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Spectrophotometric determination of the kinetic parameters of β-fructofuranosidase and the mechanism.. Zhang, Allen 2011

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Spectrophotometric Determination of the Kinetic Parameters of β-fructofuranosidase and the Mechanism of Inhibition by Copper (II) Sulfate Allen Zhang Tyson Miao Science One Program The University of British Columbia Vancouver, Canada April 2011 1 Abstract Excessive dosages of heavy metals can be hazardous to organisms and the environment. This is due to the ability of heavy metal ions to inhibit several kinds of biological enzymes. In this exper- iment, the kinetic parameters of β-fructofuranosidase, a fungal enzyme that catalyzes the break- down of sucrose, were quantified through dinitrosalicylic acid assay and nonlinear regression. Furthermore, the activity of β-fructofuranosidase was measured in 6 different concentrations of copper (II) sulfate, an inhibitor, to determine the nature of inhibition. From the experiment, the results show that the nature of inhibition is noncompetitive at [CuSO4] < 0.0022M and competitive at [CuSO4] > 0.0044M , Vmax is 0.441± 0.0354mmolmin , Km is 17.6± 2.6mM , and Ki is 0.00863± 0.00101. 2 Introduction 2.1 Background Sucrose is a disaccharide composed of α-glucose and β-fructose joined by an α-1,4-glycosidic linkage (Freeman et al., 2011). This bond can be hydrolyzed by β-fructofuranosidase, an enzyme commonly known as invertase, to yield glucose and fructose in equal proportions (Greenwood Health Systems, 2009). The presence of heavy metal ions such as Cu2+ may inhibit the activity of invertase. This is due to the high affinity of the sulfhydryl group on cysteine and methionine residues in a protein for the Cu2+ ion (Greenwood Health Systems, 2009). Soft bases, including sulfhydryl groups, have a high affinity for soft acids, such as the Cu2+ ion (Cotton et al., 1976). When copper ions form complexes with sulfhydryl groups in amino acid residues, disulfide bridges are disrupted, modifying the enzyme’s tertiary structure. Since the functions of an enzyme are highly correlated with its form, a change in the tertiary structure of invertase would likely inhibit its function (Cotton et al., 1976). As a result of the close proximity of methionine residues to the glycosylation sites of invertase, CuSO4 is suspected to be a noncompetitive inhibitor–one that slows product formation, while not affecting substrate binding (http://www.civil.uwaterloo.ca/enve375). In order to test this hypothesis, the rates of sucrose hydrolysis by invertase at various concentrations of copper sulfate were experimentally determined, and a Lineweaver-Burk plot was used to determine the mechanism of inhibition. 2.2 Michaelis-Menten Kinetics Michaelis-Menten kinetics were used throughout the experiment to model the reaction rate as a function of the inhibitor concentration, and to test whether the mechanism of inhibition was noncompetitive, competitive, or mixed. Noncompetitive inhibition refers to the process by which an inhibitor binds to a non-active site in an enzyme, and slows down the rate of product formation. However, noncompetitive inhibitors do not change the affinity, as measured by the Michaelis-Menten constant, Km, of an enzyme for its substrate. On the other hand, competitive inhibition refers to the type of inhibition in which an inhibitor binds to an enzyme’s active site and lowers the affinity of the enzyme for its substrate. Competitive inhibition does not change the maximal rate of reaction, measured as Vmax, due to the fact that the presence of a large 1 amount of substrate can saturate the active sites, preventing the inhibitor from binding. Mixed inhibition is a combination of competitive and noncompetitive inhibition; both Vmax and Km are changed. The general kinetic scheme for noncompetitive inhibition is as follows (http://www.civil.uwaterl- oo.ca/enve375): The reaction velocity, v, is: v = Vmax[S] (Km + [S])(1 + [I] Ki ) where Ki is the kinetic constant of inhibition, which measures the ratio between the concentra- tion of the enzyme-inhibitor complex and free inhibitor (http://www.civil.uwaterloo.ca/enve375). Rearranging, 1 v = (1 + [I] Ki ) Km Vmax 1 [S] + (1 + [I] Ki ) Vmax is a linear equation in terms of [S] and v. This equation serves as the basis for the Lineweaver- Burk plot (1/v vs. 1/[S]), for noncompetitive inhibition. Similar equations for competitive and mixed inhibition can be found in literature (http://www.civil.uwaterloo.ca/enve375). 