curiously as a "temporary" designation) which means cellular pigment but also proposed a nomenclature system based on the wavelength of maximal absorption for 1 the reduced proteins (Kei l in 1966). He was also the first one to obtain a pure preparation of cytochrome c (around 1930) isolated from yeast. A t that time, this protein was normally recognized by the unusual position of its a-band, suggesting a modification of the heme group. Such modification was later identified by Theorell (Ehrenberg and Theorell 1955) as the product of reaction between the a-carbon of the vinyl substituents of the prosthetic group and the thiol sulfurs of two cysteine residues. These results in turn, led to the later finding of the conserved protein motif C y s - X - X - C y s - H i s for this class of proteins. A s a result of these pioneering experiments and subsequent work by many others, the proteins involved in oxidative phosphorylation (Ferguson 2001) and the role of cytochrome c as the penultimate electron acceptor in this process have been defined so that the prominent role of cytochrome c in the "respiratory mechanism of the cel l" described by K e i l i n (Kei l in 1966) is now generally recognized. B. Classification Cytochromes can be found not only in cells of higher organisms, where they were originally discovered, but also in yeast and bacteria. In the case of bacterial cytochromes, a greater variety of structures and wider range of cellular roles is apparent relative to the mitochondrial cytochromes (Ferguson 2001). The term cytochrome, as introduced by Ke i l i n , encompasses a group of hemeproteins that undergoes reversible oxidation-reduction reactions that are readily detected by distinct changes in the visible electronic absorption spectrum. More recently, a more specific but essentially equivalent definition describes a cytochrome as a 2 "hemeprotein whose characteristic mode of action involves transfer of reducing equivalents associated with a reversible change in oxidation state of the prosthetic group" (Palmer and Reedijk 1991). A t present this term is applied to most intracellular electron transfer proteins that possess a heme group. Heme proteins responsible for dioxygen transport or heme enzymes (e.g., peroxidases, catalase, sulfite reductase) are not regarded to be cytochromes (Palmer and Reedijk 1991). Cytochromes are generally classified into four general groups and are referred to as cytochromes a, b, c and d, depending on the type (Figure 1) and binding characteristics of their heme group: a-Type Cytochromes: The members of this group contain a molecule of Heme A (Figure 1) and some examples include cytochrome acii, a membrane bound protein complex also referred to as cytochrome c oxidase, and cytochrome ci\ present in Nitrobacter agilis. b-Type Cytochromes: Cytochromes in this group possess an iron protoporphyrin LX prosthetic group. This prosthetic group is not covalently attached to the protein, so the binding to the apo-cytochrome is mediated by the protein ligands to the heme iron, by hydrophobic interactions of the heme with the apo-protein, and by H -bonding interactions of the heme propionate groups with the protein. Some examples include cytochrome b, cytochrome bs, cytochrome bsa, cytochrome 6559, and cytochrome &562-c-Type Cytochromes: Cytochromes c possess one or more iron protoheme groups covalently bound.to the apo-protein by thioether bonds. These thioether bonds form between the heme vinyl substituents and Cys residues in the heme binding site. 3 Heme C Heme D Figure 1. Examples of various heme groups distributed among the cytochrome c family. 4 In most species, these bonds are formed by two highly conserved Cys residues though Euglena and Crithidia cytochromes c are notable exceptions (Pettigrew et al. 1975) insofar as they each possess just one thioether bond to the heme group. The mitochondrial cytochrome c group includes cytochrome c and cytochrome c\. The former is the protein described by Ke i l i n and moves freely in the intermembrane space, while the latter is a membrane bound electron transfer protein. The wide diversity of c-type cytochromes has now been recognized, particularly in bacteria, and has led to the identification of four classes (or types) of cytochromes c (Moore 1990) as described below. Class I cytochromes c are small, soluble proteins of high midpoint potential that usually contain a low-spin iron. The heme group is attached to the protein near the N-terminus while the sixth ligand, normally Met80, is situated near the C-terminus. Three main a-helices form a "basket" surrounding the heme group which is exposed to the solvent at the surface of the protein along one of its edges. Mitochondrial cytochrome c and cytochrome c 2 are included in this class. Class II ferricytochromes c contain primarily a high-spin iron, but in a few members of this family (e.g., cytochrome C556) is low-spin. The heme group is also attached to the apo-protein near the JV-terminus, and the main protein fold consists of a four a-helical bundle (Ambler 1991). Class III cytochromes c are normally high molecular weight proteins with low-spin iron and multiple heme groups of a wide range of midpoint potentials. Examples of this class include cytochrome c-j and C3. 5 Class IV cytochromes c are tetraheme proteins that normally function as subunits and that have heme groups with either bis-His or His-Met coordination. The typical example of this class is the subunit of the photosynthetic reaction center. Cytochromes d: This group of cytochromes is distinguished by the presence of a partially reduced tetrapyrrole prosthetic group that possesses additional substituents not normally observed in proteins possessing protoheme LX-based prosthetic groups. This class of prosthetic group is characterized by the presence of one or two reduced pyrrole rings and they are referred to as chlorins or bacteriochlorins, respectively. Representative members of this group of cytochromes include cytochrome d, cytochrome bd and cytochrome cd\ (nitrite reductase). C. Structural characteristics of c-type cytochromes Despite this structural variability, elements of the secondary structure (e.g., surface exposed residues, components of the hydrophobic core) as well as the tertiary structure of c-type cytochromes are conserved (Borsari et al. 2000). The backbone structure of cytochromes c consists primarily of four to five a-helices, varying with species, that are connected through several loops that together enclose a covalently attached protoheme group at the active site (Figure 2). A s mentioned previously, this prosthetic group is attached to the protein covalently by thioether bonds to a highly conserved C X X C H motif although some examples of C X X X X C H and C X X C K have been found (Stevens 2004). The iron atom coordinated by this cyclic tetrapyrrole is coordinated to two axial ligands provided by amino acid side chains that are typically methionine and histidine but 6 other amino acid residues can be involved depending on the species and type of cytochrome. Methionine can be substituted by another histidine in bacterial cytochromes, while histidine can be replaced by lysine in some cases (Stevens 2004). Figure 2 . Ribbon view of yeast iso-l-cytochrome c. The heme group in the center (lateral view) is coordinated to the axial ligands, His 18 (right) and Met 80 (left). 7 The biological function of c-type cytochromes is to act in electron transfer reactions in which the oxidation state of the heme iron atom undergoes a reversible change in oxidation state between F e + 2 and F e + 3 . In view of the similarities in prosthetic group and axial ligands between various classes of c-type cytochromes, the range in midpoint reduction potential exhibited by this family of proteins is striking. For bacterial cytochromes c, this range covers -800 m V , from -400 m V to +400 m V (Moore 1990; Lett and Guillemette 2002). For mitochondrial cytochrome c (Class III), the high potentials of+200 to +350 m V result from stabilization of the ferrous state by the methionine ligand, the hydrophobic nature of the heme binding site and the low dielectric constant of this environment, and the limited solvent accessibility of the heme (Moore 1983; Churg and Warshel 1986; Mauk and Moore 1997; Battistuzzi et al. 2002). Besides the type of axial ligands, additional factors that influence the potential of heme proteins include substituents on the porphyrin ring, degree of exposure of the heme group to the solvent, and the hydrophobic environment and electrostatic effects, including the interaction with the propionate groups (Moore 1983; Lett and Guillemette 2002; Paoli et al. 2002). The contributions of specific amino acid residues to the midpoint potential has been explored through the systematic study of variants prepared by chemical modification (Rees 1980; Wallace 1984; Wallace and Corthesy 1987; L i u et al. 1995), site-directed mutagenesis (Komarpanicucci et al. 1992; Lett et al. 1996; Lett and Guillemette 2002), semi-synthesis (Wallace and Proudfoot 1987; Wallace 1993) or a combination of mutagenesis and semi-synthesis (Wallace and Rose 1983; Wallace et al. 8 1986) by means of spectroscopic, electrochemical and kinetic methods (Moore 1983; Moore et al. 1984; Cutler et al. 1989; Moore 1990). Other conserved structural features among class I c-type cytochromes include the presence of interacting TV-terminal and C-terminal helices, contiguous hydrophobic patches in the C-terminus between position 94-98, conserved hydrogen bonds between the heme propionate groups and several amino acids around the heme group (including residues Arg38, Tyr48, Trp59 and Thr78), and a large number of lysyl residues distributed asymmetrically on the surface of the protein in the region where the edge of the heme prosthetic group is partially exposed to solvent. The positively-charged electrostatic surface created by these lysyl residues is generally believed to facilitate electrostatic interaction with the other electron transfer proteins with which the cytochrome reacts physiologically (Scott 1996). A s mentioned above, the functional roles of many individual amino acid residues have been studied by characterization of modified cytochromes in which specific residues have been chemically modified or replaced by mutagenesis or semi-synthesis. The bases for selecting some of these substitutions are readily apparent (e.g., the residues providing axial ligands to the heme iron, residues that anchor or interact with the heme group), but other residues have been selected for study on the basis of several factors. B y its proximity to the heme binding site, by sequence analysis or selected by screening (e.g., Tyr67, which forms an H-bond with the axial ligand Met80 (Frauenhoff and Scott 1992)); Trp59, Arg38 and Tyr48 which participates in an internal H-bond network that involves a heme propionate (Schweingruber et al. 1977; Caffrey and Cusanovich 1993); A r g 38, Tyr48 and Trp59, which participate in an internal H-bond network involving a heme propionate (Thurgood 9 et al. 1991); Asp52, Ile75, Thr78 and Phe82, all of which have been implicated in the stability or electron transfer reaction of the protein (Rafferty et al. 1996). Other residues like Lys73, Lys79, Lys86 and Lys87, have been evaluated with regard to the alkaline conformational change (Rosell et al. 1998); Cys 102, which allows dimerization of the yeast protein (Zuniga and N a i l 1983); Met65, which relates to use of semi-synthetic strategies with yeast cytochrome c (Schejter and Avi ram 1970; Bren and Gray 1993); C y s l 4 , C y s l 7 , Leu34, Leu37, Trp59 and Met80 in the expression and in vivo function (Hampsey et al. 1986); and replacement of the (Q)-loop (residues 18-32) on the overall protein stability (Fetrow et al. 1997). Despite this substantial body of work, the specific roles of many residues in determining various functional properties of cytochrome c remain to be evaluated rigorously. D. Function Cytochrome c is an essential component of the respiratory chain that facilitates electron flow between redox proteins during cellular respiration, either in the periplasm of various bacteria or in the intermembrane space of the mitochondrion. In mitochondria (Figure 3), cytochrome c transfers electrons from Complex III (a coenzyme Q-cytochrome c oxidoreductase) to complex IV (cytochrome c oxidase) both located in the inner membrane, h i the intermembrane space, cytochrome c also interacts with sulfite oxidase and cytochrome bs (in animals) and with cytochrome b2 and cytochrome c peroxidase (in yeast). 10 NADH NAD + 311111 lliP '.M'/^r':'. l'7, ' > so 2 e" from Complex I and II Lactate Pyruvate | ^ QH 2 Q Outer Membrane •*©: H 20 H 20 2 + 2H + Intermembrane space Complex Complex IV 0 2 + 4H + 2H 20 Inner membrane Matrix Figure 3. Role of cytochrome c in mitochondrial electron transport. The direction of electron flow is indicated by broken arrows. Cytochrome c (c) transfers electrons between complex III and IV and also interacts with cytochrome b5, sulfite oxidase (SO), cytochrome b2 (b2) and cytochrome c peroxidase (CCP). Modified from (Moore 1990). Despite extensive study over 80 years, the physiological roles of cytochrome c continue to be a source of enquiry. As mentioned above, although cytochrome c is recognized as a crucial component in electron transfer reactions, the detailed mechanism by which electron transfer between proteins occurs is a topic of continuing study (Moore 1990; Rafferty et al. 1996; Stevens et al. 2004). Similarly, cytochrome c serves as a model for the investigation of the mechanisms of protein folding and thermal stability 11 (Caffrey and Cusanovich 1994; Liggins et al. 1994; Fetrow et al. 1997; Winkler et al. 1997; Lett et al. 1999; Liggins et al. 1999; Mi lne et al. 1999; Yamamoto et al. 2002; Bhuyan et al. 2005; Sherman 2005). More recently, cytochrome c has been recognized in many cell lines as a major participant in. apoptosis. In these cell lines, signaling pathways utilize the mitochondrial machinery to effect apoptosis by releasing various proteins including cytochrome c (Iverson and Orrenius 2004; Orrenius 2004). Once released from the mitochondrion, cytochrome c takes part in a complex that is referred to as the apoptosome. In turn, this complex triggers an activation cascade involving several caspases that eventually leads to characteristic morphological changes such as cytoplasmic blebbing, and condensation and fragmentation of chromatin. Cytochrome c has also been implicated in other cellular processes under conditions such as oxidative stress where the concentration of oxidizing species, including H 2 O 2 , are significantly elevated. Under such conditions, cytochrome c can catalyze the oxidation of model membrane structures such as phosphatidylcholine liposomes, very likely by various mechanisms, and, thus, promote "site-specific" mitochondrial l ipid peroxidation during oxidative stress (Radi et al. 1991b). Cytochrome c has also been reported to act as a scavenger of reactive oxygen species in neurons under conditions of oxidative stress and has been proposed to function as a defense against such species (Atlante et al. 2000). Similarly, the reduction of ferricytochrome c by superoxide has been suggested to serve an antioxidant function (Korshunov et al. 1999). 12 II. Catalytic properties of cytochrome c A. Overview and applications In recent years, realization that cytochrome c possesses a variety of catalytic activities in vitro has led to increasing interest in understanding the mechanisms by which these activities arise and in the potential use of cytochrome c as an industrial catalyst. The catalytic activity of cytochrome was reported as early as 1962 in rat kidney and liver (Flatmark 1962), but more recent examples were proposed to be involved in membrane peroxidation (Florence 1985; Radi et al. 1991b). Several catalytic activities that normally involve reaction with H 2 O 2 have been attributed to cytochrome c and some examples are listed in Table I. h i addition to these examples, cytochrome c can promote the nitration of proteins by nitrite through a mechanism that involves the formation of tyrosyl radicals and nitrogen dioxide (Castro et al. 2004), and it reacts with hypochlorous acid (Prutz et al. 2001), ethanol radicals (Anni and Israel 1999) and peroxynitrite (Cassina et al. 2000). In sum, these demonstrate the catalytic potential and diversity of cytochrome c and suggest several potential applications especially for the peroxidase activity of cytochrome c. These possible applications provide considerable motivation for an improved understanding of the mechanism and structural basis for the peroxidase activity of cytochrome c and for the development of new cytochrome derivatives with enhanced peroxidase activity. 13 Table I . Catalytic activities displayed by cytochrome c Substrate Reaction Reference A B T S , 4 amino antipyrine and Oxidation (Radi etal. 1991a) luminol Tyrosine Oxidation (Chen et al. 2002) m-aminophenol Hydroxylation and dimerization (Zhu etal. 1998) o-phenylendiamine . Polymerization (Zlmetal. 1998) o-phenylendiamine Oxidation (Ono et al. 2001) N,N-dimethylaniline N-demethylation (Fujitaef a/. 1994) Diphenylacetaldehyde and 3- Oxidation (Nantes etal. 1998) methylacetoacetone Diphenylacetaldehyde and 3- Peroxidation methylacetoacetone Catecholamines Oxidation (Roseie^fl/. 1998) Anthracene, pyrene, Oxidation (Torres etal. 1995) anthraquinone, etc Pinacyanol chloride Oxidation (Vazquez-Duhalt 1993) Dibenzothiophene Oxidation (Vazquez-Duhalt R 1993) 7,12-dimethylbenzanthrace, Oxidation (Tinoco 1998) azulene and anthracene Pho sphatidylcho line Peroxidation (Radi et al. 1991b) N-methylcarbazole and Oxidation (Akasaka et al. thioanisole 1993) Oleofines Epoxidation N,N,N',N'-tetramethyl-1,4- Oxidation (Zhou et al. 2004) phenylendiamine 14 B. P e r o x i d a s e ac t i v i t y o f c y t o c h r o m e c The catalytic mechanism of peroxidases has been studied extensively, establishing many general principles of the mechanism (Abelskov et al. 1997; Banci 1997; Dunford 1999; Moffet et al. 2000; Nielsen et al. 2001). Nevertheless, detailed understanding of the contributions of amino acid residues in the active site or some distance from the heme prosthetic group to regulating the kinetic properties of such enzymes, as well as alternative pathways in the mechanism, remains a subject of continuing investigation (Rasmussen et al. 1995; Ghibaudi and Laurenti 2003). Similarly, understanding of the precise mechanisms by which peroxidases become inactivated during turnover remains poor. (Valderrama et al. 2002). The general peroxidase catalytic cycle (Figure 4) involves a series of intermolecular electron transfer reactions that start with the conversion of the ferric form of the prosthetic group to a higher oxidation state, ferryl heme (Fe (IV)=0) to produce a cationic porphyrin radical intermediate referred to as compound I (CI). This two electron transfer reaction requires H 2 O 2 as a substrate and results in the oxidation of the protein and the reduction of the peroxide to water. Compound I can react with an electron donor, usually an aromatic moiety, to recover one electron and produce a second intermediate, compound II (CH), and a free radical on the donor molecule. To complete the cycle, CII reacts with a second electron donor to regenerate the Fe(III)-heme state (Banci 1997; Dunford 1999). 15 Figure 4. Peroxidase cycle showing the path for compound I (CI) and compound II (CH) formation starting from the ferric form of the enzyme. A H represents any electron donor molecule, while ' A H is the corresponding radical. 16 Several steady-state kinetic analyses of this mechanism have been reported, and more than one possible peroxidase mechanism is recognized. Perhaps the most widely accepted analysis is the one developed by Dunford (Dunford 1999) in which the peroxidase reaction is regarded as a special (irreversible) case of a ping pong mechanism. This concept of irreversibility implies that the rate of enzyme-complex formation is much faster than the rate of its breakdown. For this reason, when the peroxidase cycle is represented in the form of a ping-pong mechanism (Figure 5), the products are shown to be formed as soon as the substrates are in contact with the enzyme because the enzyme-substrate complexes are so short lived that they are extremely difficult to observe. A s a consequence, the rate of the reaction seems to lack an upper limit. In other words, the rate of the reaction wi l l increase with increased substrate concentrations until saturation or inactivation of enzyme occurs (Dunford 1999). H 2 0 2 H 2 0 A H A A H A V V Native C o m p o u n d I C o m p o u n d II Native Figure 5. Intermediates considered for the kinetic analysis of the peroxidase cycle (Dunford 1999). 17 The individual steps of this mechanism are shown below (modified from (Dunford 1999): Fe(III)-cytochrome + H 2 0 2 — - — • C o m p o u n d I + H 2 0 (1) Compound I + A H — — • Compound II + A» (2) Compound II + A H ^ 3 •Fe(III)-cytochrome + A» + H 2 0 (3) Where A H denotes a reducing substrate and A» is the corresponding free radical species. These steps are consistent with the overall reaction for this process shown below: H 2 0 2 + 2 A H = 2 H 2 0 + 2 A . (4) If the reaction rate (v) is defined in terms of disappearance of reducing substrate and ki is assumed to be much smaller that fo, the steady-state approximation simplifies to the following expression: 2rEnl 0MH2O2]) (fc[AH]) Therefore, under conditions that the starting concentration of reducing reagent is held constant, a plot of E/v versus 1/[H 2 0 2 ] allows calculation of the second order rate constants k\ and £3 (M" 1 sec"1) for the rates of compound I formation and compound II reduction, respectively. 