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An investigation of the mechanism of the Cellulomonas fimi exoglucanase

University of British Columbia

The exoglucanase from Cellulomonas fimi catalyses the hydrolysis of cellobiose units from the non-reducing terminus of cello-oligosaccharides with overall retention of anomeric configuration. Its mechanism of action is therefore thought to involve a double displacement reaction, involving as the first step, formation of a glycosyl-enzyme intermediate (glycosylation) and as a second step, the hydrolysis of this intermediate (deglycosylation). This mechanism is investigated here through the study of the kinetics of hydrolysis of aryl β-glucosides and aryl β-cellobiosides and by employing the mechanism-based irreversible inactivators, 2', 4'-dinitrophenyl 2-deoxy-2-fluoro-β-D-glucoside (2F-DNPG) and 2", 4"-dinitrophenyl 2-deoxy-2-fluoro-β-D-cellobioside (2F-DNPC). The study with the aryl β-glucosides revealed that this enzyme is indeed active on glucosides, a feature that had previously been undetected. A linear relationship was found to exist between the logarithm of Vmax for hydrolysis and the phenol pKa as well as between the logarithm of Vmax/Krn and me phenol pKa, showing that glycosylation is both the rate determining step and the first irreversible step for all substrates. The reaction constant calculated, ρ = 2.21, indicates a considerable amount of charge build up at the transition state of glycosylation. The linear free energy relationship study of the aryl β-cellobiosides revealed no significant dependence of the logarithm of Vmax on the pKa of the phenol, indicating that deglycosylation is rate determining. However, the slight downward trend in this Hammett plot at higher pKa values may suggest that the rate determining step is changing from deglycosylation to glycosylation. However, the logarithm of Vmax/Km does correlate with the pKa of the phenol, thus showing that the first irreversible step is glycosylation. The reaction constant (ρ = 0.60) which reflects the development of charge at the glycosylation transition state for the cellobiosides is less than that calculated for the glucosides, thus suggesting a glycosylation transition state with either a greater degree of acid catalysis or less C-O bond cleavage than that for the glucosides. The inactivators, 2F-DNPC and 2F-DNPG, are believed to inactivate the exoglucanase by binding to the enzyme and forming covalent glycosyl-enzyme intermediates. The inactivated-enzyme was stable in buffer but reactivated in the presence of a suitable glycosyl-acceptor such as cellobiose, presumably via a transglycosylation reaction. These results indicate that covalent 2F-glycosyl-exoglucanase intermediates are stable and are catalytically competent to turn over to product, thus supplying further evidence for the Koshland mechanism. The exoglucanase is inactivated more rapidly by 2F-DNPC than by 2F-DNPG. However, both inactivated forms of the enzyme reactivated at comparable rates in the presence of cellobiose, showing that the second glucosyl unit present on the cellobiosides increases the rate of glycosylation relative to that found for the glucosides but not the rate of deglycosylation. The stable covalent nature of the 2F-glycosyl-enzyme intermediates provided an excellent opportunity to identify the enzymic nucleophile. This was accomplished by radiolabelling the exoglucanase with a tritiated analogue of 2F-DNPG cleaving the protein into peptides and purifying the radiolabelled peptides. Sequencing of this peptide resulted in the identification of the active site nucleophile as glutamic acid residue 274. This residue was found to be highly conserved in this family of β-glycanases, further indicating its importance in catalysis.

Text

2010-12-14

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http://hdl.handle.net/2429/30402

Chemistry

University of British Columbia

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