2.3 Applications The study of enzyme kinetics is essential in the fields of medicine, biology, and chemistry. Baker’s yeast (scientific name: Saccharomyces cerevisiae), a species of fungi that produces invertase, can be used to detect trace amounts of heavy metal ions, like cadmium and mercury, by analyzing the yeast’s enzymatic activity over a fixed period of time, in solution (Narayana & Kumar, 2010). Detecting heavy metal concentrations is vital in testing water quality. Enzyme inhibitors can also be used in antibiotics. Most antibiotics are administered with β- lactamase inhibitors, such as tazobactam and clavulanic acid, to prevent the bacterial enzyme β-lactamase from degrading antibiotics (Kuck et al., 1989). By using an approach similar to the one carried out in this experiment, the enzyme kinetics of certain types of β-lactamases can be studied in order to elucidate the amount of tazobactam or clavulanic acid that should be administered to patients so as to minimize the probability of bacterial survival while maintaining reasonable production costs. 2 Figure 1: Diagram of the experimental setup for the treatment with no inhibitor. DNS reagent was added to each of the test tubes, and the test tubes were immersed in the 95◦C water bath. 3 Methods 3.1 Beer’s Law Calibration Various amounts of deionized water and glucose-fructose solution were added to 7 labelled test tubes to generate the glucose-fructose concentrations specified in Table 1. 1.5mL of DNS reagent was then added to each test tube, and all of the test tubes were transferred to a 95−100◦C water bath. The test tubes were collected after 10 minutes and transferred to a cold water bath. After the test tubes returned to room temperature, the contents in each of the test tubes were diluted 100 times by mixing 0.1 mL of test tube solution with 9.9mL of deionized water. An aliquot of the diluted content for each test tube was transferred into individual borosilicate test tubes in order to measure the absorbance of the diluted contents at 540nm with a spectropho- tometer. The blank used was the same as that for Measuring Enzyme Activity without Inhibitor (see below). All empty borosilicate test tubes were chosen within a range of 0.073- 0.077A absorbances at a wavelength of 540nm for consistency. Table 1. Summary of the concentrations of glucose-fructose solution in each of the test tubes. Test tube 1 Test tube 2 Test tube 3 Test tube 4 Test tube 5 Test tube 6 Test tube 7 [Glucose- Fructose] (M) 3.5× 10−5 7.0× 10−5 1.75× 10−4 2.34× 10−4 3.51× 10−4 8.78× 10−5 2.0× 10−3 3 3.2 Measuring Enzyme Activity without Inhibitor Various amounts of deionized water and 50mg/mL sucrose solution were added to 6 labelled test tubes, exact quantities are specified in Table 2. 1.5 mL of 0.04 mg/mL invertase solution (from Sigma-Aldrich I4504 Grade VII invertase) was then added to each test tube before the test tubes were transferred to a 55◦C water bath. The sudden rise in temperature facilitated the reaction between invertase and sucrose. After 5 minutes, the test tubes were taken out of the 55◦C water bath and 1.5 mL of the DNS Reagent was added to each test tube. Upon the addition of the DNS Reagent, the test tubes were immediately transferred to a 95− 100◦C water bath; the second rise in temperature was intended to denature invertase and facilitate the bonding of DNS Reagent to glucose and fructose. The test tubes were collected after 10 minutes and transferred to a cold water bath. When the test tubes returned to room temperature, the content of each test tube was diluted 100 times by mixing 0.1 mL of the content with 9.9 mL of deionized water. An aliquot of the diluted content of each test tube was transferred into individual borosilicate test tubes in order to measure the absorbance of the diluted contents at 540nm, using a spectrophotometer; Test tube 1 was