18 C. Influence of cytochrome structure on peroxidase activity Recent studies have correlated the structural stability of ferricytochrome c with the catalytic activity of the protein (Diederix et al. 2001; Diederix et al. 2002; Diederix et al. 2003) and the effect of specific mutations on the thermal stability of the protein (Berghuis et al. 1994; Lett et al. 1996). A major objective of the current work was to identify and correlate changes in the structure of designed variants of yeast iso-l-cytochrome c that- exhibit varying peroxidase activities. For example, the N52I /Y67A variant is known to exhibit increased stability to thermal denaturation (Berghuis et al. 1994) while the W 5 9 A substitution was expected to destabilize the protein by disrupting an internal hydrogen bonding network (Caffrey and Cusanovich 1993; Black et al. 2001). These mutations may also directly influence the catalytic mechanism of the peroxidase reaction because the N52I/Y67F double substitution has been found to delay the bleaching of the heme group in the presence of H 2 O 2 without fully preventing the inactivation of the protein (Villegas et al. 2000). On the other hand, replacement of Trp59 eliminates a likely inactivation target in the catalytic mechanism by removing a residue prone to free radical formation. Finally, the N52I /Y67F/W59A triple variant was constructed to determine whether the combination of these two opposing but desirable properties in a single protein would result in a more active and robust peroxidase activity. D. Spectroscopic methods for characterization of cytochrome c Spectroscopic methods provide powerful tools for the study of subtle structural changes in chemically and genetically modified proteins. In the present work, several spectroscopic techniques were used to characterize the kinetic behavior of the variants 19 under study and to study the structural and spectroscopic consequences of substitutions introduced in proximity to the heme prosthetic group. Specifically, U V and visible electronic absorption spectroscopy, circular dichroism (CD), magnetic circular dichroism ( M C D ) and nuclear magnetic resonance ( N M R ) spectroscopies are established methods for the characterization of heme proteins in general and cytochrome c in particular. Some examples of the application of these methods to cytochrome c include the following: the study of the conformation of cytochrome c from various species and the effect of denaturants and p H by C D spectroscopy (Myer 1968a; Myer 1968b; Vinograd and Zand 1968; Greenwood and Wilson 1971); the study of the effect of mutations involving axial ligands to the heme iron and other substitutions by electronic absorption spectroscopy (Pielak et al. 1986; Santucci and Asco l i 1997; Zheng et al. 2000); evaluation of the effects of denaturants on the conformation of the protein by C D spectroscopy (Tsaprailis et al. 1998); assessment of the coordination state of cytochrome c variants by M C D spectroscopy (Gadsby et al. 1987; Gadsby and Thomson 1990; Hawkins et al. 1994; Fedurco et al. 2004). In addition, an extensive use of the N M R technique has been applied for the assignment of the ' H - N M R resonances of several heme methyl and ligand protons in horse and yeast ferricytochrome c (Mcdonald and Phillips 1973; Keller and Wuthrich 1978; Moore and Will iams 1984; Will iams et al. 1985; Feng et al. 1990); the study of the effects of temperature and ionic strength on the structural dynamics of the protein (Burns and Lamar 1979; Burns and Lamar 1981; Moore 1990; Turner and Will iams 1993; Yamamoto 1996); characterization of the influence of mixed solvent systems on the conformational equilibria of ferricytochrome c (Sivashankar and Mabrouk 1998; Sivakolundu et al. 2001; Sivakolundu 2003); the correlation of mutation-induced 20 structural changes to cytochrome c with the electrochemical properties of the protein (Moore et al. 1984; Cutler et al. 1989; Moore 1990); determination of the structure of yeast wo-1-ferricytochrome c in solution (Gao et al. 1990; Banci et al. 1997), including determination of the structure of one alkaline ferricytochrome conformer (Assfalg et al. 2003); and the assessment of the structural consequences of several amino acid replacements (Gao et al. 1991; Moench et al. 1991; Thurgood et al. 1991; Davies et al. 1993). E. Practical advantages of a cytochrome c-based catalyst In comparison with true peroxidase enzymes, the main disadvantage of using cytochrome c to catalyze oxidative reactions is the low activity. The rate of formation of CI for a typical peroxidase is between 10 5-10 7 M ' V 1 (Abelskov et al. 1997; Moffet et al. 2000), while for some cytochromes a second rate constant in the order of 10-200 M ' V 1 has been reported (Prasad et al. 2002). On the other hand, cytochrome c possesses several advantages relative to true peroxidases, the most obvious of which is the covalent attachment of the prosthetic group to the protein. Although the physiological advantage(s) of a covalently-bound prosthetic group in this protein is not yet apparent (Stevens 2004), this characteristic endows cytochrome c with significant stability to thermal denaturation and with considerable tolerance to exposure to organic solvents and extremes of p H (Vazquez-Duhalt 1999). Notably, cytochrome c can promote the oxidation of dibenzothiophene over the p H range of 2 to 10 with maximum activity at pH~7 (Vazquez-Duhalt 1993). Similarly, cytochrome c can catalyze the oxidation of pinacyanol chloride, a model peroxidase substrate, in the presence of varying proportions 21 of water miscible solvents with 18% of activity retained in a 90% solution of tetrahydrofuran (Vazquez-Duhalt et al. 1993). Finally, the peroxidase activity of cytochrome c can be observed over the temperature range of 30 to 80 °C (Vazquez-Duhalt 1999). h i comparison, exposure of the widely available and readily accessible plant and fungal peroxidases to even mild forms of these environmental perturbations normally results in the loss of the heme group, which is bound only through non-covalent interactions (N.B. some mammalian peroxidases also possess covalently bound heme prosthetic groups, but these are not abundant proteins and often exhibit other properties that render them challenging to study (i.e., membrane binding)). To exploit the structural advantages of cytochromes and overcome their catalytic limitations, a detailed understanding of the catalytic oxidative mechanism is required. The current work begins this undertaking by characterizing the spectroscopic, catalytic and thermodynamic consequences of selected amino acid substitutions that are expected to increase the probability of developing an effective peroxidase based on the cytochrome c structural scaffold. III. Design of cytochrome c variants. h i this work, we used as a starting point recent studies that have correlated the stability of cytochrome c with its peroxidase activity by demonstrating an increased catalytic activity upon unfolding of the protein (Diederix et al. 2001; Diederix et al. 2002; Diederix et al. 2003) a s w e U a s studies concerning the effects of specific mutations on the thermal stability of the protein (Berghuis et al. 1994; Lett et al. 1996). B y 22 designing variants with opposing effects on the stability of the protein and investigating the resulting consequences on the peroxidase activity as well as oh the overall structure of the protein, new and useful insight was sought into the mechanism by which cytochrome catalyzes these type of reactions. A. Mutation sites The amino acid residues substituted in the variants studied in the present work are involved in a complex hydrogen-bonding network (Figure 6A) that includes residues Thr78, Met80, the heme propionate groups and at least two buried water molecules (Louie and Brayer 1990). This internal hydrogen-bonding network has been identified as a crucial region of the protein with regard to some of the structural differences between oxidation states of the wild-type cytochrome (Berghuis and Brayer 1992; Lett et al. 1996) and the double variant N52I/Y67F (Berghuis et al. 1994). In the oxidized wild-type protein, reorientation of an internal water molecule (WAT 166) eliminates the hydrogen bond between Ile52 and Watl66 that occurs in the wild-type protein, and the hydrogen-bond that normally forms between heme propionate A (or HP7) and Ile52. This latter interaction has been associated with an increase in the dynamics of a substructure of the protein upon oxidation (Berghuis and Brayer 1992). Tyr67 on the other hand, is located on the same side of the heme, and normally forms a hydrogei>bond with the Met80 axial ligand to the heme iron in the oxidized cytochrome (Berghuis and Brayer 1992) though the structural contributions of this tyrosyl residue are only partially understood (Berghuis etal. 1994). 23 Figure 6. Mutation region in (A) WT, (B) N52I/Y67F, (C) W59A, and (D) N52I/W59A/Y67F variants of yeast iso-l-cytochrome c in the oxidized state. In A - D , mutated residues are represented as ball and stick models while nearby residues, including the heme group are shown as stick models. The heme group, in thinner black lines, is facing the plane with the propionate groups 6 and 7 in the bottom. The residue number in each structure, starting from the top residue (Met 80) right in front of the heme group, in a clock wise direction are: Thr 78; residue 52 (Asn in A and C, He in B and D); residue 59 (Trp in A and B , A l a in C and D); residue 67 (Tyr in A and C, Phe in B and D). The water molecule W A T 1 6 6 (present only in A and C) is represented as a big sphere. Hydrogen bonds are shown as thick broken lines. A and B structures were taken from the P D B bank (I.D. 2 Y C C (Berghuis and Brayer 1992) and 1RCI (Berghuis et al. 1994) respectively), while C and D were simulated by introducing the mutation W 5 9 A in A and B respectively. 24 One of the main structural changes to occur in the N52I/Y67F variant is the loss of Wat 166, which is caused by the compression of an internal cavity by as much as 8 cubic Angstroms, rendering it too small to accommodate a water molecule (Bergliuis et al. 1994). This structural change results in the disruption of the interaction of Wat l66 with residues Ile52 and Phe67, leaving only the hydrogen bond between Thr78 and the heme 6-propionate and the interaction between Trp59 and the heme 7-propionate (Figure 6B). A s the consequence of these rearrangements, Met64 is displaced toward Leu68 to result in formation of a new hydrogen bond. The other residue substituted in the present work is the sole tryptophanyl residue in the protein, Trp59. This phylogenetically conserved residue is located at the bottom of the heme crevice on the distal site. The principal roles proposed for this unique residue are the stabilization of the hydrophobic core of the protein by means of the aromatic indole side chain and stabilization of the heme prosthetic group through formation of a hydrogen bond with the heme 7-propionate group. (Hampsey et al. 1986; Caffrey and Cusanovich 1993; Black et al. 2001). Because crystallographically determined structures are not available for the W 5 9 A and N52I /W59A/Y67F variants, the likely structural changes resulting from these substitutions were initially simulated by introducing the W 5 9 A substitution into the corresponding proteins (WT and double variant respectively) using molecular graphics software (Swiss P D B viewer v.3.7b2 and D S ViewerPro v.5.0). Consequently, the discussion of this structure represents an approximation of the true structure. In these models, the overall protein backbone is conserved, and many of the interactions observed in the parent proteins are also preserved. In both, variants, one of the 25 more evident alterations is the lack of a hydrogen bond between the side chain of Trp59 and the oxygen of HP7. A s mentioned previously, this hydrogen-bonding interaction has been proposed to help interaction of the heme group with the protein (Caffrey and Cusanovich 1993; Black et al. 2001). The replacement o f Trp59 by an alanyl residue (Figures 6C and D) potentially introduces a substantial cavity in the region of the substitution. Despite the extreme structural modification introduced, this variant was expected to be functional in part because several studies have shown that this structurally important residue can be replaced by several natural and synthetic amino acids with retention of function (Schweingruber et al. 1978; Caffrey and Cusanovich 1993; Black et al. 2001). Examples of residues used for replacement of Trp59 include amino acids incapable of forming hydrogen bonds with the heme propionates. In addition, the introduction of an alanine close to the heme iron and propionate groups should impose less electrostatic disorder than the introduction of any charged residues. IV. Directed evolution One highly effective means of identifying useful amino acid substitutions for the modification of catalytic activities or the introduction of catalytic activities into proteins that has emerged in recent years is an approach often referred to as directed evolution (Stemmer 1994; Brakmann 2001; Kolkman and Stemmer 2001). Most simply, directed evolution involves the combined use of random mutagenesis and high-throughput screening to identify amino acid substitutions that produce proteins with desired properties (Bornscheuer 1998; Harayama 1998; Iverson and Breaker 1998; Schellenberger 1998; Valetti and Gilardi 2004; Hibbert et al. 2005). .Among the 26 advantages of this strategy is the fact that knowledge of the protein structure is not required and detailed insight concerning the catalytic mechanism of the protein is not required. In some cases, this approach has been reported to result in enzyme variants with activities several orders of magnitude greater than that exhibited by the wild-type protein (vide infra). A s a result, directed evolution has at times proven to be a very powerful means of "customizing" either an existing catalytic or binding activity or, less commonly, to introduce novel functions into proteins. A crucial point o f any artificial evolution strategy is the availability or design of the selection or screening method to be used. A s well , efficient expression of the protein in a suitable host and an effective means of random mutagenesis are also important. A . E x p e r i m e n t a l s t r a t e g i e s t o i d e n t i f y f u n c t i o n a l l y u s e f u l s t r u c t u r a l v a r i a n t s Several methods have been proposed for efficient generation of large numbers of variants o f a target protein, but most can be divided into two major groups that differ in the underlying principle involved (Bornscheuer 1998). One general approach involves creation of sequential generations of mutated genes, and the other approach involves the recombination in vitro of homologous genes (Figure 7). These strategies are usually complementary to each other. The primary approach to mutant generation in the first group is to introduce sequence variability by random mutagenesis. This method has often proven to be a useful means of improving pre-existing activities of proteins, particularly for situations in which homologous genes are not available or in which genes are too small to afford effective in 2 7 vitro recombination. One of the simplest and perhaps the most commonly used method of random mutagenesis is error-prone P C R (Cadwell 1991). This method is based on the controlled introduction of random errors by a D N A polymerase during a non-optimal P C R in the presence of elevated concentration of divalent metals ions, usually D i r e c t e d e v o l u t i o n by p r o d u c t i o n o f s e q u e n t i a l g e n e r a t i o n s E r r o r - p r o n e 1 " ° 1 S e l e c t i o n i i • i n n n i • • i o o ~ m P a r e n t I m p r o v e d g e n e M u t a t e d g e n e s v a r i a n t ^ ( l ibrary) : • m f • • • • • • ' • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • * • • • • F u r t h e r c y c l e s D i r e c t e d e v o l u t i o n by in v i t ro r e c o m b i n a t i o n IS P o o l s o f h o m o l o g o u s p a r e n t g e n e s • 1. D N A d i g e s t i o n a ? o n i : o i 3. S e l e c t i o n • ni' 0 ' i r~m » (U~Dcr 2. R e c o m b i n a t i o n rr~~i i a i un 3 i—P~ In v i t ro r e c o m b i n e d f r a g m e n t s I m p r o v e d v a r i a n t F u r t h e r c y c l e s F igure 7. T w o strategies for the in vitro evolut ion of e n z y m e s . M o d i f i e d f r o m . magnesium and/or manganese (Bornscheuer 1998). 28 Once the sequence variation has been achieved, the modified sequences are expressed, and the resulting proteins are screened for the desired activity or property to identify, effective mutations. A n extremely critical point in this strategy is the level of mutagenesis or mutation frequency, which ultimately means the average number of mutations introduced per gene. Typically, advantageous mutations are rare, while deleterious mutations are more frequent, so it is important to ensure that the number of beneficial mutations is maximized. If the mutation frequency is too high, deleterious substitutions may overwhelm the effects of beneficial substitutions so that the beneficial substitutions are not recognized. A t the same time, i f the frequency of mutation is too low, the wild-type sequence w i l l dominate, and too few variants w i l l be generated for screening. Usually, a rate of 1.5-5 mutations per gene is preferred (Cadwell and Joyce 1991; Chen and Arnold 1993) ; however, this number also depends on the capacity of the screening method, so the mutation rate must be adjusted to suit the capabilities of the screening or selection method (Kuchner and Arnold 1997). The principal means of controlling this rate is to evaluate a variety of conditions, particularly the concentration of magnesium and manganese, because the activity and fidelity of most of the D N A polymerases can be affected by the concentration of these ions. The rate of errors introduced by the enzyme can also be affected by the proportions of nucleotides employed during the amplification reaction. Furthermore, these proportions can be also varied to provide a "bias" toward specific types of desired substitutions. The second major approach to efficient generation of mutant genes is frequently referred to as D N A shuffling or molecular breeding (Stemmer 1994). For use of this 29 approach, either naturally occurring or engineered homologous genes are required. As depicted in Figure 7, random segments are created from a pool of parent genes usually by digestion with the enzyme Dnase I. These fragments are recombined in vitro as the result of sequence homology during a polymerase chain reaction (PCR) and are then subjected to a selection step that produces improved variants that are able to enter subsequent cycles of recombination. Several variations of this general strategy have been reported. For example, methods have been devised for the use of single stranded D N A (Kikuchi et al. 2000) or for the addition of a peptide tail to enhance the sequence space that can be explored (Matsuura et al. 1999). Despite the wide range of successful examples obtained with this second general strategy (Minshull and Stemmer 1999), some limitations remain such as the need for pre-existing groups or families of related D N A sequences and occasional difficulty in controlling the size and specificity of fragments produced by the Dnase I. The latter problem is a particular concern in the use of small genes for which advantageous changes may be adjacent to or very close to each other in the sequence and are less likely to be combined in different proteins. B. Activity screening strategies As mentioned previously, the screening or selection method is a critically important component of any directed evolution strategy. The set up and complexity of such methods clearly depend on the enzymatic activity or functional property (e.g., improved thermal stability, greater tolerance of extreme pH, improved binding affinity, increased or decreased substrate specificity, enhanced tolerance of organic solvents) for which improvement is sought. The complexity of the assay is greater when the library of 30 enzymes cannot be assayed directly, for example i f they are intracellular or i f a coupled activity assay is required. The methods for screening and/or selecting clones from a pool of random mutants can be divided into three general groups (Schellenberger 1998), each of which has advantages and drawbacks. The first group is the standard screening method that has been mentioned previously. This method is the most flexible and may have the principal limitations of cost and the number of clones that can be analyzed, with a limit of around 10 4 clones. The second group of screening methods involves the expression and display (e.g., on the surface of a phage (phage display)) of a large number of variant proteins. This approach is frequently used to screen large populations of proteins for specific binding recognition capability and, thus, is often used for screening antibodies. The main limitation of this method is that it is oriented primarily to the optimization of binding rather that catalytic capability. Finally, the third group of strategies involves selection of useful, transformed cultures rather than screening of enzymes or extracts, h i this way, i large populations, usually in the order of 10 5 or bigger can be evaluated with a high level of sensitivity because only advantageous conditions, even i f they are marginal, w i l l allow growth of colonies expressing proteins with activity characteristics that permit growth under a variety of conditions. The main limitation of this strategy is that it succeeds only when the desired catalytic activity is linked to the