used as the blank for calibration. Furthermore, all empty borosilicate test tubes were chosen within a range of 0.073A to 0.077A absorbances at 540nm for consistency. Table 2. Summary of the amounts of reagents added to each of the test tubes (see Measuring Enzyme Activity without Inhibitor). Test tube 1 Test tube 2 Test tube 3 Test tube 4 Test tube 5 Test tube 6 Deionized H2O (mL) 1.5 1.375 1.25 1.125 1.00 0.75 50mg/ml Sucrose (mL) 0 0.125 0.25 0.375 0.5 0.75 3.3 Measuring Enzyme Activity with Varying Concentrations of In- hibitor (CuSO4) Similar to the setup of the previous section, various amounts of deionized water and 50mg/mL sucrose solution were added to labelled test tubes with new additions of 0.1 M CuSO4 at dif- ferent treatments levels. During the experiment, this section was conducted in sets of 6 test tubes; all sets contained the amounts of sucrose specified in Table 2 and the amount of CuSO4 specified for each treatment level in Table 3. Although the amount of sucrose in each test tube remained constant from the last section, as illustrated by Table 2, the amount of deionized water required by each test tube varies between different treatments levels of CuSO4. After sucrose and CuSO4 were added, the volume of each test tube was adjusted up to 1.5 mL with deionized water. For each set, an additional 1.5 mL of 0.04 mg/mL invertase solution was added before the test tubes were transferred to a 55◦C water bath. The sudden rise in temperature facilitated the re- action between invertase and sucrose. After 5 minutes, the test tubes were taken out of the 55◦C water bath and 1.5 mL of the DNS Reagent was added to each test tube. Upon the addition of the DNS Reagent, the test tubes were immediately transferred to a 95− 100◦C water bath; the second rise in temperature was intended to denature invertase and facilitate the bonding of DNS Reagent to glucose and fructose. The test tubes were collected after 10 minutes and transferred into a cooling water bath. When the test tubes returned to room temperature, the 4 content of each test tube was diluted 100 times by mixing 0.1 mL of the content with 9.9 mL of deionized water. An aliquot of the diluted content of each test tube was transferred into individ- ual borosilicate test tubes in order to measure the absorbance of the diluted contents at 540nm, using a spectrophotometer; Test tube 1 was used as the blank for calibration. Furthermore, all empty borosilicate test tubes were chosen within a range of 0.073A to 0.077A absorbances at 540nm for consistency. Table 3. Summary of the volumes of 0.1M CuSO4 in each of the treatment levels with inhibitor. Treatment level (mLs of CuSO4) 0.025 0.050 0.1 0.2 0.3 4 Results & Discussion 4.1 Measurements Table 4. Beer’s Law Analysis. The following data was taken with a spectrophotometer by pouring the invertase solutions into borosilicate test tubes, and recording the absorbances at 540nm. The solutions in the test tubes were diluted 100X from the values listed below. The borosilicate test tubes had absorbances that ranged from 0.073A to 0.077A. The uncertainties were determined by taking half of this range (0.002A). [Glucose] (M ) Absorbance Uncertainty [Glucose] × Absorbance [Glucose]2(M) (Model(Abs.)-Actual)2(M2) 3.5× 10−5 0.007 0.002 2.46× 10−7 1.24× 10−9 3.24× 10−6 7.0× 10−5 0.01 0.002 7.03× 10−7 4.94× 10−9 1.61× 10−7 1.8× 10−4 0.027 0.002 4.74× 10−6 3.09× 10−8 9.92× 10−7 2.3× 10−4 0.033 0.002 7.73× 10−6 5.49× 10−8 2.79× 10−6 3.5× 10−4 0.057 0.002 2.00× 10−5 1.23× 10−7 2.56× 10−5 8.8× 10−4 0.128 0.002 1.12× 10−4 7.71× 10−7 3.70× 10−6 2.0× 10−3 0.296 0.002 5.92× 10−4 4.01× 10−6 2.21× 10−9 The linear regression line has to go through the origin (the blank), so slope (m) and the uncer- tainty in slope (σm) were determined by minimizing χ 2 with: m = ∑N i=1 xiyi∑N i=1(xi) 2 = 7.38× 10−4M 4.99× 10−6M2 = 148.0M −1;σm = √ 1 N ∑N i=1(yi −mxi)2∑N i=1(xi) 2 = 1.2M−1 5 Table 5. Rates of sucrose hydrolysis by invertase under different concentrations of CuSO4. The sucrose solutions were diluted 100X from the values listed. The rate of hydrolysis were determined using the absorbances and Beer’s Law calibration techniques (see below). All rates listed below are in units of mM/min. Absorbances listed below are the means of three replicates for each treatment. The uncertainties (δ) were determined by taking the standard error for each of the sets of three replicates. No inhibitor 25µl CuSO4 50µl CuSO4 100µl CuSO4 [Sucrose] (M ) Abs. δ Rate Abs. δ Rate Abs. δ Rate Abs. δ Rate 0 0 0.001 0.000 -0.002 0.002 -0.002 -0.002 0.001 0.001 0.001 0.001 0.002 0.004 0.064 0.001 0.086 0.053 0.001 0.072 0.050 0.001 0.067 0.041 0.002 0.056 0.008 0.114 0.002 0.154 0.095 0.001 0.128 0.085 0.002 0.115 0.073 0.000 0.099 0.012 0.151 0.002 0.204 0.137 0.003 0.185 0.119 0.006 0.161 0.093 0.002 0.126 0.016 0.168 0.001 0.227 0.149 0.004 0.201 0.146 0.003 0.197 0.110 0.003 0.148 0.024 0.174 0.001 0.235 0.178 0.001 0.241 0.161 0.003 0.218 0.146 0.003 0.197 200µl CuSO4 300µl CuSO4 [Sucrose] (M ) Abs. δ Rate Abs. δ Rate 0 0.002 0.000 0.000 0 0.001 0.000 0.004 0.036 0.001 0.048 0.031 0.001 0.042 0.008 0.069 0.002 0.093 0.058 0.002 0.079 0.012 0.083 0.003 0.112 0.076 0.001 0.103 0.016 0.104 0.002 0.140 0.093 0.001 0.126 0.024 0.133 0.003 0.179 0.124 0.002 0.168 The rates of hydrolysis were calculated from the absorbances with the slope from the Beer’s Law plot (Table 4), as follows: R = Abs (148.0M−1)× 5min × 1000mM M 4.2 Uncertainty Calculations Table 6. Summary of the uncertainty calculations for the rate of sucrose hydrolysis under different concentrations of copper (II) sulfate. These uncertainties were obtained from propagating the uncer- tainties listed in Table 5 with the results from the Beer’s Law calibration in Table 4. Uncertainty in Rate of Hydrolysis (mmol/min) [Sucrose] (M ) No inhibitor 25µl CuSO4 50µl CuSO4 100µl CuSO4 200µl CuSO4 300µl CuSO4 0 0.00197 0.00316 0.00197 0.00119 0.000451 0.000781 0.004 0.00208 0.000977 0.00131 0.00278 0.00167 0.00200 0.008 0.00302 0.00171 0.00257 0.000808 0.00285 0.00259 0.012 0.00291 0.00440 0.00764 0.00293 0.00411 0.00214 0.016 0.00270 0.00556 0.00435 0.00388 0.00317 0.00187 0.024 0.00226 0.00251 0.00385 0.00425 0.00412 0.00303 The rate uncertainties (δR) were calculated in the same way as the rates of hydrolysis R were calculated, with a few changes. A sample calculation for the no inhibitor, 0.004M sucrose 6 treatment, using the method of partial derivatives, is shown below: δR = √ ( δAbs m )2 + ( (Abs)(δm) m )2 × 1000mM 5min×M = √ ( 0.001 148.0M−1 )2 + ( (0.064)(1.2M−1) 148.0M−1 )2 × 1000mM 5min×M = 0.00208 mmol min Figure 2: Graph of the changes in the hydrolysis rate of sucrose by invertase at 6 different concentrations of sucrose and 6 different concentrations of CuSO4 (n = 3). This graph was plotted with the data in Table 5, and the uncertainties in Table 6. 4.3 Lineweaver-Burk Plot A Lineweaver-Burk plot is a method for linearizing the Rate vs. [Substrate] plots in Figure 2. If the lines intersect at the x-axis ( 1 [S] ), then copper (II) sulfate is a noncompetitive inhibitor. This is due to the fact that the x-intercept will be − 1 Km (http://www.civil.uwaterloo.ca/enve375). Likewise, if the lines intersect at the y-axis ( 1 Rate ), then copper (II) sulfate is a competitive inhibitor, as the y-intercept will be 1 Vmax (http://www.civil.uwaterloo.ca/enve375). If the lines intersect somewhere in the (−x,+y) quadrant, then the inhibition method is mixed. Table 7. Summary of the calculations used to construct the Lineweaver-Burk plots (Figure 2). The uncertainties were calculated through propagation; smaller uncertainties indicate higher precision. All values for 1V (1/rate) are in units of min mmol . 1 S (M −1) No inhibitor 25µl CuSO4 50µl CuSO4 100µl CuSO4 200µl CuSO4 300µl CuSO4 1 V 95% C.I. 1 V 95% C.I. 1 V 95% C.I. 1 V 95% C.I. 1 V 95% C.I. 1 V 95% C.I. 246 11.6 0.282 14.0 0.190 14.9 0.291 17.9 0.890 20.7 0.719 23.6 1.11 123 6.51 0.128 7.78 0.104 8.66 0.193 10.1 0.0829 10.8 0.330 12.7 0.416 82.1 4.91 0.0701 5.40 0.128 6.21 0.295 7.92 0.184 8.94 0.329 9.69 0.201 61.6 4.41 0.0526 4.97 0.138 5.06 0.119 6.74 0.177 7.13 0.161 7.95 0.118 41.1 4.25 0.0410 4.15 0.0434 4.58 0.0892 5.08 0.109 5.57 0.128 5.96 0.108 7 Figure 3: Linearized plots of 1/Hydrolysis Rate vs. 1/[Sucrose], commonly referred to as Lineweaver-Burk plots. Plots were constructed from the data in Table 7. 