survival of the cell that is expressing the protein of interest. Although the current screening and selection methods are frequently able to produce the desired result, they are nevertheless limited to evaluation of small numbers of variants relative to the immense number of variants that could, in principle, be created 31 (Iverson and Breaker 1998). In time, perhaps additional strategies w i l l be identified that eliminate this remaining barrier to creation of "custom made" enzymatic activities. Numerical methods have been described in some reviews (Kuchner and Arnold 1997; Moore et al. 1997) to determine the number of clones that must be screened to assure that an optimized combination of mutations has been evaluated. The complexity of this problem is indicated by the estimate that even for a protein of -250 amino acids, approximately 5,000 single amino acid substitutions and 22 mil l ion double mutations are possible. In spite of these discouraging numbers, many successful examples of the use of current strategies have been reported even with libraries as small as 10 4 clones (Schellenberger 1998). h i addition, when previous information suggests that particular areas of a protein may be important for the desired property, random mutagenesis can be delimited by the use of specific D N A primers towards such areas to create "smart libraries" that can also be recombined (Chica et al. 2005). C. Examples df activity enhancement through directed evolution A t present, an ever increasing number of successful applications of directed evolution are being published. Examples concerning such work with oxidative enzymes are relatively recent and involve primarily cytochromes P450, biphenyl dioxygenases and peroxidases (Valetti and Gilardi 2004). One of the main constraints in the use of many of these oxidative enzymes, particularly at the industrial scale, is the requirement of expensive cofactors as electron donors (Hibbert et al. 2005). In the case of the hydroxylation of naphthalene by cytochrome P450, this problem was approached by 32 evolving this enzyme not only to exhibit a 20-fold greater activity relative to the wild-type enzyme but to use H 2 O 2 as an electron donor rather than NADH (Joo et al. 1999). Another example of directed evolution involving this same family of enzyme involves cytochrome P450 from Bacillus megaterium. This protein possesses a monooxygenase and a reductase domain in a single polypeptide and is able to hydroxylate fatty acids (between C12 and C is) , amines and alcohols but not alkanes. By a combination of random mutagenesis focused around the heme domain sequence and an automated screening protocol, a variant capable of hydroxylating the hydrocarbon octane was identified (Farinas et al. 2001). The thermal stabilities of cytochromes P450 have also been improved by this approach. For example, studies where the thermostability of a cytochrome P450 variant, also known as BM-3 peroxygenase 21B3, was increased by several rounds of random mutagenesis to produce a variant containing 15 additional substitutions that retained much of the original peroxygenase activity (Salazar et al. 2003). Examples of the use of directed evolution to modify the activity of biphenyl dioxygenases include the use of DNA shuffling on the genes from Pseudomonas pseudoalcaligenes KF707 and Burkholderia xenovorans LB400 to modify substrate specificity to include benzene and toluene (Kumamaru et al. 1998). Similarly, DNA shuffling of specific segments of genes from Burkholderia xenovorans LB400, Rhodococcus globerulus P6 and Comamonas testosteroni B-356 produced biphenyl dioxygenase variants that unlike the original enzymes, displayed greater ability to transform the 2,2'-, 3,3'-, and 4,4'- dichlorobiphenyls and were capable of oxidizing the very persistent compound 2,6-dichlorobiphenyl (Barriault et al. 2002). 33 In some applications, substrate specificity broader than that of the wild-type enzyme is desired. This goal was achieved by use of directed evolution to produce a variant amine oxidase from Aspergillus niger with altered affinity for the substrate L-ot-methylbenzylamine. Following random mutagenesis and the use of a colorimetric activity screen, a single variant with increased activity and selectivity towards the L -enantiomer was identified (Alexeeva et al. 2002). Peroxidases have also been engineered by directed evolution, especially for improved stability. In the case of the heme peroxidase from Coprinus cinereus, variants were generated by random and site-directed mutagenesis and screened for improved thermal stability and resistance to inactivation by H2O2. Recombination of the favorable mutations obtained by both methods produced a variant containing seven mutations that displayed dramatic improvements in thermal and oxidative stability (Cherry et al. 1999). In related studies, horseradish peroxidase expressed in yeast was subjected to random mutagenesis and recombination. This approached was used to obtain a double variant with a three-fold greater half-life at 60 °C relative to that of the wild-type protein. This variant also exhibited increased stability toward the addition of SDS, N a C l arid urea as wel l as to conditions of extreme p H (Morawski et al. 2001). A final example of another heme protein subjected to directed evolution is horse heart myoglobin. Through combined use of PCR-based random mutagenesis and a colorimetric plate assay using the substrate A B T S , a triple variant with 25-fold increased activity was identified. Characterization of the mechanistic basis for this increase in activity through use of steady-state kinetics and stopped-flow spectroscopy established that this increased activity resulted primarily from an increase in formation of the 34 myoglobin equivalent of compound I (i.e., k\) as represented in the Dunford peroxidase ping-pong mechanism discussed previously (Wan et al. 1998). V. Limitations of oxidative catalysis A. Catalytic inactivation and degradation products A s described above, a major limitation in the industrial application of peroxidases is the inactivation of the enzyme by H 2 O 2 or any organic peroxide and the limited understanding of the structural and mechanistic basis for this inactivation. Exposure of the enzyme to peroxide is, o f course, unavoidable because peroxide is a substrate. Rigorous understanding of the catalytic cycle and its relationship to inactivation during turnover is complicated by the variation of the mechanism from peroxidase to peroxidase and its variation with reaction conditions. Several inactivation pathways have been proposed, and they vary with the enzyme, the reaction products obtained and the kinetic behavior observed (Nicell and Wright 1997; Prasad et al. 2002; Valderrama et al. 2002; Diederix et al. 2003). Nevertheless, from the regular mechanism described in Figure 4, it has been proposed for some peroxidases, the conversion of the intermediate CII into compound III (CIII) which is also known as oxyperoxidase (Hiner et al. 2002). The reactivity of the CfJI intermediate has led to the proposal that it can react further to produce oxidative damage to the protein moiety and thereby inactivate the enzyme by various pathways (Valderrama et al. 2002). Such pathways include the opening of the tetrapyrrole heme ring to produce a biliverdin-like compound by oxidizing one of the meso carbon bridges. This reaction results in bleaching of the Soret absorbance band and loss of the heme iron 35 as reported for several peroxidases (Valderrama et al. 2002). In an alternative pathway, CHI could return to the resting Fe(III)-enzyme following the catalytic oxidation of an amino acid residue. A third and final option is that CIH may decay spontaneously to produce a peroxyl radical that can be converted to an even more reactive hydroxyl radical that is capable of modifying any susceptible protein residue. Most of these intermediates have not been observed in the reaction of ferricytochrome c with hydrogen peroxide except in the case of carboxymethylated horse hearth cytochrome c. In this cytochrome derivative, the sulfur atom of the Met80 axial ligand to the heme iron is carboxymethylated and no longer binds the heme iron. As a result, the reactivity with peroxide is sufficiently altered that a Cl-like intermediate has been identified by UV-Vis spectroscopy after addition of an excess of H 2 O 2 (Prasad et al. 2002). While such an intermediate has never been observed to form following similar treatment of native ferricytochrome c, extensive studies on the reaction of this protein with various hydroperoxides resulted in the identification of protein-based free radicals centered on tyrosine residues around the heme group (Barr et al. 1996; Qian et al. 2002). In addition, it has been shown by mass spectrometry and the use of the spin trap DBNBS, that a free radical originated on horse heart cytochrome c by its reaction with H 2 O 2 , can be transferred to tyrosine and tryptophan residues of synthetic peptides (Deterding et al. 1998). In accord with this report, cytochrome c and several other heme proteins, including myoglobin and hemoglobin, can be radioactively labeled by [3H]benzo[a]pyrene and [3H]17(3-estradiol after treatment with H 2 O 2 (Rice et al. 1983). In the same work, gel electrophoresis experiments established that some heme proteins treated with peroxide can form covalently crosslinked dimers and polymers, thereby 3 6 suggesting additional reaction products for the peroxidative reaction of cytochrome c. In other work, the extent of bleaching of the absorbance of the heme group of different heme proteins after exposure to H 2 O 2 was shown to be proportional to the amount of iron released from the protein during the reaction (Harel et al. 1988). In agreement with this information, and due to the possible differences between the inactivation mechanism of peroxidases and cytochrome c during oxidative catalysis, a general inactivation mechanism can be proposed for the latter. In this model, the decrease in activity as a function of time is assumed not to result from substrate depletion, and the oxidized product produced by the peroxidase activity is assumed to be stable (N.B.: a condition not met by many substrates). With these assumptions, the reaction progress curve at a specific concentration of H 2 O 2 can be fitted to the single exponential function shown below: P t = Pa (1 -e - / ( i n a c t t ) + Pi (6) Where Px is the concentration of product at time t, P a is the concentration of product generated during the assay, P\ is the initial concentration of product and kinact is the inactivation rate at a given H 2 O 2 concentration in sec"1. Usually with this parameter and the value of the catalytic constant, it is possible to calculate a partition ratio: Par t i t i on rat io = / c c a t / / C i n a c t (7) This ratio, which has no dimensions, represents the number of effective catalytic turnovers per inactivation event (Tudela et al. 1987). However, in the particular case of peroxidases and peroxidase-like catalysts like cytochrome c, this analysis cannot be 37 applied so that an alternative approach is necessary. The partition ratio can also be calculated from the plateau in the progress curve, which represents the maximum amount of substrate transformed under specific reaction conditions. If the moles of substrate transformed are divided by the moles of catalyst used in the reaction, this ratio will yield the moles of substrate transformed before the enzyme is completely inactivated. A similar analysis has been applied for the case of cytochrome c 550 (Diederix et al. 2001). B . Principal challenges to the conversion of cytochrome c into an efficient catalyst The principal challenges to the use of ferricytoclrrome c as an oxidative enzyme are as follows: (a) the low catalytic activity of the cytochrome relative to a true peroxidase is problematic, particularly on the scale of commercial applications; (b) the instability of the protein to the presence of H2O2, a characteristic exhibited by any peroxidase catalyst, shortens the life expectancy of the catalyst; (c) recombinant expression of cytochrome c as an intracellular protein complicates development of activity screening strategies required for application of directed evolution; and (d) the limited understanding of the catalytic and inactivation mechanisms compromises structure-based design of a more active and robust peroxidase based on the cytochrome c scaffold. Based on this background information, the objectives of the current work were to (a) construct cytochrome c variants by site-directed mutagenesis to study the peroxidase activity of this protein, (b) characterize kinetically and spectroscopically the resulting variants to understand further the catalytic mechanism, (c) improve the peroxidase 38 act iv i ty o f cytochrome c b y directed evo lu t ion , and (d) study the mechanism b y w h i c h w i ld - type and var iant fo rms o f cytochrome c are inact ivated dur ing perox idat ive turnover. 39 2. METHODS I. Computer simulation methods Structures previously reported (Louie and Brayer 1990; Berghuis et al. 1994) for wild-type yeast wo-1-cytochrome c and a double variant (PDB accession numbers 2 Y C C and 1CRI respectively) were used to simulate the structures of the two other variants W 5 9 A and N52I /Y67F/W59A, with the programs Swiss P D B viewer v.3.7b2 (Guex and Peitsch 1997) and DS ViewerPro v.5.0 (Accelerys) running on a personal computer. II. Molecular biology methods A. Expression of yeast /'so-1-cytochrome c Yeast /so-1-cytochrome c was expressed in Escherichia coli with the expression system ( p B P C Y C (wt)/3) originally designed by Pollock (Pollock et al. 1998) and modified by Rosell (RoselLef al. 1998). In both systems, the cytochrome c gene (CYC1) is co-expressed with the gene encoding cytochrome c heme lyase ( C Y C 3 ) , and both proteins are produced in the cytoplasm. In the case of p B P C Y C (wt)/3, the two genes were cloned in the HindUl/Smal sites of the vector pUC18, while in Rosell 's system (pBTRI), the NcoVHindlll fragment from p B P C Y C (wt)/3 was cloned into the p G Y M vector (Guillemette et al. 1991). In the case of the latter system, attempts to improve protein production by induction with I P T G did not produce any change in the yield of the protein. High expression levels of the recombinant proteins were obtained from several E coli strains, the most efficient being the CD41 D E 3 strain (Miroux and Walker 1996). These cells produced on the order of 1-8 mg of protein per L of culture depending on the variant being purified. 40 B. P r o d u c t i o n a n d pu r i f i ca t i on o f r e c o m b i n a n t c y t o c h r o m e c. Cytochrome c variant proteins were produced according to previous protocols (Pollock et al. 1998) with minor modifications. In brief, 1.5 L of Y T a m p media (15 g of bactotfyptone, 11.25 g of yeast extract, and 5.6 g of N a C l dissolved in 1.5 L of distilled water) were inoculated with 150 m L of overnight culture, and incubated at 37 °C for 24-36 hours in 2 L Erlenmeyer flasks under intense agitation (200-250 rpm). In some cases, the expression of the proteins was so efficient that the E. coli cell's were visibly red in colour. Cells were recovered by centrifugation at 6000 rpm in a SS-34 Sorvall rotor for 20 minutes in a R C - 5 B centrifuge, and the pellet was resuspended in 20 m M sodium phosphate buffer, p H 7.2 (8.90 g of N a H 2 P 0 4 and 18.95 g of N a 2 H P 0 4 , p H was adjusted with 1 M N a O H and volume completed to 1 L) and washed several times with the same buffer. To disrupt the cells, two different methods producing similar yields were employed, h i the first method, repeated freeze-thaw (4 °C) cycles were used in the presence of an enzyme cocktail that included 2.5 mg of ribonuclease A (Sigma, R-4875), 5 mg of deoxyribonuclease I (Sigma, D-5025), and 3 g/ml of lysozyme (Sigma, L-6876) as well as 0.1 m M P M S F (Sigma P-7626) dissolved in ethanol as a protease inhibitor. Alternatively, cells were lysed with a French pressure cell at -2,000 psi with at least two passes. The cell debris were removed by centrifugation at 13,000 rpm for 30 minutes in 50 m L tubes, using the same rotor and centrifuge described above (Sorvall). The colored supernatant fluid was recovered, and ammonium sulfate (Sigma, A4915) was added to a final concentration of 326 g/L to precipitate some of the contaminant proteins. After 41 centrifugation (as above), the supernatant fluid was dialyzed overnight against 20 m M sodium phosphate buffer pH 7.2. Cytochrome c was oxidized in a desalting column by the use of NH4(Co[dipicolinate]2) (Mauk et al. 1979). After concentrating, the sample was loaded onto a Pharmacia Mono-S HR 10/10 column in an FPLC system equipped with two p-500 pumps and an LC-500 chromatography controller. Various gradient conditions were used for purification depending on the identity of the variant cytochrome. For the variants W59A and N52I/Y67F/W59A variants, the final purification step was performed using a CM-Sepharose column at 4 °C to prevent protein degradation. The proteins were loaded in the column in 20 m M sodium phosphate buffer pH 7.2, washed with the same buffer containing 75 m M sodium chloride, and eluted with 300 m M sodium chloride. After elution, the proteins were exchanged in buffer without salt, concentrated, flash frozen in liquid nitrogen and stored. The identity of each variant was confirmed by the presence of a single component of the expected atomic mass in an electrospray mass spectrometry analysis. C. S i t e - d i r e c t e d m u t a g e n e s i s o f c y t o c h r o m e c. The main mutagenic method employed to obtain site-directed variants of cytochrome c was the procedure commercialized by Stratagene. In this protocol, a thermostable D N A polymerase, Pfu turbo™, produced a full length D N A copy of the plasmid of interest in the presence of mutagenic primers by means of an optimized PCR. The clones were transformed by a standard heat shock method using competent cells treated with a CaCl 2 (Sigma C-8106) solution 0.1 M (2.94 g of CaCl 2 -2H 2 0 dissolved in 42 200 ml of water). The desired clones were screened by colony PCR and identified by the presence of additional restriction sites introduced by silent mutations in the mutagenic primer. D. R a n d o m m u t a g e n e s i s Several methods were evaluated for the introduction of random mutations into the cytochrome c gene; however, the best results were obtained using the error-prone PCR method, using different concentrations of M g 2 + in the presence of M n C l 2 (Cadwell 1991). A conventional PCR reaction was first used to estimate the number of doublings after a given number of PCR cycles. This program consisted of 20 cycles of a 1 min denaturation phase at 95 °C, 1 min of annealing at 50 °C and 1 min of extension at 72 °C. The starting concentration of plasmid D N A template (WT cytochrome c) was 20 pmoles/uL with 50 uL of a solution containing 0.3 u M of primers, 7 m M M g C l 2 (prepared from dilution of a 100 m M stock, 0.197 g in 10 ml of water), 0.5 m M M n C l 2 (dilution from a 1 M stock, 19.79 g in 100 ml of water), 1 m M DNTPs, 50 mM KC1 (dilution from a 1 M stock, 7.46 g in 100 ml of water), and 20 m M Tris buffer. From this reaction, 10 uL samples were taken every 4 cycles. After this experiment, it was determined that 10 cycles were required to obtain 10 D N A doublings; therefore, the mutagenic PCR employed similar reaction conditions using only 10 cycles. The resulting product was amplified again by conventional PCR and cloned into the vector pDrive 1 M with a PCR cloning system (Qiagen), to facilitate the recovery of the blunt PCR product obtained by random mutagenesis. After this, a second 43 cloning step in the expression vector pBTRI allowed the expression of the variants for screening. E. Screening assay The screening protocol that eventually allowed identification of several variants employed small scale liquid cultures (0.5-1 mL of Y T media) grown in multi-well (1 mL) plates (Axygen). Each plate was covered with a sealing mat (Axygen), which had been punctured in the area above each well to allow some oxygen transfer while preventing excessive evaporation during the incubation period. After sterilization, Y T a m p media (500 uL) was placed into each well with a multichannel pipette and inoculated with an overnight culture (5 uL) that had been diluted to contain a small number of cells (-5-10 cells per well). These mini cultures were grown for -24 hrs at 37 °C until the cells were visibly red, and a secondary culture was obtained by inoculating a small volume from each well into fresh media in a new plate. These plates were incubated again at 37 °C and incubated overnight to preserve the original culture. The cells in the primary culture were lysed by adding 100 ul of "Bugbuster" protein extraction solution (Novagen) and assayed for peroxidase activity using ABTS (Sigma, A-1888) and H 2 0 2 (Sigma, HI 009) as substrates. To do this, 200 ul of a solution containing 2.5 m M ABTS and 1.25 mM H 2 0 2 (0.027g of ABTS and 12.5 ul of a solution of H 2 0 2 125 mM, dissolved in 200 ul of 20 m M sodium phosphate buffer pH 7.2) were added directly to the well. Due to the limitations of this assay and the failure to observe further positive clones upon dilution of the screening solution, only 20 plates were 44 screened. Based on these results, it can be estimated that in the best case -1.8 X 10 colonies were screened. The cells with positive reaction were plated in agar plates, and at least 10 colonies from each plate were assayed individually in the liquid culture assay. III. Spectroscopic methods A. Electronic absorption spectroscopy The UV-Visible spectra of each variant protein (-1 uM in 20 m M phosphate buffer pH 7.2) were collected with a quartz cuvette (1 cm pathlength) and a Varian Cary 4000 spectrophotometer. Sample temperature was maintained at 25 °C with a thermostated circulating water bath. B. Spectropolarimetry The far U V (185-255 nm) and visible (350-550 nm) CD spectra were recorded with a Jasco J-810 spectropolarimeter equipped with a PFD-425S Peltier device to control the temperature. For the far U V spectra, the protein solution (10 uM in phosphate buffer (20 mM, pH 7.2) was placed into a water-jacketed, cylindrical quartz cell (0.1 cm cell path length). For spectra in the visible region, protein solutions (1 uM in phosphate buffer (20 mM, pH 7.2)) were placed into a rectangular, quartz cuvette (1 cm cell path length). Each measurement is an average of at least 3 scans. C. Magnetic circular dichroism For M C D spectroscopy, protein solutions (0.1 mM) were exchanged into deuterated phosphate buffer (20 m M pD 7.2) by centrifugal ultrafiltration with a 45 Mill ipore concentrator. The spectra were collected in a 1 mm cell path length cuvette and a Jasco J-730 spectropolarimeter adapted with an electromagnet (Alpha Magnetics) operated at 1 T. At least 3 scans were averaged for each variant. D. NMR spectroscopy Protein solutions for I D and 2D N M R spectroscopy were prepared by exchanging the protein into deuterated phosphate buffer by centrifugal ultrafiltration in an Amicon Centricon-10 centrifugal device. A t least 3 cycles of dilution and concentration with deuterated buffer were performed to ensure that most of the exchangeable N H protons were replaced by deuterium atoms. The final sample concentration was ~3 m M . Spectra were collected at 25 °C except in the temperature-dependent experiments for which the sample temperature was adjusted to 16, 25, 35 and 45 °C (WT only). Spectra were collected with a Varian Unity 500 M H z instrument. E. Electron spin resonance spectroscopy The X-band E P R spectrum of wild-type ferricytochrome (250 u M in 20 m M sodium phosphate buffer p H 7.2) incubated with peroxide (0.25 M in the same buffer) was collected at 4 K on a ESP 300 E Bruker instrument (Villegas et al. 2000). The sample was flash frozen in liquid nitrogen within the first 15 seconds of reaction and the spectra were collected under the following parameters: frequency 9.45 G H z ; microwave power 0.5 m W ; receiver gain 1 x 10 5; modulation frequency 100 K H z ; modulation amplitude 3.89 G . 