2a Lineweaver- Burk plot for the first four concentrations of CuSO4, with the point of intersection near the 1 [Sucrose] -axis. 2b Lineweaver-Burk plot for the two highest concentrations of CuSO4; the point of intersection is at the 1 V -axis. 4.4 Nonlinear Regression Results Due to the fact that using the Lineweaver-Burk method of linearizing to determine Km and Vmax magnifies experimental errors, GraphPad’s Prism 5 R© nonlinear regression software was used to determine the kinetic parameters of invertase. Table 8. Summary of the values of the kinetic parameters for invertase (with sucrose as a sub- strate and copper (II) sulfate as the inhibitor) as determined by nonlinear regression analysis with GraphPad’s Prism 5 R© software. Vmax (mmol/min) Km (mM) Ki Best Fit Values 0.441 17.6 0.00863 Standard Error 0.0179 1.30 0.000507 95% Confidence Intervals 0.406-0.477 15.0-20.1 0.00763-0.00964 Overall R-squared 0.978 The R-squared value of 0.978 is close to 1.00, which provides confidence in our regression lines. 4.5 Data Analysis 4.5.1 Kinetic Parameters From nonlinear regression analysis, it was found that Km = 17.6± 2.6mM (Table 8). This is comparable to the value of 18.3mM reported by Ribeiro and Vitolo (2006) for soluble invertase at pH 4.5. On the other hand, the value of Vmax obtained in this study, 0.441 ± 0.0354mmolmin (Table 8), was not in agreement with the results of the same study (Vmax = 0.0450 Units mL ) (Ribeiro & Vitolo 2006). This can be explained by the different experimental conditions in which the two studies were conducted. Ki, the kinetic constant of inhibition by copper (II) sulfate, was determined to be 0.00863± 0.00101 (Table 8). 8 4.5.2 Mechanism of Inhibition When interpreting the data in Figure 2, it is interesting to note that the shape of the rate- substrate curve differs between concentrations of inhibitor. As inhibitor concentration is in- creased, the curve gradually becomes more sigmoidally (S) shaped, as opposed to the hyperbolic shape for the noninhibited reaction curve. This indicates that there may have been coopera- tive or noncooperative binding–when the substrate of the reaction serves to facilitate or inhibit the binding of other substrate molecules–or feedback inhibition (Combes & Monsan, 1983). In fact, both the substrate, sucrose, and the products of the reaction, glucose and fructose, in- hibit invertase (Combes & Monsan, 1983). Despite this, inhibition due to glucose and fructose likely plays a minor role in the kinetics of the enzyme, as the affinity of sucrose for inver- tase (Km = 17.6 ± 2.6mM) is greater than that for fructose (Km = 128mM) and glucose (Km = 270mM) (Combes & Monsan, 1983). Furthermore, from the Lineweaver-Burk plot in Figure 3a, it is apparent that the best fit lines intersect close to the x-axis, which is indicative of the noncompetitive nature of copper (II) sulfate inhibition, as Km stays nearly constant while the inhibitor concentrations are varied; this is in agreement with the hypothesis. However, the Lineweaver-Burk plot in Figure 3b shows that the best fit lines for the 200µl copper (II) sulfate and 300µl copper (II) sulfate treatments actually intersect at the y-axis. This suggests that, at high concentrations, copper (II) sulfate competitively inhibits invertase. While this apparent discrepancy can be attributed to experimental error as well as the non-Michaelis-Menten (sigmoidal) nature of inhibition, it is also possible that Cu2+ ions interact with the active sites of invertase when the allosteric sites are saturated. Research shows that one methionine residue (residue number 40) is in close proximity to one of the active sites of invertase (residue number 42); furthermore, residue num- ber 224, cysteine, is located next to another substrate binding site in invertase (residue number 223) (Taussig & Carlson, 1983). Therefore, it can be conjectured that Cu2+ ions preferentially bind to the allosteric sites in invertase (noncompetitive inhibition), but at high concentrations of Cu2+, the ions may also bind to the active sites in invertase (competitive inhibition). 