46 IV. Kinetic methods A. Steady-state kinetics 1. Determination of k\ and fo. The peroxidase activities of the W T and protein variants were measured by following the absorbance change at 440 nm from the oxidation of dicarboxidine by H2O2. The oxidation product of this peroxidase substrate is more stable to the excess of H 2 0 2 than the corresponding product of A B T S oxidation (Paul et al. 1982). Assays were performed at 25 °C on a Cary U V - V i s 4000 or 6000 spectrophotometer and 1 ml quartz cuvettes (1 cm path length) containing 1ml of 0.5 u M protein solution in 20 m M sodium phosphate buffer p H 7.2 and 250 u M dicarboxidine substrate solution (diluted from a 250 m M stock solution prepared by dissolving 0.012 g of dicarboxidine dihydrochloride, Sigma D-5907, in 25 m l of 20 m M phosphate buffer p H 7.2). Reactions were initiated by addition of H 2 0 2 to a final concentration of 10-50 m M . Triplicates of each reaction were performed, and the initial reaction rate was calculated using software provided with the spectrophotometer. A n extinction coefficient s 4 4 0 of 13.4 m M " 1 cm"1 (Paul et al. 1982) was used to calculate the amount of product oxidized. The rate constants k\ and £3 were evaluated using the equations describing the peroxidase ping-pong mechanism (Equations 1-3) postulated by Dunford (Dunford 1999). 2. Inactivation constant During catalytic turnover, proteins exhibiting peroxidase activity undergo spontaneous inactivation by a process that is generally assumed to involve oxidation of 47 one or more amino acid residues in or near the active site (Barr et al. 1996; Valderrama et al. 2002). To calculate the inactivation rate during the peroxidase assays, the reaction progress curve was monitored for each variant (25 °C) until the absorbance change reached a maximum; such time depended on the properties of each variant. In these experiments, protein (100 nM) and dicarboxidine substrate (250 mM) solutions in 20 m M sodium phosphate buffer pH 7.2, were placed in a quartz cuvette (1 cm pathlength) (25 °C), and the reaction was initiated by addition of H 2 O 2 solution to three different concentrations: 40 mM, 60 m M and 80 mM. This curve was fitted to a mono-exponential function (Equation 6) according to the model of Duggleby (Duggleby 1986). Curves from three independent assays for each H 2 O 2 concentration, were averaged and fitted to the above equation using the program Origin (ver 7.020, OriginLab Corp). 3. Partition ratio The frequency with which the peroxidatic catalytic cycle is completed relative to the frequency which the protein or enzyme is inactivated can be expressed in terms of a partition ratio (Tudela et al. 1987). For the present experiments, this value was calculated by dividing the maximum product concentration, obtained from the reaction progress curve, by the molar concentration of the catalyst. This quotient reflects the amount of product generated by each variant prior to inactivation. The reaction conditions were identical to those used for determination of steady-state kinetics parameters described above. 48 4. Heme decay Addition of H 2 O 2 to solutions of cytochrome c is known to cause a decrease in the intensity of the electronic absorption spectrum of the protein that is often referred to as "bleaching" (Florence 1985) The kinetics of this decay of absorbance induced by the addition of hydrogen peroxide was determined for the proteins studied in the current work by monitoring the spectrum (250 to 750 nm) of a cytochrome solution (5 uM) in a quartz cuvette (1 cm path length) for at least 30 minutes following addition of a' 100-fold molar excess of H 2 0 2 . In these experiments, a Cary 300 spectrophotometer equipped with a circulating, thermostatted water bath was used. The trace of the absorbance decay at 408 nm was fitted to a single exponential function curve using the program Origin (ver. 7.0, OriginLabs, Inc.). V. Physical methods A. Thermal denaturation curves The thermal stability of wild-type and variant forms of yeast /so-1-cytochrome c was evaluated by monitoring the change in ellipticity at 220 nm with a Jasco J-810 spectropolarimeter equipped with a PFD-425S peltier device. The temperature (Tm) at which half of the protein was denatured was determined by monitoring the ellipticity (220 nm) for protein (20 uM) in phosphate buffer (20 mM, pH 7.2) placed in quartz cuvettes (1 mm path length) while the temperature of the sample was increased from 25 to 80 °C at a rate of 1 °C per minute under computer control. Tm values were calculated with 49 software provided by the instrument manufacturer (Jasco spectrum analysis) by obtaining the first derivative of the curve. B. pKa value The pATa for the alkaline conformational transition of wild-type and variant forms of ferricytochrome c was studied by monitoring the pH dependence of absorption at 695 nm of cytochrome solutions (~1 mM) following exchange by ultrafiltration into unbuffered 0.1 M NaCl. In these experiments, the solution pH was titrated from pH 5 to 11 by adding small volumes of 0.1 M NaOH or HC1 solutions (Rosell et al. 1998), monitoring the pH with a IQ240 ISFET Bench top pH meter (IQ Scientific instruments) equipped with a stainless steel micro pH probe. The resulting data were analyzed by fitting to a function for a single titratable group to obtain the pKa value of the conformational transition. C. Direct electrochemistry Cyclic voltammetry experiments were performed in a non-isothermal, two-compartment glass cell with a three-electrode system as described previously (Rafferty et al. 1990; Rafferty 1992; Rosell 1998) using a calomel reference electrode held at 25 °C and a platinum counter electrode. A gold electrode modified with a saturated solution of 4,4'-dithiodipyridine (Aldrich, 143057) was used as the working electrode, and the protein solution (500 uL, 0.4 mM) was prepared in a 100 m M KC1 solution pH 6.0. Voltammetry was performed over a potential range of +/- 250 mV at a 20 mV s"1 sweep rate. The temperature was controlled by immersing the sample compartment in a jacketed beaker connected to a thermostatted, circulating water bath. Thermodynamic parameters 50 for the reduction of cytochrome c were obtained from the temperature-dependent variation of the midpoint redox potential of cyclic voltammograms between 5 and 35 °C. VI. Protein analytical methods A. Electrophoresis Products of the reaction between ferricytochrome c and hydrogen peroxide were analyzed by SDS-PAGE gel electrophoresis (Laemmli 1970). A reaction mixture containing cytochrome c (20 uL, 32 uM) and various concentrations (10-, 100- and 1000-fold molar excess relative to cytochrome c) of H 2 O 2 were incubated for 30 minutes in the dark. In each case, half of the resulting reaction mixture was mixed with the appropriate buffer (with or without 2-mercaptoethanol), and loaded into the gel (15% acrylamide) and separated at 120 V for -90 min. The gel was stained with Coomassie blue solution (50% methanol, 10% acetic acid, 40% water and 0.05% Coomassie brilliant blue R-250, Bio-rad) for about 4 hrs under moderate shaking and destained for at least 2 h in an aqueous solution of 5% methanol, 7% acetic acid. Finally the gel was photographed using a gel documentation system. B. Exogenous ligand binding to the heme The binding of the cyanide anion to ferricytoclrrome proteins was studied by monitoring the change in electronic spectrum as a function of time following the addition of cyanide. Protein solutions (5uM) in 20 m M sodium phosphate buffer pH 7.2, were incubated with lOmM K C N and spectra were acquired every 2 min over a 40 min period on a Cary 4000 spectrophotometer at 25 °C. 51 C. Trypsin digestion Cytochrome c was hydrolyzed using trypsin as previously reported (Mauk and Mauk 1988) with few modifications. Specifically, 0.5 mg of protein were dissolved in 250 ul of 0.5 M ammonium bicarbonate buffer and the volume was completed with distilled water up to 500 ul. TPCK-trypsin solution (6.25 ul of a solution prepared with 1 mg of enzyme in 1 ml of HC1 0.1 mM) was added and the reaction was incubated for 6 h. at 37 °C. After this period, a second equal amount of TPCK-trypsin was added, and the reaction was allowed to continue for 18 h more. Hydrolysis was stopped by addition of HC1 (1 M , 50 ul), and the reaction mixture was taken to dryness with a Speed-Vac vacuum centrifuge. Water (250 ul) was added to the dried hydrolysate, and the entire digestion and drying procedure was repeated. The resulting dried sample was dissolved in TFA (0.05% (v/v)) to a concentration of ~1 umol/uL, and the sample was submitted to HPLC/MS analysis. D. Mass spectrometry The analysis of peptide fragments derived from cytochrome c following reaction of the protein with H 2 0 2 in the presence of the spin trap N-/ler/-Butyl-a-(4-pyridyl)nitrone N'-oxide (POBN, Aldrich, 215030) was based on the procedure described by Filosa (Filosa 2001). In the current study, 10 ul of a 2.5 m M solution of cytochrome c was mixed with 2.5 ul of 100 m M H 2 0 2 and 25 ul of 0.5 M P O B N solution in a 50 ul final volume. A l l solutions were prepared in 20 mM sodium phosphate buffer pH 7.2. The reaction was stopped by injecting the reaction mixture onto an HPLC reverse-phase C4 column (Econosphere 300 4.5 mm in diameter and 250 mm in length from Alltech) 52 attached to a Beckman System Gold HPLC system or by diluting with 20 m M phosphate buffer pH7.2 and concentrating the reaction mixture by centrifugal ultrafiltration prior to loading it onto the C4 column. The reaction products were separated with a gradient from 0 to 60% acetonitrile in water (both with 0.05% TFA) in 60 min., followed by a steeper gradient of 60 to 100% acetonitrile in 20 min. in a C18 reverse phase column, and the fractions isolated in this manner were collected, dried and submitted for mass spectrometry analysis. 53 3. R E S U L T S I. Kinetic parameters of cytochrome c variants A. Peroxidase substrate and progress curve The first step towards the kinetic characterization of the cytochrome c variants, was to select a suitable peroxidase substrate. For this purpose, several chemicals (Figure 8) including 2,2'-azino-bis(3-ethylbenzothiazoline-6-sulfonate (ABTS) , 2-methoxyphenol (Guaicol), 3,3'-dimethoxybenzidine (o-Dianisidine), N.N.N'.N1-tetramethyl-l,4-phenylenediamine ( T M P D ) and y,y'-(4,4'-Diamino-3,3'-biphenylylenedioxy) dibutyric acid (Dicarboxidine) were tested in terms of solubility, absorbance change and product stability. A B T S is usually the substrate of choice to characterize this type of catalysis, but it presents the disadvantage of an oxidation product unstable to the excess of hydrogen peroxide. On the other hand, in spite of being less soluble and having a minor absorbance change upon oxidation compared to A B T S , dicarboxidine yields a product stable throughout the assay (Figure 9). This characteristic is very important for the analysis of the reaction progress curve, as w i l l be discussed later, and therefore dicarboxidine was selected for subsequent experiments. 54 o II H O - C - C H 2 - C H 2 - 0 o II 0 - C H 2 - C H 2 - C - O H NH2 Dicarboxidine H 3 C — \ H3CQ tf-dianisidine HsCs ,Crfe N (HCI) 2 H 3 C ' N N C H 3 T M P D Guaiacol Figure 8. Substrates used to test the peroxidase activity of cytochrome c. A l l assays were performed in 20 m M phosphate buffer pH 7.2 using freshly prepared concentrated stocks solutions. 55 0 7 i ABTS 'i 1 1 • r '-• 1 1 1 -• 1 0 2 4 6 8 10 Time (min) Figure 9. Typical reaction curves for ABTS and dicarboxidine substrates Similar reaction conditions were used for each assay (100 n M cytochrome c, 250 u M substrate, 40 m M H2O2 in 20 m M sodium phosphate buffer p H 7.2). The numbers represent the different phases of the reaction (see text below). A representative oxidation curve for dicarboxidine consisted of three phases, as opposed to four when A B T S was used (Figure 9). In both curves, the first three phases are equivalent, with an initial relatively short linear segment (phase I), followed by a stage (phase II) where the reaction rate is lower compared to the initial rate and reaches a maximum (phase III). In the case of A B T S , the final stage (phase IV) consisted in a decline of the absorbance, very likely due to product instability; this agrees with previous studies of cytochrome c-550 (Diederix et al. 2001). However in our case, no p're-activation phase was observed in this curve with either A B T S or with dicarboxidine, even when different ratios of protein versus H2O2 and incubation times where used (data not shown). 56 With either dicarboxidine or A B T S as substrate, the initial rate of the reaction increased with increasing concentration of H 2 O 2 and the rate depended linearly on the concentration of each cytochrome c protein. The initial rates were linear for at least 30 seconds (or longer depending on the concentration of H 2 O 2 and the variant) and no rate saturation was observed as the concentration of H 2 O 2 was increased up to 100 m M (using 100 n M cytochrome c and 250 u M dicarboxidine). B. Steady state kinetics analysis Based on the analysis proposed by Dunford for the peroxidase ping pong kinetics (Dunford 1999), the initial rate was plotted against the H 2 O 2 concentration (Figure 10), and the following equation was fitted to the data: k 1 k 3 2 r E 0 ] [ H 2 Q 2 ] [ D c ] (7) V _ k 3 [ D c ] + k! [H 2 0 2 ] Where. [Eo] is the initial concentration of enzyme; [ H 2 O 2 ] the concentration of H 2 O 2 and [Dc] the initial concentration of dicarboxidine. The fitting of this curve provided the two rate constants that characterize peroxidases, k\ and fa. In this analysis k\ represents the rate of formation of the intermediate Compound I (CI, Figure 4, p. 16) while £3 is a measure of the rate of breakdown of the second intermediate, Compound II (CH, Figure 4). 57 ns a: 6.0x10"H 5.0x10'7-0) 4.0x10"M 3.0x10" 2.0x10"7H 1.0x10"7 0.02 0.04 0.06 H 2 0 2 (M) 0.08 0.10 Figure 10. Calculation of k\ and £3 kinetic parameters. Conditions: 100 n M cytochrome c, 250 uM dicarboxidine, H 20-2 from 20-100 m M in 20 m M sodium phosphate buffer p H 7.2. Each point is an average of at least three independent assays and the solid line represents the non-linear fitting of the data to the rate equation with a R 2 value of 0.90. A similar analysis was performed for all the variants, and the numerical results obtained in this manner are summarized in Table II. A s can be seen, considerable differences are evident when the two catalytic parameters are compared. 58 Table II. Catalytic parameters derived from the peroxidase ping pong analysis for cytochrome c variants. Protein ( M ' V 1 ) h ( M ' V 1 ) W T cytochrome c 20 6.5 x 10 3 W 5 9 A 130 3.3 x 10 4 N52I7Y67F 0.1 1.8 x 10° N52I /W59A/Y67F 51 2.4 x 10 4 Conditions: 100 nM protein; 250 uM dicarboxidine; 20 to 100 mM H 2 0 2 in sodium phosphate buffer (20 mM, pH 7.2). The errors for both values were equal or minor to 5%. The magnitude of k\ for all proteins is considerably smaller than that of £ 3 , especially in the case of the N52I/Y67F variant. When this variant is compared to the W T protein, the low kj and k3 values reflect a very low rate of formation of the catalytic intermediate CI and breakdown of CII respectively. B y contrast, both the single and triple substituted variants have k i and k 3 that are greater than that of the W T protein. C. /Cjnact and the partit ion ratio To quantify the susceptibility of the proteins to inactivation during catalytic turnover in the peroxidase reaction, a rate of inactivation, &jnact> was obtained from a general inactivation analysis (Duggleby 1986). To rule out the contribution of substrate depletion to the decrease in oxidation rate and therefore assure that such decrease was mainly caused by inactivation of the protein, the reaction rate was calculated at the time 59 point were the enzyme is inactivated (maximum absorbance in the progress curve) and it was shown to vary less than 10 % (~ 3%, data not shown). The resulting values for this parameter derived from numerical fits to the progress curves are shown in Table III for all variants at three different concentrations of H2O2. Table III. Inactivation rate (s 1) for cytochrome c variants at three concentrations of H 2 O 2 . [ H 2 0 2 ] 40 m M [H2O2] 60 m M [H2O2] 80 m M W T 1.1 x 10"2 1.7 x IO"2 2.2 x IO"2 W 5 9 A 5.7 x IO"2 9.7 x IO"2 1.4 x 10"1 N52I/Y67F 1.1 x IO"4 2.1 x IO"4 2.3 xlO" 4 N52I /W59A/Y67F 2.3 x IO"2 4.0 x 10"2 5.3 x IO"2 Calculated from the non-linear fit of the progress curve to a mono exponential function. The error was estimated to be equal or smaller than 10% with a R 2 value of at least 0.91. Conditions: 100 nM protein; 250 uM dicarboxidine; H 2 0 2 40, 60 or 80 mM in sodium phosphate buffer (20 mM, pH 7.2). The lowest value of & j n a c t at any concentration of H2O2 was obtained for the double variant (1.1 to 2.3 x 10"4 s"1), indicating that this variant is the most resistant of these proteins to inactivation during catalytic turnover. When comparing the rates at constant H 2 0 2 concentration (e.g., 60 m M ) , the value for the W T protein (1.7 x 10 s") is increased 6-fold when the destabilizing substitution W59A.(9.7 x 10~2 s"1) is introduced. If the stabilizing substitutions N52I/Y67F are added to the destabilizing W 5 9 A mutation 60 i i i the triple variant, the protein is still inactivated about twice as fast (4 x 10~2 s"1) as the W T protein but about half as rapidly as the W 5 9 A single variant. A partition ratio that provides a measure of the number of catalytic turnovers per inactivation event can be derived by dividing the total amount (moles) of product oxidized (obtained from the progress curve) by the amount of catalyst (moles) used in the assay. For the proteins studied in this work, the partition ratio varies slightly with H 2 O 2 concentration (Table TV) however the differences do not appear to be significant and essentially this value is independent of peroxide concentration. Table IV. Partition ratio for cytochrome c variants calculated from the coefficient of product oxidized versus the amount of catalyst at three concentrations of H2O2. [ H 2 0 2 ] 40 m M [ H 2 O 2 ] 60 m M [ H 2 0 2 ] 80 m M W T 178 155 150 W 5 9 A 385 387 389 N52I/Y67F 80 82 67 N52I /W59A/Y67F 186 171 160 Conditions: 100 nM protein; 250 uM dicarboxidine; H 2 0 2 40, 60 or 80 mM in sodium phosphate buffer (20 mM, pH 7.2). The estimated error calculated from at least 5 independent experiments for each value was estimated to be equal or smaller than 6%. Comparison of partition ratios at constant peroxide concentration (e.g., 60 m M ) demonstrates that the double variant turns over the least number of times (82) prior to inactivation while the single variant turns over the greatest number of times (387) prior to inactivation. Interestingly, combining the stabilizing N52I /Y67F substitutions with the 61 destabilizing W 5 9 A substitution results in a turnover number (171) similar to that of the wild-type protein (155). Under these conditions, the most destabilizing substitution (W59A) produces a partition ratio more than double that of the wild-type. II. Stability of variants towards temperature, pH and H 2 0 2 To correlate the kinetic characteristics of the variants with their structural properties, various parameters related to the stabilities of the ferricytochromes were evaluated from three perspectives. First, the thermal stability of each ferricytochrome was determined by spectropolarimetry (Figure 11) and represented by the Tm, the temperature at which half of the protein is denatured. Second, the pATa for the alkaline conformational change of each ferricytochrome was determined by spectrophotometric pH-titration (Greenwood and Wilson 1971). Finally, the percentage of the heme bleached by incubation with H2O2 (1:100 ratio) represents a measure of oxidative stability. The results of these experiments are shown in Table V . From these results, it can be seen that the N52I/Y67F substitutions increase the Tm by 11 °C in agreement with a previous report (Berghuis et al. 1994). On the other hand, the W 5 9 A substitution decreases the Tm by 22 °C. Combining this destabilizing substitution in a variant with the stabilizing N52I/Y67F substitutions results in only partial restoration of thermal stability as indicated by the observation that the Tm for the triple variant is ~18 °C lower than that of the wild-type protein. 