4.5.3 Biological and Chemical Applications From the results of this study, CuSO4 has been classified as a noncompetitive inhibitor of in- vertase, even when [CuSO4] is low. While there are inherent differences among invertase and other biological enzymes, other sources have reported that heavy metal ions are noncompeti- tive inhibitors for most enzymes (Narayana & Kumar, 2010). Therefore, it is likely that the ingestion of a substantial amount of Cu2+, and possibly also other heavy metal ions may be life-threatening for organisms (heavy metal poisoning), as the essential enzymes that support life may be denatured in the presence of metal ions. With the kinetic parameters determined in this experiment (see Kinetic Parameters), a pre- cise value for the decrease in the activity of invertase in the presence of a certain amount of CuSO4 can be elucidated. Consequently, [Cu 2+] can be determined by adding known amounts of invertase and sucrose into a solution, assuming it does not contain any other inhibitors. A similar experiment and analysis to the ones used in this study can be carried out, for other heavy metal ions. Through this process, the qualities and metal ion contents of water samples can be ascertained. Furthermore, due to the noncompetitive nature of invertase inhibition by CuSO4, the Michaelis- Menten model of enzyme inhibition can be extended to an enzyme-inhibitor system involv- 9 ing certain types of β-lactamases and tazobactam, a noncompetitive inhibitor of particular β- lactamases (Kuck et al., 1989). After determining the various kinetic constants–Km, Vmax, and Ki–for the β-lactamase-tazobactam system, the degree of inhibition at various concentrations of tazobactam can be related to the reaction rates between β-lactamases and their substrates. 5 Conclusion Throughout the experiment, the kinetic parameters of invertase with respect to its substrate, sucrose, were determined as Km = 17.6 ± 2.6mM and Vmax = 0.441 ± 0.0354mmolmin . Further- more, it was found that CuSO4 is a noncompetitive inhibitor of invertase at low [CuSO4] (below 0.0022M), and CuSO4 is a competitive inhibitor of invertase at higher [CuSO4] (above 0.0044M). The inhibition constant, Ki, was found to be 0.00863 ± 0.00101. However, further studies should be conducted, with more precise equipment, to determine the kinetic constants to a greater degree of accuracy, and to confirm or refute our conjecture (see Data Analysis). 6 Acknowledgements We would like to thank Dr. Carl Douglas for his guidance and assistance throughout the con- ception and presentation of this project; Mrs. Xueqin Huang and Mrs. Mingxin Guo for the use of various equipment in the Biology Lab; ; Dr. Eric Cytrynbaum for his help in deriving the ap- propriate mathematical equations; and Meiying Zhuang and Christie Chan for their assistance in editing this report. 7 References Combes, D. & Monsan, P. (1983). Sucrose hydrolysis by invertase. Characterization of products and substrate inhibition. Carbohydrate Research, 117, 215-228. doi:10.1016/0008-6215(83)88088- 4. Cotton, F.A., Wilkinson, G. & Gaus, P.L. (1976). Basic Inorganic Chemistry. New Baskerville: John Wiley & Sons. Freeman, S., Harrington, M. & Sharp, J.C. (2011). Biological Science. Toronto: Pearson Canada. Greenwood Health Systems. (2009). Invertase. Retrieved March 10, 2011, from http://greenwo odhealth.net/np/invertase.htm. Kuck, N.A., Jacobus, N.V., Petersen, P.J., Weiss, W.J. & Testa, R.T. (1989). Comparative In Vitro and In Vivo Activities of Piperacillin Combined with the 1-Lactamase Inhibitors Tazobac- tam, Clavulanic Acid, and Sulbactam. Antimicrobial Agents and Chemotherapy, 33, 1964-1969. Narayana, N.N.H. & Kumar, N.V.N. (2010). Detection of Compounds of Mercury, Cadmium and Copper by Baker’s Dry Yeast Enzyme Inhibition. Middle-East Journal of Scientific Edu- cation, 6, 152-156. Ribeiro, R.R. & Vitolo, M. (2006). Anion exchange resin support for invertase immobilization. Journal of Basic and Applied Pharmaceutical Sciences, 26, 175-179. Taussig, R. & Carlson, M. (1983). Nucleotide sequence of the yeast SUC2 gene for invertase. Nucleic Acids Research, 11, 1943-1954. doi: 10.1093/nar/11.6.1943. 10 University of Waterloo. (2001). Enzyme kinetics. Retrieved March 10, 2011, from http://www. civil.uwaterloo.ca/enve375 /Background.htm#Enzyme. 11

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