62 WT W59 N52I/Y67F N52I/Y67F/W59A / 40 60 80 Temperature (°G) Figure 11. Thermal denaturation curves of cytochrome c variants. Conditions: 20 uM protein on 20 m M sodium phosphate buffer pH 7.2. To facilitate comparison, the curves were normalized using the Jasco instrument software (Spectra analysis V . 1.53.04). 63 T a b l e V . P a r a m e t e r s re la ted to s tab i l i t y o f f e r r i c y t o c h r o m e c v a r i a n t s . Tm Heme bleached ( ° Q (%)a W T 59.2 (0.8) 7.8 (3.8) 62 (9) W 5 9 A 36.6 (1.0) n.d. 78 (3) N52I/Y67F 70.5 (0.7) 11.0 (2.7) 2 (6) N52I /W59A/Y67F 40.8 (1.2) 8.4 (3.6) 74 (3) Conditions used for each type of measurement are described in the Methods section. Calculated errors (%) are indicated in parenthesis and were calculated from the non-linear fit (7m and p/Ca values) and from the results of 5 independent experiments (% of heme bleached) a This value represents the percentage of decrease in intensity of the Soret maximum after incubation with 100-fold excess of H 2 0 2 for 30 minutes. A s thermal stability is often correlated with susceptibility of the ferricytochrome to undergo the alkaline conformational transition (Berghuis et al. 1994) (exchange of the Met80 axial ligand for Lys73 or Lys79 (Pollock et al. 1998; Rosell et al. 1998), the piC a for this transition has been determined by spectrophotometric pH-titrations with disappearance of the absorption maximum at 695 nm that is characteristic of native ferricytochrome c is monitored as p H is increased. A s expected (Berghuis et al. 1994), the pKa obtained for the N52I/Y67F double variant is significantly greater than that of the W T protein. On the other hand, the value exhibited by the W 5 9 A variant could not be determined because the absorbance at 695 nm could not be detected at the p H values that were tried. Combining these stabilizing and destabilizing substitutions in one protein (the triple variant) restored the alkaline pKa to a value very similar to that of the wild-type protein. 64 The susceptibility of the heme in each of these proteins to oxidation by H 2 O 2 was measured as the percentage decrease in the Soret absorbance that resulted following incubation of the protein with peroxide. A s seen, the W 5 9 A substitution significantly increases the vulnerability of heme to destruction by peroxide as reflected by a decrease in the Soret absorbance by as much as 75%. Interestingly, a nearly identical result was obtained for the N52I /W59A/Y67F triple variant even though the N52I/Y67F double variant was more resistant to peroxide than was the wild-type ferricytochrome. III. Reduction potential and thermodynamic parameters To investigate further the structural stability of each variant, the reduction potential (Figure 12) and its dependence on temperature and p H (Figure 13) were determined for each of the proteins studied in this work. From these results, the standard thermodynamic parameters A G , A H and AS were obtained (Table VI) . These values are very useful to compare the effect of mutations on the structure of proteins, especially i f they have similar fold (Rafferty et al. 1990; Bixler et al. 1992; Komarpanicucci et al. 1992; Battistuzzi et al. 2001a). 65 N52I/Y67F/W59A , ~ N N52I/Y67F \ WT :'t \  x V / i 1 i — r ~ -0.15 0 0.15 Potential (Volts) -0.30 0.30 Figure 12. Cyclic voltammograms of cytochrome c variants using a three electrode system with a modified gold electrode as working electrode. Measurements were performed at 25 °C over a potential range of -300 to +350 mV at 20 mVs"1 sweep rate. 0.4 m M protein solution were prepared in 100 m M KC1 pH 6.0. 28(h 260H WT N52I/Y67F/W59A N52I/Y67F 24(H Figure 13. Temperature-dependent variation of the midpoint redox potential for cytochrome c variants. Conditions: 0.4 m M protein in 100 m M KC1 p H 6.0. Temperature was varied from 5 to 35 °C. The data for each protein was fitted to a linear model with R value of at least 0.90 and P O . 0 0 0 1 . The reduction potentials of double variant (218 m V vs. N H E ) and the triple variant (227 mV) were both significantly lower than the potential of the wild-type cytochrome (266 mV) . A similar value has been reported previously for the double variant (Berghuis et. al. 1994). Despite repeated attempts, acceptable cyclic voltammograms could not be obtained for the W 5 9 A variant, presumably because this protein was not sufficiently stable to the conditions of the measurement or as the result of ineffective interaction of the protein with the electrode surface. The greater changes in thermodynamic parameters were observed for the double variant, which exhibited an increase in the standard enthalpy of ~4 kcal/mol and an increase in standard entropy of - 10 eu. The less negative value of A H 0 observed for N52I /Y67F is consistent with a significant increase in stability (Berghuis et al. 1994; Terui et al. 2003). Table VI . Electrochemical properties of wild-type /so-l-cytochrome c and variants. "Versus NHE, 298.15 K, 0.1 M KC1. Uncertainties: E°, 0.8%; AG°,1.9%; AS 0 5%; and AH 0 , 5%. E° A G 0 A S 0 A H 0 - A H / F TAS/F (mV) a (kcal/mol) (eu) (kcal/mol) (mV) (mV) W T 266 -6.1 -20.1 -12.1 +524.3 -258.6 W 5 9 A n.d. n.d n.d. n.d. n.d. n.d. N52I/Y67F 218 -5.0 -10.2 -8.0 +348.2 -130.6 N52I /W59A/Y67F 227 -5.2 -15.5 -9.8 +426.0 -199.0 68 Previous authors have interpreted the thermodynamic parameters by factoring the contributions of the enthalpic and entropic terms toward the midpoint potential in each variant. When these values are compared between similar proteins (e.g. variants from a native protein) and significant changes are observed, this allows to determine whether the changes in the potential are mainly due to enthalpic or entropic effects (Battistuzzi et al. 2001b; Battistuzzi et al. 2002). A s observed in Table V I , the contribution of both terms changed to a considerable extent, especially when the W T protein is compared to the double variant in terms of both, the contribution of enthalpic and the entropic terms. The same is true for the comparison with N52I /W59A/Y67F, but the main difference is observed in the value of the enthalpic term. These data suggest that the reduction in the potential of N52I/Y67F is due to a contribution of both the enthalpic and entropic terms, while the decrease in the value of N52I /W59A/Y67 is mainly enthalpic and may be produced by stabilization of the oxidized form of the protein (Battistuzzi et al. 2002). Finally for the case of the tryptophan variant, the redox potential could not be determined in spite of various attempts. IV. Spectroscopic characteristics of cytochrome c variants A. Electronic absorption spectroscopy In an attempt to identify structural characteristics related to the differences in activity and stability exhibited by these cytochromes, these proteins were studied by a variety of spectroscopic methods. For example, the electronic absorption spectra of the ferricytochromes are shown in Figure 14. 69 0.3 I ' 1 ' 1 ' r 240 260 280 300 320 380 400 420 440 • , I ' • ' - ' • -> 500 525 550 575 660 690 720 750 X (nm) Figure 14. Electronic spectra of cytochrome c variants (sodium phosphate buffer (20 m M ) p H 7.2, 25 °C). A varying ordinate scale was used for each region of the spectrum to assist comparison of the spectra. A l l spectra were normalized to the same absorbance intensity at the Soret maximum (12B). 70 The characteristic Soret band for heme proteins occurs at -408 nm and is observed at this position for all the variants (12B). Nevertheless, several differences between these spectra (12A and 12C) are apparent, particularly in the region of the a and P bands (500-575 nm (12C). These latter bands are particularly sensitive to the environment around the heme group (Pollock et al. 1998). A s mentioned earlier, the main difference between these spectra observed in the near-infrared region is the absence of the 695 nm charge-transfer band in the spectrum of the W 5 9 A variant, which is generally interpreted as indicating the absence of the Met 80-iron interaction. B. Spectropolarimetry Comparison of the far U V and visible C D spectra of the ferricytochrome variants reveals differences from which consequences of the mutations on the secondary and tertiary structures the proteins as well as modification of the heme environment can be inferred. For example, the far U V C D spectra of the variants reveals the two negative Cotton effects (-210 and -222 nm) and one positive Cotton effect (-195 nm) that are characteristic of the a-helix-rich secondary structure of cytochrome c (Myer 1968c) (Figure 15). Notably, the far U V C D spectrum of the W 5 9 A variant does not exhibit the negative Cotton effects, consistent with significant change in the secondary structure of this variant. 71 8.0 Figure 15. Far U V circular dichroism spectra of ferricytochrome c variants. 10 u M protein solutions were prepared in 20 m M sodium phosphate buffer p H 7.2 and spectra were measured at 25 °C on a 185-255 nm wavelength. The protein concentration was the same for each sample The negative Soret Cotton effect present in the C D spectrum of ferricytochrome c provides a sensitive indicator of the integrity of the heme environment (Fetrow et al. 1997) in that this feature is less negative when the heme crevice is more open (Pielak et al. 1986; Chottard et al. 1987). The spectra of the wild-type and N52I/Y67F and N52I/W59A/Y67F-variants exhibited a positive Cotton effect at -405 nm and a negative Cotton effect at -415 nm (Figure 16). This negative feature had different relative 72 intensities being less negative for the W T band, followed by N52I /W59A/Y67F and N52I/Y67F with the most negative value. However, the feature undergoes the greatest change in the spectrum of the W 5 9 A variant in which this feature has been replaced by a single, positive Cotton effect with a maximum at -407 nm. Similar spectroscopic changes have been reported in the C D spectrum of ferricytochrome c that has been partially unfolded by the action of a denaturing agent (Myer 1968b; Fetrow et al. 1997; Santucci and Asco l i 1997), suggesting a similar structural state for this variant without addition of a denaturant. 24 - • - • W59A WT N52I/Y67F N52I/Y67F/A59 / \ O) T3 CD E 4 A 2 A 04 350 400 450 500 A, (nm) Figure 16. Visible circular dichroism spectra of cytochrome c variants. 1 u M protein solutions in 20 m M sodium phosphate buffer p H 7.2 at 25 °C were scanned on a wavelength between 350-550 nm. (sodium phosphate buffer (20 m M ) p H 7.2, 25 °C). 73 C. Magnetic circular dichroism The near-infrared M C D spectra of low-spin, six-coordinate ferriheme proteins provide a good indication of the Fe(III) coordination environments (Gadsby and Thomson 1990; Hawkins et al. 1994). The near-infrared M C D spectra for the ferricytochromes studied in the current work are shown in Figure 17. The N52I/Y67F and N52I /W59A/Y67F variants, as well as the W T protein, displayed a maximum at -1700 nm that is consistent with Met-His axial coordination of the heme iron. The corresponding spectrum of the W 5 9 A variant, however, exhibited a peak at -1500 nm and generally observed for heme proteins and model compounds with o/s-nitrogen coordination (Gadsby and Thomson 1990; Fetrow et al. 1997; Fedurco et al. 2004). 74 1200 1400 1600 1800 2000 A, (nm) Figure 17. Near I R magnetic circular dichroism spectra of the proteins studied in this work. 0.1 M protein solutions were exchanged in duterated 20 m M sodium phosphate buffer pD 7.2 and measured on a wavelength between 1300 and 2000 rim using an electromagnet operated at 1. T. obtained in deuterated phosphate buffer (20 m M , p H 7.2, 25 °C). D. NMR spectroscopy 1.1H NMR spectra Another highly sensitive means of assessing the heme environment, especially for the region around the ligands and the propionate groups, is the ' H N M R spectrum. The chemical shifts observed in this type of experiment have been thoroughly characterized by others and can be used to assess some of the changes introduced by the mutations in this region of the protein (Moore and Will iams 1980; Cutler et al. 1989; Gao et al. 1990; Gao et al. 1991b; Thurgood et al. 1991). In Figure 18, the nomenclature convention for the heme group is shown (Nc-Iub 1991) and it w i l l be used further for the description of the N M R results. The I D ' H N M R spectrum for each of the ferricytochromes studied here is shown in Figure 19. In the particular case of the W 5 9 A mutation (Figure 19, last trace), very broad signals were obtained that complicated the assignment of resonances. Figure 18. Nomenclature conventions for the heme group. The numbers denote the substituents in each pyrrol ring indicated by the letters. 76 35 30 25 20 15 10 5 (ppm) Figure 19. 1 H N M R spectra of ferricytochrome c variants. (20 mM. phosphate in D 2 0 , pD 7.2, 25 °C. (A) wild-type cytochrome c; (B) the N52I/Y67F variant; (C) the N52I/W59A/Y67F variant; (D) the W59A variant. Only the down field region of the spectrum is shown. Assigned resonances are labeled above each spectrum and correspond to protons of heme methyls HM8 and HM3, heme propionate-7 (HP7-la and b) and heme methyl 5 (HM5). Equivalent signals in the spectra of each variant are connected by dotted lines. The main differences between consecutive spectra (starting with spectrum A) are highlighted with asterisks while the signals that could not be assigned were not labeled. 77 Some resonances observed in these spectra were assigned initially by comparison with previous reports of the spectra of the wild-type yeast cytochrome (Bums and Lamar 1981; Williams et al. 1985; Gao et al. 1990; Turner 1993; Banci et al. 1991a) and included the hyperfme shifted heme methyl resonances HM8, HM3, and HM5 as well as the broad signal for the s i proton of His 18 and some of the protons for heme propionate 7. Other characteristic signals of cytochrome c were also observed in the high field region (data not shown) and consisted of protons S - C H 2 of Pro30 and the y and s protons of Met80 (Gao et al. 1990; Banci et al. 1997a). Notably, the spectrum of the W59A variant exhibited the greatest difference from the other three spectra. The spectrum observed for this protein suggests considerable structural perturbation relative to the other proteins and suggests that the much of the protein is significantly unstructured under these conditions. Following acquisition of this spectrum, no further N M R studies of the W59A variant were undertaken. 2. 2D NOESY spectra To confirm the assignments and to study these signals further, additional spectra were recorded to consider the dependence of the spectrum on temperature and pH (data not shown) and 2D NOESY spectra were obtained (Figure 20a and 20b). The H ^ H 1 NOESY (Nuclear overhause effect spectroscopy) experiment, is a two dimensional N M R technique that is useful for determining i f signals arising from specific protons are close to each other in space even i f they are not bonded. 78 W T E a a I i 30-32 -3 3 4 - B f 3 6 -35 30 25 20 15 w 2 - 1 H (ppm) 10 Figure 20a. Upfield region of the N O E S Y spectra of wild-type and variant ferricytochrome c . Only a section of the spectra is shown for clarity. The heme methyl resonances, HM8 and HM3 are indicated by numbers, and the cross peaks with specific protons are indicated by letters and arrows. Each mark represent a cross peak of HM8 with (A) H P l - a ; (B) H P l - b ; and (C) with proton C7 of residue W59. Equivalent resonances in each spectrum are connected by dotted lines 79 WT I52F67 to T3 T3 3 I52F67A59 31 32 33 34 35H 36 -8 36 ~$5 S 3 Y 32 31 36 3s S 3Y w1- 1 H 32 8 31 36 35 34 33 32 f\ Figure 20b. Expanded view of Figure 20a showing the area around H M 8 and H M 3 for each variant. The labels are the same as for Figure 16a, except for R8 and R3 in N52I /W59A/Y67F and their corresponding cross peaks (A and B , see text). Comparison of these ' H and N O E S Y spectra reveals several differences among them; however only significant variations around specific protons are considered below. 3. HM8 The H M 8 resonance observed in the spectra of all three proteins exhibits normal Curie behavior (i.e., upfield displacement as temperature is increased), and undergoes small shifts as p H is changed from 7 to 6 or to 9 (data not shown) that agree with previous reports (Moench et al. 1991). The N O E S Y spectra (Figure 20a) establish that these protons interact (form a cross peak) with protons of heme propionate-7 (HP7- lb and HP7- la ) and with the proton C2 from Trp59 (Thurgood et al. 1991), except in the triple variant where this latter residue has been replaced. Two additional, unidentified signals overlap the peak for 80 H M 3 , but not that of H M 8 (Figure 19C). However, in the N O E S Y spectrum of the N52I /W59A/Y67F variant (Figure 20b), it is evident that the extra resonance at 32.9 ppm (designated R8) is in close proximity to H M 8 and results in a strong cross peak. Moreover, R8 also fonns a cross peak with both HP7 protons, confirming the proximity of this proton to H M 8 . Two more cross peaks are observed in the N O E S Y spectrum of the triple variant (data not shown). One of these cross peaks is an unassigned signal at 15.9 ppm that also interacts with protons H P 7 - l b and H P 7 - l a . The other cross peak is a signal at 10.9 ppm that very likely corresponds to the interaction with H M 1 (see discussion of H M 1 below). 4. HM3 The resonance for H M 3 also exhibits normal Curie behavior (Turner 1993), and it shifts downfield as the p H is increased (Moench et al. 1991). From the N O E S Y experiments, it was concluded that the H M 3 protons of both variants interact with protons CAH and PH (~6 and ~4 ppm, respectively) from Phe 82 (data not shown) and with protons 4 p C H 3 (~3 ppm) in the heme group (Moench et al. 1991). A s mentioned previously, the spectrum of the N52I /W59A/Y67F variant exhibits two additional resonances that overlap the resonance of H M 3 and are absent from the corresponding spectra of the wild-type protein and the N52I/Y67F variant. The resonance at 31.9 ppm (R3) forms a cross peak with H M 3 at 32.5 ppm (Figure 20a). In addition, R3 forms a cross peak with the proton 4pCH3 (data not shown) that again suggests proximity of this group to H M 3 . 81 5HP7-1b In the spectra of all three cytochromes, the resonance for the H P 7 - l b proton exhibits Curie behavior and no change with p H over the range studied. Nevertheless, differences in the behavior of this resonance are apparent in the N O E S Y spectra. Specifically, in the spectrum of the wild-type cytochrome, this proton exhibits a strong cross peak with another heme propionate proton, H P 7 - l a (at 13 ppm) and weaker interactions with the 8-2, C-2 r\-2 protons of Trp59. A l l of these interactions are maintained in the spectrum of the N52I/Y67F variant but are not evident in the spectrum of the N52I /W59A/Y67F variant as expected by the lack of Trp59 in the latter protein, h i the triple, variant, however, additional, unassigned cross peaks are observed at 15.9, 9.7, 8.9, 8.0 and 7 ppm (data not shown) that are not observed in the spectra of either the wild-type or double variant protein. 6. HP7-1a - A s observed for H P 7 - l b , the chemical shift for H P 7 - l a exhibits normal Curie behavior and no dependence on p H . Interaction of this group with Trp59 is evident in the spectra of the wild-type and double variant proteins, and interaction of H P 7 - l a with H M 1 is apparent with all three proteins. A s observed in the spectrum of the triple variant, the extra resonance R8 interacts not only with this proton but also with H M 1 and with HP7- lb . 82 7. HM5 Two significant differences between the N O E S Y spectrum of wild-type cytochrome c and the corresponding spectra of the variants, are the two strong signals that are most likely attributable to heme methyls H M 5 and H M 1 . A s seen in Figure 19 (B and C), both signals shift downfield significantly. Specifically, the resonance of H M 5 shifts from 10.5 ppm in the spectrum of the wild-type protein to 12.5 ppm in the spectrum of the variant. The resonance of H M 5 is not readily observed in the spectrum of the wi ld-type protein, but it appears at -10 ppm in the spectrum of both variants. In addition, both signals exhibit anti-Curie behavior (Turner 1993). Differentiation between H M 5 and H M 1 in Figures 19 and 20 was based on the observation that in the N O E S Y spectra the signal assigned as H M 5 exhibited no cross peaks with H M 8 , H P 7 - l a or H P 7 - l b and, therefore, could not be that of H M 1 . 8. HM1 The resonance for H M 1 undergoes a substantial up field shift from 8 ppm in spectrum of the wild-type cytochrome to 10.6 and 10.9 ppm in the double and triple variant respectively (Figure 19B and C). Analysis of the N O E S Y spectrum of the N52I /W59A/Y67F variant revealed strong cross peaks of H M 1 with H M 8 , H P 7 - l a and H P 7 - l b (data not shown) as observed in the spectrum of the wild-type protein, but in addition, a weaker cross peak is observed with H M 3 at 12.5 ppm (also observed in the spectrum of the N52I/Y67F variant at 12.4 ppm) and with R8 at 31.9 ppm, suggesting a significant modification in the environment of this proton in the variant. 83 Summarizing these results, two significant changes are evident from the mutations as observed by the N M R experiments: First the effect of substituting Asn52 and Tyr67 in both variants, N52I/Y67F and N52I /W59A/Y67F, produced changes in the electronic environment of protons H M 1 and H M 5 , in both proteins, since these signals are significantly displaced upfield in comparison with the W T protein. The other evident effect of the mutations is observed only in the more active variant N52I /W59A/Y67F, where additional cross peaks around H M 8 , H M 3 , and HP7 protons are observed. These differences are not likely to be cause exclusively by the substitution of Trp59, since the variant bearing only this mutation had very broad spectrum from which was not possible to obtain significant information by the N M R techniques employed. V. Random mutagenesis A n alternative approach to studying the peroxidase activity of cytochrome c is to create a collection of random variants that are then screened for improved activity. Variants identified in this manner have the potential of being more useful in a specific catalytic reaction (better activity, stability, expression, etc.). A t the same time, such variants can provide insight into the general mechanism of enzymatic catalysis exhibited by the wild-type protein. In the case of cytochrome c, two crucial limitations can be addressed in this way: the low catalytic activity of the cytochrome relative to authentic peroxidases and the low stability of cytochrome c to heme oxidation by the substrate H 2 0 2 . In this work we employed random mutagenesis in combination with a plate screening assay in an attempt to isolate additional active variants. 84 A. Mutagenic reaction The first step in this process consisted of determining the number of cycles of a mutagenic P C R reaction that were required to achieve the desired level of mutagenesis. The level of mutagenesis required is a function of the number of times the starting amount of D N A is doubled (number of doublings) and of the length of the sequence to be mutated (Cadwell 1991). In this work, we chose to introduce an average of 2 mutations in the 400 bp Bam HI- Nhe I fragment from our expression vector. Under the reaction conditions that we used for the error-prone P C R , an average of 2 mutations per 300 base pair fragment would be observed after 10 D N A doublings (Cadwell 1991). Samples were withdrawn from the mutagenic P C R reaction taken after a given number of cycles and were loaded onto an agarose gel (Figure 21 A ) . The product begins to be visible after 4 cycles, and it was estimated that after 10 cycles, ten D N A doublings were obtained. The next step involved promoting the introduction of errors by the polymerase during the P C R reaction by the addition of Mn(II). The optimal concentration of Mn(II) for this purposes was determined by adding varying concentrations of M n C f : to the P C R reaction mixture and evaluating the effect on the amount of product amplified. The results obtained with 0, 0.2, 0.5, and 1 m M M n C ^ are shown in Figure 21B. 85 A B ?*> t* i* #* §•* u § **s| NI u 8 «*H **H L 0 0.2 0.5 1.0 H M 400 200 800 400 Figure 21. Agarose gels (2%) to estimate the amount of D N A produced per cycle of mutagenic P C R (A) and the yield at different concentrations of M n C h (B). In 21 A , the subscripts represent the number of cycles (c) while 400 is the approximate size in base pairs of the D N A fragment. In 2 I B , the different numbers in the wells denote the concentration of M n C b (mM) while L the ladder in base pairs. From this result, it is evident that the amount of D N A produced decreased with increasing concentrations of MnCl2, to the point that no product was observed at a concentration of 1 m M . Based on this result, we subsequently used 0.5 m M M n C l 2 because at this concentration a significant effect is observed on the D N A yield, but a useful amount of product is generated. This product was cloned back into the expression vector, transformed into E. coli cells (with a low transformation efficiency, estimated to be around 10 3 clones per jug of D N A ) and purified. Fifteen clones were sequenced to estimate the rate and type of mutation generated by this approach (Table VII). A l l the clones sequenced contained insert, but not all were expressed. 86 As indicated, seven single mutations (-50%), two double substitutions (14%), one triple mutant (7%) and four clones with no mutations (29%) were obtained. Only two transitions were observed (-13%), and half of the mutations were missense. Table VII . Results from random mutagenesis of the cytochrome c gene. Original sequence Mutated sequence Amino acid substitution Type of mutation Type of mutation (A) .(B) G A C A A C D90N Missense Transition C C A T C A P25S a a A A G A G G K22R a a TTT CTT F36L a tt GGT TGT G23C a Transversion GCC A C C A81T a Transition G G G G A G G84E u A A A G A A K5E a " A A G A A A K-2 Silent A A C A A T N31 a " A A G ' A A A K54 a A A C A A T N56 a " TTC_ TTT F10 u " A C T A C C T69 a A C C A C A T78 Transversion Underlined letters indicate the base that was changed. Single letter amino acid codes are used to indicate the original residue (first letter) and the new amino acid (letter after the number) resulting from mutagenesis. . 87 B. Screening methods To establish the activity screening method, several parameters were evaluated to determine the optimal assay conditions. Specifically, a serial dilution of a ferricytochrome c solution was adsorbed onto a filter paper, and several combinations of peroxidase substrate and H 2 O 2 were evaluated. The reagent concentrations identified in this manner (Figure 22) to permit detection of the smallest amount of wild-type cytochrome (1 u M ) were 12.5 m M A B T S and 25 m M of H 2 0 2 . Figure 22. Minimum amount of cytochrome c for which peroxidase activity was detectable with peroxidase substrates A B T S and H 2 O 2 . The numbers written on the filter paper indicate the concentration of ferricytochrome c that was placed on the paper at that position. C" is the control without protein and C + indicates a solution with a small amount of myoglobin as positive control. 88 On the basis of this information and the screening protocol reported previously for detecting the peroxidase activity of myoglobin (Wan et al. 1998), the screening method used in the current study was designed. A schematic diagram of this method is provided in Figure 23. Expression cells Filter paper Figure 23. Principal steps in the initial peroxidase activity screening strategy. In this method, a suspension culture of E. coli cells expressing the protein was spotted onto a disc of filter paper and exposed to chloroform fumes. In principle, this treatment induces cell lysis and the release of the cytochrome variants to permit detection of peroxidase activity. This activity assay produced a dark green colour (for a positive clone) that results from the oxidation of A B T S , so variants with greater activity produced detectable amounts of this green colour as the concentration of peroxide in the reaction mixture was decreased. After repeated attempts involving a variety of conditions (e.g., various lysis reagents, incubation conditions, peroxidase substrates, and E coli strains 89 used to express the protein), no clones with detectable levels of peroxidase activity were identified. This result led to the conclusion that the expression level of the cytochrome combined with the intrinsically low peroxidase activity of the protein prevented detection of a positive clone. Thus, an alternative screening strategy was required. In an attempt to improve the culture conditions that would allow a better expression of the protein, very small volumes of liquid culture (150 ul) were grown in P C R microplates. A general scheme of this method is presented in Figure 24. Culture . di lut ions p . . . . . . ^ .., . . PCR w Small l iquid w Released Microplates cultures protein 1 ABTS + H 20 2 Individual ^ Culture in ^ C o [ o u r r e a c t i o n colonies ^ agar plates ^ Figure 24. Modified screening protocol that permitted detection of peroxidase activity in E coli cells expressing cytochrome c variants. After transforming E. coli with the products of the mutagenic P C R mixture, consecutive dilutions were used as inocula to set up "minicultures" in microtitre plates containing L B media. Ha l f of each plate was used as a "duplicate" and was not assayed for peroxidase activity. Each plate was sealed with a special plastic mat to prevent excessive evaporation during incubation at 37 °C. After incubation, the cells were isolated by centrifugation, excess media was discarded and a commercial lysis reagent 90 was added. The resulting lysate could then be assayed for peroxidase activity by addition of a mixture of ABTS and H 2 O 2 . When a positive reactions was observed, the equivalent tube in the duplicate cultures were spread on agar plates to obtain individual colonies that would be subjected to repeated activity assay by the same procedure. Finally, D N A isolated from individual colonies that exhibited activity was submitted for sequence analysis. In several cases, the positive clones turned out to be producing wild-type protein, so the stringency of screening was increased by using diluted substrate solution which would produce a positive result only for those clones expressing more active variants. An average of 15 microplates per concentration of substrate solution was screened and only two clones, both with a single mutation, were identified as exhibiting improved activity by this method. D N A sequence analysis identified the variant cytochromes leading to this improved activity as G84E and K5E. Both mutations are situated in the surface of the protein, above the heme group. G84E is located relatively close to the heme group (on the top part of the heme cavity), while K5E is localized on the opposite side as part of the first a-helix of the protein. G84E was chosen for further characterization because it was the only clone that gave a positive reaction after repeated dilutions of the assay solution (Figure 25) and because of its proximity to His82 and Argl3, two residues considered to be relevant for the structure and function of cytochrome c (Rafferty et al. 1990; Berghuis and Brayer 1992). In Figure 25, a representative plate clearly shows a dark green colour only in the tube with the E84 variant, while the other samples have no reaction. 91 WT A G84E C Figure 25. P C R plate showing the positive reaction of variant G84E after repeated dilutions of the substrate solution. The positive reaction of G84E was indicated by a dark green colour (darker tone in the Figure) absent in all the other tubes. The W T protein was included for comparison, while A and C indicate colonies with negative reaction. C. G84E variant The G84E variant was characterized further in terms o f its kinetic and structural properties for comparison primarily with the wild-type cytochrome. 1. Kinetic parameters The same analytical procedure used to calculate the kinetic parameters for the previously discussed variants (page 57) was applied to the G 8 4 E variant. The kinetic parameters derived in this manner for the peroxidase ping-pong mechanisms are compared with the corresponding values for the wild-type protein in Table VIII. 92 Table VIII. Rate constants k\ and &3 constants obtained from steady-state kinetics analysis of the peroxidase activity of wild-type ferricytochrome c and the G84E variant. Error Error k\ ratio &3 ratio ( M ' V 1 ) (%) ( M ' V ) (%) (Variant/WT) (Variant/WT) G84E 59 3 1.4 X 10 4 2 2.9 2.2 W T 20 5 6.4 X 10 3 5 — • — Conditions are the same as in Table II, page 59. A s is apparent from these results, the catalytic parameters resulting from the single G84E substitution are distinctly different from those of the wild-type protein. The value of k\ is ~3-fold greater than observed for wild-type protein and &3 is about doubled, consistent with the greater catalytic activity detected for the cell lysate in the screening assay. Nevertheless, the rate enhancements observed for the G84E variant are not as great as those observed for the variants described earlier in this work (Table II, page 59). In terms of inactivation, this protein resembles the W 5 9 A and N52I /W59A/Y67F variants in that it is less stable than the wild-type cytochrome to inactivation during turnover. This characteristic is reflected in the values of ^ I i act obtained at three concentrations of H 2 O 2 (Table IX). In this case however, the differences are marginal, and therefore the partition ratio observed in the same Table, is still higher due to an increased activity (Table IX). In a similar way, i f this ratio is compared against the other variants, is interesting to notice that G84E is more efficient that the triple variant N52I /W59A/Y67F showing a higher ratio at the three concentrations of W2O2 tested. 93 Table IX. Inactivation and partition ratio comparison of the G84E variant versus the WT protein. [ H 2 0 2 ] [ H 2 0 2 ] 60 [ H 2 0 2 ] 40 m M m M 80 m M G84E fcinact (s"1) 1.7 xlO" 2 2.4 x 10~2 2.8 x 10"2 G84E Partition ratio 206 193 183 W T * i n a c t (s"1) 1.1 x IO"2 1.7 x IO"2 2.2 x IO"2 W T Partition ratio 178 155 150 Conditions described in Table III (for k i n a c t , page60) and Table I V (for partition ratio, page 61). Error were calculated to be no more than 3% (partition ratio) and no more than 8% ( k i n a c t ) • 2. Stability parameters The parameters determined to characterize the structural stability of the cytochrome variants produced by site-directed mutagenesis (Table V , page 63) were also determined for the G84E variant identified by random mutagenesis (Table X ) . From the 0.5 decrease in the pi^a for the alkaline conformational transition, it is apparent that this variant is less stable toward formation of one or both of the alkaline conformers than is the wild-type protein. Similarly, the Tm for thermal denaturation of the protein as determined by spectropolarimetry is significantly lower than that of the wild-type protein. Finally, the G84E substitution also renders the cytochrome more susceptible to oxidation of the heme prosthetic group by H 2 0 2 . A l l three parameters indicate that this variant is significantly less stable than the wild-type cytochrome. These observations extend the general correlation of decreased protein stability with increased activity in the peroxidase activity. 94 Table X. Parameters related to stability of the randomly generated ferricytochrome c. Error Error Heme Error CO (%) (%) (%)a (%) G84E 54.3 0.2 7.3 1.1 50 1 WT 59.2 0.8 7.8 3.8 38 9 Conditions used for each type of measurement are described in the Methods section and are similar to those for related values shown in Table V . 3. Reduction potential and thermodynamic parameters The reduction potential of the G84E variant and the related thermodynamic parameters are compared with the corresponding values for the wild-type cytochrome in Table XI. Notably, the reduction potential of the variant is significantly lower (48 mV) than that of the wild-type protein, and the potential obtained is very similar the potential obtained for the double variant N52I/Y67F (Table VI, page 66). Despite the significant decrease in reduction potential, there is no significant difference in the entropy of reduction between proteins; however, there are differences of around 1 kcal/mol in the values of AG and AH. In terms of the factorization of the entropic and enthalpic terms, is possible to infer that the differences in the redox potential are mainly due to the later term (Battistuzzi etal. 2001b). 95 Table XI. Reduction potential and thermodynamic parameters for the G84E variant. E° A G 0 A S 0 A H 0 A H / F TAS/F ( m V ) a (kcal/mol) (eu) (kcal/mol) (mV) (mV) G84E 218 -5.03 -20.3 -11.1 +480.5 -262.5 W T 266 -6.1 -20.1 -12.1 +524.3 -258.6 Versus N H E , 298.15 K, 0.1 M KC1. Uncertainties: E °, 2%; A G 0 , 1%; AS 0 , 2%; and A H 0 , 1%. 4. Spectroscopic characteristics of G84E Despite the altered kinetic and stability properties exhibited by this variant and in contrast the variants discussed earlier, the spectroscopic properties of the G84E variant were found to be essentially identical to those of the wild-type cytochrome. For example, the electronic absorption spectrum of the ferricytochrome exhibited absorption maxima and intensities unchanged from those of the wild-type protein (Table XII). The only changes evident in the U V or visible C D spectra of the variant relative to the spectrum of the wild-type protein were some small shifts in the visible region. Specifically, a shift in the positive Cotton effect at 409 nm was observed (to 405 nm) and in the negative Cotton effect at 417 (to 415 nm). A minor shift in the absorbance maximum observed at 1744 nm in the near infrared M C D spectrum of the wild-type protein to 1741 nm in the spectrum of the variant was also observed. The I D ' H - N M R spectrum of the variant ferricytochrome was unchanged from that of the wild-type protein. Taken together, these results suggested the absence of dramatic changes in the overall structure of the variant. 96 Table XII . Electronic Absorption Spectroscopy of the G84E Variant Circular Dichroism Electronic Absorption far-UV visible M C D Protein Soret3 <x,p C-T ^ M a x 'Winl ^ M i n 2 ^ M a x ^ M i n ^ M a x G84E 409 529,560 695 196 209 219 409 417 1741 WT 408 529,560 695 195 210 221 405 415 1744 Conditions for each measurement were similar to ones for the WT protein (page 67-72) and therefore were included for comparison. VI. T h e m e c h a n i s m of c y t o c h r o m e c inactivation by hydrogen peroxide As discussed above, the peroxidase activity of cytochrome c is inactivated during turnover, presumably owing to oxidative modification of the cytochrome by hydrogen peroxide. As was discussed before, several inactivation pathways have been proposed, and the kinetics of such pathways seem to vary with the enzyme and the reaction conditions involved (Nicell and Wright 1997; Prasad et al. 2002; Valderrama et al. 2002; Diederix et al. 2003). Therefore, to gain insight into the mechanism by which this inactivation reaction occurs, the reaction of ferricytochrome c with hydrogen peroxide was studied by using several spectroscopic and chemical methods. The objective of this work was to identify the products of the reaction and to provide sufficient insight into the inactivation mechanism that strategies could be developed to prevent this inactivation and improve the catalytic performance of the cytochrome for subsequent applications. 97 A. Heme b l e a c h i n g It has been known for many years that the electronic absorption spectrum of ferricytochrome c and other heme proteins {e.g., myoglobin, hemoglobin) decreases dramatically on incubation of the protein with H 2 O 2 (Florence 1985; Valderrama et al. 2002). Because this change in spectrum can be observed as a loss in colour of the protein solution, this process is often referred to as "bleaching." The susceptibility to bleaching varies from protein to protein presumably because the mechanism of the reaction is characteristic of each protein. The change in electronic spectrum induced by incubating wild-type yeast iso-l-ferricytochrome c and the N52I/Y67F double variant with peroxide is shown in.Figure 26. The inset shown in the upper panel of this Figure is the X-band E P R spectrum (4 K ) of wild-type ferricytochrome (250 uM) incubated with peroxide (0.25 M ) and frozen within 15 seconds from the start of the reaction. A s seen, this spectrum clearly demonstrates the presence of a free radical (Villegas et al. 2000) that was not observed prior to addition of peroxide (data not shown). The corresponding spectrum obtained following identical treatment of the N52I/Y67F, is shown in the inset for the lower panel. This result combined with the resistance of the double variant to bleaching by peroxide suggests that the bleaching mechanism involves a radical intermediate. 98 0.25 -, 0.20 A 0.15 0.10 0.05 o o c JS 0.00 O £ 0.25 < 0.20 A 0.15 0.10 A 0.05 0.00 N52I/Y67F [*10 A, 350 400 450 500 X (nm) 550 550 Figure 26. Decrease in absorbance at the Soret maximum ("bleaching") following addition of H2O2 to ferricytochrome c. A 2.5 u M protein solut ion i n 20 m M s o d i u m phosphate buffer p H 7.2, was incubated for 2 0 m i n i n the presence o f 2 5 0 u M H2O2, and scans were taken every two minutes . The arrows represent the d i rec t ion o f the change. The inset o n the right shows the presence o f a free radica l , detected by l o w temperature E P R , observed o n l y i n the W T reaction but not w h e n the double variant is used ( V i l l e g a s et al. 2000) . 99 B. Exogenous ligand binding to the heme To investigate further the involvement of a free radical in the bleaching reaction, experiments of the type shown in Figure 27 were performed. In this experiment, the binding of the cyanide anion to the wild-type ferricytochrome and to the double variant was studied by monitoring the change in electronic spectra as a function of time following the addition of cyanide. Ferriheme proteins are well known to have high affinities for binding of CW (Horecker 1946; Wallace and Clarklewis 1992; Yao et al. 2002). The formation of this complex can be observed by monitoring changes in the Soret absorbance as a function of time (Blumenthal and Kassner 1980) as shown in Figure 27. Notably, the spectrum of the double variant is virtually unchanged following identical (H . , . , ' • ' 1 ' ' 350 400 450 350 400 450 X (nm) Figure 27. Change in Soret spectrum following incubation wild-type ferricytochrome or the double variant with K C N . 5 u M protein solutions in 20 m M sodium phosphate buffer p H 7.2, were incubated with l O m M K C N . Spectra were acquired every 2 min over a 40 min period. The direction of the change in the spectra as the reaction progressed is indicated (arrows). The initial and final scans are labeled as I and F, respectively. 100 treatment, indicating that this protein binds little or no cyanide under these conditions. Exposure of either protein to excess H 2 O 2 after incubation with CN" completely prevents bleaching of either protein (data not shown). These results suggest that heme bleaching and formation of the free radical intermediate (indicated by the EPR results) are inhibited as the stability of axial ligand binding increases. C. Peroxide-induced oligomerization of ferricytochrome c In addition to these results, it has been reported that, as a consequence of the inactivation reaction, horse cytochrome c forms protein oligomers as observed by SDS-P A G E electrophoresis (Harel et al. 1988). The possibility that yeast ferricytochrome c also forms larger species upon treatment with peroxide was evaluated in experiments represented by. Figure 28, which provides the results obtained from experiments with the wild-type cytochrome. As can be appreciated from this result, dimers, trimers, and larger oligomers were obtained. It was also apparent, that under the conditions used in this experiment (2-20 min reaction time) that the identity and concentration of products were independent of the incubation time, suggesting that the products are formed rapidly. Notably, identical results were obtained in the presence of the chelating agent DTPA or buffer prepared after removing trace metal ions, thus ruling out the involvement of metal ions (e.g., Cu(II)) and radical species resulting from the presence of metal ions in this reaction. 101 10 20 Otfftk jflUfc Trimer Dimer Monomer Figure 28. Oligomerization of cytochrome c in the presence of H 2 0 2 observed by SDS P A G E electrophoresis. 250 u M protein solutions in 20 m M sodium phosphate buffer p H 7.2 were incubated with 100 molar excess peroxide (2.5 mM) . The numbers on the left represent the sizes (in kDa) of the protein ladder (lane L ) , and the apparent oligomer size assigned to the reaction products are indicated on the right, to, t2, ts, tio and to t2o were samples taken at 0, 2,5 10 and 20 min. respectively. Further studies demonstrated that the extent of oligomer formation depends on H 2 0 2 concentration (Figure 29). A s is apparent from this Figure, only a small amount of dimer was produced with a 10-fold excess of peroxide, and the yield of dimer and trimer increased with higher oxidant concentrations (100- and 1000-fold excess). 102 L OX 10X 100X 1000X 56 38 30 21 Figure 29. SDS P A G E electrophoresis of the products resulting from reaction of wild-type ferricytochrome c with various ratios of [Ff202]/[ferricytochrome c] . Solutions of protein 250 u M in 20 m M sodium phosphate buffer p H 7.2, were incubated with the corresponding molar excess of H2C>2 (0-0.25 M ) . The excess of H 2 O 2 was removed by centrifugal ultrafiltration prior to loading the gel. The ratio of reactants relevant to each lane (0 to 1000 molar excess peroxide) is indicated at the top of the photograph. The molecular weights (kDa) for the reference protein ladder (lane L ) are indicated on the left. The difference in the yield of oligomers at equivalent H 2 O 2 molar ratios (compare lanes to to t2o in figure 26 versus figure 27 lane 100X) can be explained by the chemical incompatibility between H2C>2 and the membrane of the filtration device, producing a lost of material. Interestingly, kinetic analysis of the same experiment monitored by electronic spectroscopy (Soret absorbance as a function of time (Figure 30)) showed that the shape of the curve for the reaction varies with peroxide concentration. These observations may 103 suggest the involvement of different reaction intermediates and or mechanisms in the inactivation reaction depending on the concentration of the oxidant. I — ' — - i — • — i — ' — i — • — i — ' • — i — ' — i — i — i — i — i — • — i — i ^ — i — , — , — , — i — , — , — 0 10 20 30 40 0 10 20 30 40 0 10 20 30 Time (min) Figure 30. Absorbance change versus time for the heme absorbance of W T cytochrome c (2 uM) in the presence of I X , 10X and 100X excess (2 u.M, 20 and 200 u M , respectively) of H 2 O 2 . The insets on the right are the individual traces at different scale for the y axis. 104 D. Identification of inactivation reaction products Based on these results, an attempt was made to prepare modified cytochrome products resulting from treatment of the protein with hydrogen peroxide and to identify the chemical modifications involved. Chemical definition of these modifications could, in principle, provide an understanding of the inactivation mechanism that would be crucial to the design of cytochrome variants that are less vulnerable to inactivation during catalytic turnover. One strategy for understanding such modifications that has proved useful in the identification of transiently formed, protein-centred free radical intermediates (Fenwick and English 1996; Zhang et al. 2002; Davies and Hawkins 2004) is described in Figure 31. In this strategy, peroxide is added to a solution containing the ferricytochrome and a free-radical trapping reagent. In principle, the trapping reagent w i l l react with protein-centred radical intermediates that form even transiently as the result of the reaction. Many radical trapping reagents have been reported, and a large number of them are available from commercial sources (Rosen 1999). In the current study, 7V4ert-butyl-a-(4-pyridyl) nitrone N'-oxide (POBN) was selected as the trapping agent (Stoyanovsky and Cederbaum 1998) based on a previous report that this reagent is effective in such experiments with cytochrome c (Filosa 2001b). Modif ied cytochrome produced by this method could, in principle, be characterized by tryptic peptide mapping and mass spectrometry to identify the residue(s) modified by hydrogen peroxide in this reaction. 105 Protein (Spin trap) Trypsin digestion + ^ Protein-adduct Peptides H 2 0 2 HPLC Mass values MS Isolated peaks Figure 31. General strategy to identify protein modifications resulting from the reaction of ferricytochrome c with H 2 O 2 in the presence of a spin trap by H P L C / M S peptide mapping (see text). The first step in this strategy was to establish the formation of the spin-labeled protein adduct (Figure 32) and to optimize the yield of the modified protein. Also shown in Figure 32 is the formation of modified cytochrome produced in this manner as illustrated by the mass spectrum obtained for the reaction of wild-type yeast iso-1-ferricytochrome c. The mass difference obtained (+138 m.u.) agrees with previous reports in which the spin-trap looses the t-butyl group (-57 m.u.) after reaction with radical centre (Stoyanovsky and Cederbaum 1998). Several additional attempts were made to increase the amount of product obtained with the spin-trap by varying reaction conditions; however, the best yield (estimated from relative peak intensities) was - 1 3 % conversion to spin-trapped adduct. 106 Figure 32. Reaction of cytochrome c with H 1 O 2 in the presence of P O B N analyzed by L C / M S . In the upper part, the general reaction is defined, while in the bottom, the result of the LC/MS analysis of the reaction (0.25 m M ferricytochrome c; 2.5 m M H 2 0 2 and 50 m M POBN in 20 m M sodium phosphate buffer pH 7.2) in a Perkin Elmer SCIEX API 300 Triple Quad spectrometer, is observed. 107 Unfortunately, repeated attempts to isolate modified peptides for further analysis by H P L C tryptic peptide mapping failed to detect any modified peptides in digests of the modified protein. This difficulty could have resulted from the involvement of multiple-modified peptides of which none formed in sufficient amount to permit detection- or from instability of the modified peptides to the conditions required for analysis. 108 4. DISCUSSION Among the numerous catalytic activities of cytochrome c (Table I), the peroxidase activity is one of the most interesting owing to the enormous potential for application of this activity in diagnostic tests, chemical synthesis and bioremediation (Conesa et al. 2002; Burton 2003; Torres et al. 2003). The potential advantages of using cytochrome c as a peroxidase in such applications arise from the stability of the protein to extremes of p H and temperature and to organic solvents. This potential, however, is hindered by two main factors: low catalytic activity compared to natural peroxidases and an oxidative instability towards one of the substrates, hydrogen peroxide, that is inherent not only to cytochrome c but to any peroxidase-like catalyst (Seelbach et al. 1997; Valderrama et al. 2002; Diederix et al. 2003). To address these limitations, it is essential to understand the relationship between the protein structure and the catalytic mechanism. In this work, we used as a starting point recent studies that have con-elated the stability of cytochrome c with its peroxidase activity by demonstrating an increased catalytic activity upon unfolding of the protein (Diederix et al. 2001; Diederix et al. 2002; Diederix et al. 2003) as well as studies concerning the effects of specific mutations on the thermal stability of the protein (Berghuis et al. 1994; Lett et al. 1996). B y designing variants with opposing effects on the stability of the protein and investigating the resulting consequences on the peroxidase activity as well as on the overall structure of the protein, new and useful insight into the mechanism by which cytochrome catalyzes these type of reactions was sought. 109 I. Site-directed variants Three variants were designed for the purpose of comparing the contribution of structural stabilization on cytochrome peroxidase activity to that of structural destabilization and the ability of stabilizing mutations to offset the contributions of destabilizing mutations on this activity. Yeast z'so-1-cytochrome c was used in this work because a large number of relevant variants of this cytochrome have been characterized structurally by X-ray diffraction analysis. The structurally stabilized variant selected for this work was the N52I/Y67F double variant (Berghuis et al. 1994; Lett et al. 1996) while the W59A variant was selected as the structurally destabilized variant (Caffrey and Cusanovich 1993; Black et al. 2001). Notably, the current study represents the first successful isolation of the W59A variant of cytochrome c. Previous attempts to purify this variant in this laboratory following expression in transformed yeast failed, presumably owing to the instability of the protein to exposure to ethyl acetate as required in the standard protocol for purification of cytochrome c from yeast. The relative contributions of stabilizing and destabilizing mutations on peroxidase are of interest in light of studies by Canters and co-workers concerning the effect of denaturing agents on the peroxidase activity of the bacterial cytochrome C550 (Diederix et al. 2001; Diederix et al. 2002). The N52I/Y67F variant was selected for this phase of the work because the three-dimensional structure of this protein has been determined by X-ray diffraction analysis (Berghuis et al. 1994) in an effort to understand the structural basis for the greater thermal stability and elevated pATa for the alkaline conformation transition that result from these substitutions (Das et al. 1989; Luntz et al. 1989), two parameters often used as 110 measures of cytochrome c stability. Moreover, initial studies of the catalytic properties of this variant indicated that these substitutions stabilize the heme group to bleaching by H 2 O 2 (Villegas et al. 2000), suggesting an involvement of these residues in the oxidative mechanism. Replacement of the phylogenetically invariant Trp59 with Ala was undertaken with the expectation that this substitution would destabilize the protein by removing a key internally-located residue that is involved in a crucial and complex hydrogen bonding network (Cutler et al. 1989; Davies et al. 1993; Liggins etal. 1994). The kinetic analysis reported in this study demonstrates clear differences between the peroxidase activities of each variant. In addition, the absence of a pre-activation phase in our oxidation curve suggests that the model developed for the same reaction catalyzed by cytochrome c55o (Diederix et al. 2001) is not appropriate for the yeast cytochrome used here. Species-specific differences in cytochrome functional properties have been noted previously (Wan et al. 1998; Prasad et al. 2002), so it is not altogether surprising that the peroxidase activity also varies with the species of the cytochrome. In the case of the stabilizing mutation N52IY67F, the values for k\ (rate constant for formation of the first catalytic intermediate) and k3 (rate constant for oxidation of the second intermediate) are very low. When the destabilizing substitution W59A was introduced into either the wild-type protein or the double variant, both rate constants increased significantly. However, even in the case of the most active variant, W59A, these values are much lower than those exhibited by true peroxidases such as horseradish peroxidase (k\ = 9 x 106 M _ 1 s _ 1 ; (Hiner et al. 1996)) and even other non-peroxidase heme proteins such as myoglobin (^=540 M ' V 1 ; (Wan et al. 1998)). At the same time, it is interesting to observe that the value of k3 for all variants studied in this work was 111 considerably greater than the corresponding value for k\, contrary to the relative values of these rate constants that are generally observed for true peroxidases (Hiner et al. 1996; Moffet et al. 2000; Nielsen et al. 2001). In the specific case of the W59A variant, the magnitude of k3 is surprisingly high not only with dicarboxidine as reducing substrate but also with ABTS as substrate. In this latter case, the value for k3 (9.5 x 1(T M " V ; data not shown), is four-fold greater than the same parameter determined for horseradish peroxidase (2 x 103 M ' V 1 ; (Moffet et al. 2000)). These results imply that the active site of the tryptophan variant is highly accessible to the reducing agent, as has been observed for other synthetic peroxidases (Moffet et al. 2000). In this regard, it was unfortunate that a direct comparison between the catalytic activity of the proteins studied in this work and other cytochromes was not possible, not only because different substrates were used (dicarboxidine versus guaiacol or ABTS) but also because the kinetic analysis was different for each case (Diederix et al. 2001; Prasad et al. 2002). As demonstrated by the results reported above, the rate constants k\ and k3 are frequently insufficient to describe fully the peroxidase activity of this protein under a variety of conditions, thus necessitating the consideration of additional parameters. Notably, the inclusion of a rate constant for inactivation of the cytochrome peroxidase activity is crucial owing to the instability of these proteins to reaction with hydrogen peroxide. Nevertheless, the complex and incompletely understood chemical nature of this reaction has resulted in the interpretation of such results through use of various models (Nielsen et al. 2001; Prasad et al. 2002; Diederix et al. 2003). In the present work, the good fits obtained to the data allowed us to interpret the results in terms of a general inactivation model (Duggleby 1986). In this work, it is 112 essential that attention be drawn to careful selection of the peroxidase substrate. Normally, ABTS is by far the first choice for the kinetic characterization of peroxidases and peroxidase-like catalysts because it is highly soluble, the product exhibits high molar absorptivity, and it is available commercially at modest cost. Unfortunately, the product of ABTS oxidation exhibited considerable instability under the conditions of greatest interest to the current study. While this characteristic is not a problem for analysis of initial rate constants, it is a critical problem in determination of progress curves because it adds an additional, complicating phase to the reaction curve. In our case, the use of dicarboxidine, an established (Paul et al. 1982) but less-widely used peroxidase substrate than ABTS eliminated this problem, and we submit that this substrate should be the reagent of choice in any future studies of this catalytic activity of cytochrome c. The calculation of kinetic parameters is very important to compare the different behavior among variants during the oxidative reaction. However, as discussed before, this task is intricate for several reasons: First, the complexity of the peroxidase reaction among the different enzymes, due mainly to the high reactivity of the chemical species involved, makes it difficult to identify the common intermediates and reaction products as well as the proposal of a general mechanism, with some reaction products still under investigation, this complicates the analysis not only in terms of effective catalysis but also in terms of catalytic inactivation, with several models proposed to date (Rasmussen et al. 1995; Nicell and Wright 1997; Diederix et al. 2001; Valderrama et al. 2002; Ghibaudi and Laurenti 2003). 113 Second, one of the most generalized kinetic analysis of this reaction, the one proposed by Dunford (Dunford 1999) proposed that the typical kinetic parameters used to characterize enzyme catalysis, like k c a t , k M , kj, etc and their correlations, have no obvious meaning in peroxidase chemistry. This is caused by the intrinsic characteristics of the reaction and is experimentally observed by the apparent absence of an upper limit in the reaction rate when the concentration of any of the substrates is increased (Dunford 1999). Third, even though cytochromes from different species have been reported to catalyze peroxidative reactions, no intermediates similar to the ones observed for peroxidases (e.g. compound I, compound II, etc) have been found except for the case of chemically modified horse heart cytochrome, where a compound I-like species has been identified (Prasad et al. 2002). In spite of these limitations, the inactivation constant determined by this study is useful in assessing the behavior of our catalysts during turnover where factors such as enzyme protection by the reducing substrate or the partition ratio play an important role in determining the overall catalytic performance of the protein. From the results obtained, it is apparent that the activity of the variant is directly related to the instability of the variant as indicated by thermal stability or pKa for the alkaline conformational transition. Thus, additional parameters are required to define the relative contributions of the catalytic and inactivation rates of peroxidase-like catalysts. One such parameter is the partition ratio, which reflects the number of effective catalytic turnovers per inactivation cycle (Tudela et al. 1987). From the values obtained in this study, including those from the variant G84E obtained by random mutagenesis (discussed later), it can be concluded that the single variant (W59A) is the most efficient catalyst of those proteins studied. This 114 result reflects a significant increase in the rate of catalysis resulting from the destabilizing mutation despite the corresponding increased rate of inactivation. The structural and. electronic differences identified for this variant by UV-Vis , CD, M C D and N M R spectroscopy suggest an overall more disordered structure for the W59A variant as expected from the relatively drastic nature of this substitution. This disorder correlates with an increase in the catalytic efficiency during the peroxidase reaction, as observed for denatured cytochrome c-550 (Diederix et al. 2002). On the other hand, the stabilizing substitutions in N52I/Y67F produced a protein with increased stability as indicated by the values of pKa, Tm and thermodynamic parameters that can be associated with a very low catalytic efficiency. The N52I/Y67F/W59A variant represents an interesting situation in which the effect of the destabilizing substitutions and the stabilizing effects of the double mutation are combined in one protein. The spectroscopic results and the higher pKa and AH values relative to those of the wild-type protein indicate that the N52I/Y67F substitutions can stabilize the Fe-Met linkage and the heme environment of the protein significantly even in the presence of the highly destabilizing W59A substitution. Nevertheless, the triple variant does exhibit diminished thermal stability (lower Tm) and therefore the proposed influence of the Fe-Met linkage on this value (Yamamoto et al. 2002) is less significant in this protein. In addition, the considerable difference in AS for the reduction of the proteins suggests that structural disorder is one of the factors responsible for the observed differences in reduction potential (Komarpanicucci et al. 1992). The results of this balance, in kinetic terms, are higher values of fa, fa, partition ratio and & ; n a c t constants relative to the corresponding values for the wild-type protein. In this context, it is interesting to observe the presence of 115 additional resonance peaks observed exclusively in the ! H N M R spectrum of the N52I/Y67F/W59A variant. Such variations suggested a modified electronic environment for the HM3, HM8 and HP7 heme substituent protons that could be related to the modified kinetic characteristics of the protein. Interestingly, the computer analysis of the structure simulation for the N52I/W59A/Y67F variant (Figures 33a and 33b), suggest changes in the electronic environment of such protons that could potentially be related to the differences in the structure and/or catalytic behavior. Specifically in the region around HM8 (Figure 33a) the orientation of Argl3 and the absence of a water molecule (W107) produces the rupture of a hydrogen bond between those molecules in the N52I/W59A/Y67F variant. In the other region, around HM3 and HP7 (Figure 33b) the replacement of Trp59 by an alanine, in addition to the creation of a cavity as discussed earlier, causes the side chain of Ala59 to be in place to form a novel hydrogen bond interaction with Arg38. These changes are also likely to affect the interaction between these residues and the propionate groups, which as observed also in the same Figure, are slightly changed in their orientation within the heme group. Notably, electrostatic interactions between the heme propionates, Arg38 and the iron atom have been argued to influence the reduction potential of the heme iron and related conformational changes of cytochrome c (Moore 1983). However, more extensive structural studies employing X-ray crystallography or more detailed N M R experiments are required to characterize the structure and heme electronic environment of N52IY67FW59A in sufficient detail to permit definitive structural correlations with the catalytic properties reported here. 116 Figure 33a. Stereo drawing showing the region around H M 3 in the W T and N52I/W59A/Y67F variants. The structure for the variant (dark gray) was simulated by computer analysis, while the structure of the W T protein (light gray) was taken from the PDB bank (file 2 Y C C ) . A broken line indicates the hydrogen bond discussed in the text. Figure 33b. Stereo drawing of the region around H M 8 and HP7 in the W T and N52I/W59A/Y67F proteins. Same structure analysis as before (fig 31a), showing some of the potential differences upon the introduction of the mutations. 117 II. Random mutagenesis Optimization of enzyme catalytic activity by site directed mutagenesis requires at the least a detailed understanding of the structural elements that are required for efficient catalysis as for maintenance of the three-dimensional structure of the protein. Even with this knowledge, optimal design requires simultaneous adjustment of a great number of structural and functional issues that is, realistically, not possible to achieve. This situation is further complicated by the fact that it is highly unlikely that all relevant structural and functional issues can even be identified. The identification of useful mutations by generation of a large number of randomly-generated variants followed by selection of desirable variants by activity screening provides a strategy by which the limitations in knowledge, time, and labor that impede the use of structure-based design can be overcome. The success of this approach, often referred to as directed evolution, as represented by the work of Arnold and co-workers (Kuchner and Arnold 1997; Joo et al. 1999; Salazar et al. 2003) has resulted in its adaptation for use with a variety of enzymes. The goal of the present implementation of directed evolution in the use of cytochrome c as a scaffold for development of a robust peroxidase was to overcome two crucial limitations exhibited by the peroxidase activity of the cytochrome. The first of these limitations is the instability of the protein to inactivation by hydrogen peroxide, and the second is the intrinsically low peroxidase activity of cytochrome c. Considering that even true peroxidases are inactivated during turnover by oxidative damage resulting from exposure to the peroxide substrate, this first goal was regarded as the more challenging to overcome. In part for this reason, we decided to focus on selection of variants with greater catalytic activity. Optimizing this characteristic of the protein seemed more 118 promising owing to the great range of chemical activities exhibited by the heme prosthetic group in various protein environments (Dawson 1988). In addition, success with screening assays to detect peroxidase activity has been reported previously in the literature (Wan et al. 1998; Cherry et al. 1999; Joo et al. 1999; Farinas et al. 2001) while the design of an assay for stability to heme degradation by peroxide was less readily apparent. The first step in this procedure involved selecting and adjusting the desired mutagenesis level. An average of 2 mutations per gene was chosen based on the small size of the cytochrome c gene and from concern regarding the high probability of introducing non-fuctional mutations in such a short D N A sequence. A low mutagenesis rate is normally preferred in such work because the number of variations in the sequence is greater that the actual number of variants that can be screened (Valetti and Gilardi 2004). The results obtained from the sequencing of representative clones exhibited a high degree of silent mutations and transitions, suggesting creation of a biased library of variants. These limitations could be overcome by using various DNTP mixtures to prevent the bias towards transitions and by increasing the mutagenesis rate by increasing the concentration of MnCb or by related methods (Vartanian et al. 1996). After achieving the desired number of mutations per variant, the next challenge was to obtain a positive result from the screening reaction. Consequently, a series of peroxidase substrates were tested, and it was concluded that in terms of sensitivity, solubility, and colour difference, the best combination consisted in the use of H2C>2 and ABTS. Other substrates such as o-dianisidine provided sensitivity comparable to that of ABTS, but the latter substrate exhibited a lower background colour. Even though this 119 substrate has limitations for kinetic analyses (as discussed previously), it was the best choice for the screening reaction. An additional challenge to the development of an effective screening assay was related to the expression of the protein and the necessity of releasing the proteins from the cells to allow reaction with the (extracellular) substrate to proceed. The initial screening protocol for cytochrome c was designed on the basis of the previous use of the method with myoglobin (Wan et al. 1998). However, after repeated failed attempts, it was concluded that this procedure is inappropriate for use in the current application as the combined result of a very low catalytic activity and a relatively low protein expression level. Thus, the screening protocol required modification. The screening protocol presented several limitations especially in terms of the Tabor required and the number of clones that could be screened; however a similar method for evolving the hydroxylation activity of cytochrome P450 has been applied successfully (Schwaneberg et al. 2001). In addition, the restrictions imposed by our method could potentially be solved through use of robotics, which have been effectively applied in other screening strategies (Valetti and Gilardi 2004). Despite these problems, this method permitted identification of two variants of cytochrome c with increased peroxidase activity. One of these variants, G84E, produced a positive reaction after repeated dilutions of the screening solution and was, therefore, selected for further study. A. The G84E variant To gain insight into the origin of the greater peroxidase activity of the G84E variant, the same kinetic and spectroscopic methods used to study the previous site-120 directed variants, were used to characterize this variant and compare it with the wild-type cytochrome. The first comparison was made in terms of the kinetics constants, in part to confirm that this variant is, in fact, more active than the wild-type protein. The values obtained for the rate constants k\, k-$, the partition ratio as well as kmzct established that, indeed, this variant is more active than the wild-type protein. The increases in the rate of formation of compound I and breakdown of compound II are not as great as those observed for W59A; nevertheless, the value of k3 for the G84E variant is much greater than the value observed for the wild-type protein, suggesting an increase in the accessibility of the active site of this variant to the reducing substrate (Moffet et al. 2000). It is also interesting to note that the G84E variant also exhibits greater values for k\ and the partition ratio than does the N52I/Y67F/W59A triple variant but lower values for k3 and &j n a c t . The larger partition ratio observed for the G84E variant probably results from a combination of the increase in the rate of formation of compound I and the decrease in the inactivation rate constant relative to the corresponding values for the triple variant. Comparison of the spectroscopic properties of the G84E variant and the wild-type protein that were evaluated in the current study reveals no obvious differences. The electronic absorption, CD, M C D and even the N M R spectra of the two proteins were virtually identical, implying that the overall conformation of the polypeptide chain, the coordination of the iron atom and the electronic environment of the prosthetic heme group are not perturbed significantly by this substitution. On the other hand, the G84E substitution did result in some distinct changes in the stability of the protein towards thermal denaturation, altered pH, and the presence of 121 H 2 O 2 , such that the variant exhibited a decreased value for Tm and the pKa for the alkaline conformation transition and a decreased stability toward exposure to peroxide. Surprisingly, despite the notable difference in the reduction potential of the variant, no significant change was observed in the thermodynamic parameters for the oxidation-reduction equilibrium. The small difference in the standard free entropy (AS) caused by the substitution suggests that the altered potential does not result from altered protein structure or disorder (Komarpanicucci et al. 1992) in agreement with the small change observed in Tm. The decreased reduction potential (which is of the same magnitude observed for the N52I/Y67F variant) suggests a relative stabilization of the oxidized form of the protein that presumably results from introduction of a negatively charged residue adjacent to the heme prosthetic group (Lett and Guillemette 2002). Overall, these results point toward small, discrete changes around specific areas of the protein rather than major structural differences as the result of the G84E substitution. This absence of significant structural changes as suggested by these results could be explained at least partially by the location of Gly84 on the protein surface in a region where the heme becomes partially exposed to the solvent; this is located near Phe82 and Argl3 and on the same side of the heme as the Met80 ligand (Figure 34). In the wild-type protein, Gly84 forms a hydrogen bond with Argl3 and possibly to a solvent water molecule. Introduction of a glutamyl residue at position 84 will alter this H-bonding interaction and could result in formation of alternative interactions. 122 Figure 34. Stereo-drawing from the region around A r g l 3 in the W T protein. A r g l 3 (top part of the figure) hydrogen bonds (broken lines) to the oxygen atom of W107 (black molecule) and to the oxygen of the carbonyl group (amide backbone) of Gly84 (gray residue on the left). A section of the heme group (lighter gray on the right) is shown for orientation purposes. Simulation of the effects of G84E on the structure of cytochrome c, and its comparison with the W T protein (Figure 35), raise important points especially in terms of the differences around three main areas of the protein. First, the angle of Pro71 in G84E is significantly altered in comparison with its position in the W T protein residue (Figure 35). This amino acid is directly adjacent to residue Lys73 which has been identified as an alternative ligand to the heme during the alkaline transition of ferricytochrome c (Rosell 1998). Therefore, modifications in the orientation of Pro71 may affect the flexibility of this polypeptide region (required for the rearrangement of the ligand) that could be reflected in the differences of p ^ a value observed in our experiments. 123 Second, the orientation of the carbonyl group in the amide backbone of Tyr67 changed notably (Figure 35). Even though only the peptide backbone of this residue seems altered, and the role of Tyr67 in the peroxidase reaction is still not clear, such changes could potentially affect the behavior of this amino acid in the catalytic mechanism and explain some of the kinetic differences observed for G84E. In addition, and as a consequence of the introduction of a much bulkier residue in G84E, the orientation of the two water molecules around Argl3 is expected to change considerably (Figure 35). This anticipated change is very likely to affect the hydrogen network around these groups, perhaps modifying the access of the substrate during the catalytic reaction, as suggested by our kinetic data. Whatever structural change results, it is reasonable to propose that one outcome would be a modified environment for the region around Argl3 that results in improved access of a reducing substrate to the heme prosthetic group. Notably, the region around Argl3 has been proposed to be involved in some of the oxidation-state linked structural differences of the cytochrome (Berghuis and Brayer 1992) that could also be related to the observed alteration in catalytic behavior of this variant. 124 Figure 35. Structural changes produced by the G84E as estimated by molecular simulation. The residues for the G84E variant are colored in black while the W T amino acids are represented in gray. A ) Superposition of residues Gly84 and Glu84 showing the similar backbone alignment of both residues. B) Difference in the orientation of Pro 71. C) Changes in the backbone of Tyr 67. D) Different orientation of the water molecules HOH303 (bottom left) and HOH107 (right) close to A r g 13 (top right). III. Inactivation reaction To characterize the peroxidase activity of cytochrome c or any heme-containing peroxidase catalyst adequately, the inactivation reaction that occurs in the presence of hydrogen peroxide must be understood. As discussed above, although the inactivation of 125 many heme enzymes that use H 2 O 2 as a substrate has been recognized for many years, the specific products and detailed mechanisms for their generation remain uncharacterized. For this reason, one of the objectives of this work was to identify at least some of these products in an effort to gain greater insight into the mechanism of inactivation and the manner in which this process influences the overall catalytic mechanism of the protein. One of the consequences of the oxidative reaction on the catalyst is the significant bleaching or decrease in absorbance of the heme prosthetic group as reflected by the absorbance intensity of the Soret band (Florence 1985). Notably, heme proteins and heme enzymes appear to vary considerably in their susceptibility to heme bleaching (Valderrama et al. 2002). In the case of cytochrome c and other non-natural peroxidases, the influence of this reaction on the overall catalytic activity of the protein is usually more significant than for true peroxidases owing to the low level of catalytic activity exhibited by the non-natural peroxidases. In general, peroxide-induced heme bleaching is assumed to be the result of direct modification (presumably oxidation) of the heme prosthetic group, particularly for those proteins such as cytochrome c in which the heme group is bound covalently to the protein and, thus, is not readily removed from the protein. This expectation is consistent with the finding that iron is released from cytochrome c in the presence of a peroxide-generating system (Harel et al. 1988; Villegas et al. 2000); however the chemical nature of the modification to the heme group and/or to the protein induced by exposure to peroxide has not been clearly established. In the present work, reaction of hydrogen peroxide with ferricytochrome c resulted in the formation of a free radical species during the oxidation of the protein that is absent (or non-detectable) under identical conditions when the N52I/Y67F double 126 variant is substituted for the wild-type protein (Villegas et al. 2000). Several previous reports have, in fact, demonstrated that free radical species can be formed at tyrosyl residues following exposure of horse heart cytochrome c to H 2 O 2 (Barr et al. 1996; Qian et al. 2002). Even though the radical species observed in our work could be similar to those observed for the horse heart protein, the use of slightly different reaction conditions, the use of yeast rather than horse heart cytochrome, and the inability to observe an immobilized nitroxide by ESR spectroscopy after performing this reaction in the presence of various nitroso spin-traps reagents (data not shown) could also imply that the chemical functionalities involved in this case are not the same. It is worth noting that chelating agents and care in eliminating or avoiding contamination by trace metal ions made no difference to our attempts to identify a peroxide-induced protein-centered radical. The fact that the bleaching reaction is inhibited by the presence of the CN" ion suggests a direct involvement of the heme iron in this process. This latter point, however, needs to be examined more carefully by performing additional experiments. Another reported consequence of oxidizing horse heart cytochrome c with H 2 O 2 is the oligomerization of the protein (Harel et al. 1988). This result was also observed for the yeast cytochrome in the current work, and it was observed that the extent of oligomerization depends on the molar ratio of H 2 O 2 used. This finding was consistent with the kinetics experiments where the shape of the reaction progress curve, and presumably the mechanism of the bleaching reaction, also varies as the ratio of H 2 O 2 to cytochrome is varied. This latter conclusion is consistent with the report that the identity of the radical centres generated in ferricytochrome varies depending on whether H 2 O 2 is generated enzymatically (low concentration) or added directly to the reaction (high 127 concentration) (Harel et al. 1988). For this reason, a detailed kinetic analysis of this reaction under a variety of conditions that includes the characterization of the free radical centres is essential for a comprehensive understanding of the reaction of peroxide and ferricytochrome c. In the present studies, numerous attempts were made to isolate the free radicals produced by the reaction with diverse spin traps. The most promising results were obtained with the spin-trapping reagent POBN, which led to the observation of a protein adduct of the anticipated molecular weight. Unfortunately, repeated attempts to isolate a modified tryptic peptide containing this modification were unsuccessful. This difficulty is presumably a consequence of the low yield of POBN-modified cytochrome c under the wide range of conditions evaluated in this work or it results from unexpected instability of the modification to the conditions of the analytical strategy used in this work. An optimized protocol that allows the isolation of the adduct from the unmodified protein would very likely solve this problem. 128 IV. Future directions Even though some insight into the peroxidase reaction of wo-1-yeast cytochrome c has been obtained from this work, several important aspects require additional investigation for the catalytic activity of this protein to be understood in a more comprehensive way. One of the aspects that may require further work is the kinetic analysis of the peroxidase reaction, especially to unify or to clearly establish the differences between various heme proteins. Our analysis seemed adequate to compare the variants studied in this work and to compare them with "natural" peroxidase enzymes. However, differences with other examples of peroxidase activity among cytochromes and heme proteins suggest that such a mechanism could be more complicated and requires further study. This complexity is also reflected in the inactivation reaction. Again, even though our analysis allowed us to compare the properties of the different variants, the lack of results in the isolation of radical reaction products did not permit us to go further into the study of such reaction. The spin-trapping analysis, combined with liquid chromatography and mass spectrometry has proven to be very useful in the case of other cytochromes. In our case, perhaps the use of alternative spin-traps or a more detailed EPR analysis, including measurements at very low temperature, would provide more insight into this very important aspect of the peroxidase chemistry. In terms of the details in the protein architecture that dictate the differences in the catalytic activity of each variant, more specialized structural methods such as 2D N M R and crystallography are likely to provide a further understanding of the basics of such 129 dissimilarities. Either of the variants N52I/W59A/Y67F or G84E would be suitable candidates for such studies. Finally, regarding the possible physiological relevance of this peroxidase reaction involving cytochrome c would require modified set of experiments. In general the reaction conditions used in this study were chosen to optimize the catalytic process, and they were not selected to resemble the conditions encountered inside the mitochondrial membrane. 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