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Research article2018Peer reviewedOpen access

The role of catalytic residue pK(a) on the hydrolysis/transglycosylation partition in family 3 beta-glucosidases

Geronimo, Inacrist; Payne, Christina M.; Sandgren, Mats

Abstract

beta-Glucosidases (beta gls) primarily catalyze the hydrolysis of the terminal glycosidic bond at the non-reducing end of beta-glucosides, although glycosidic bond synthesis (called transglycosylation) can also occur in the presence of another acceptor. In the final reaction step, the glucose product or another substrate competes with water for transfer to the glycosyl-enzyme intermediate. The factors governing the balance between the two pathways are not fully known; however, the involvement of ionizable residues in binding and catalysis suggests that their pK(a) may play a role. Through constant pH molecular dynamics simulations of a glycoside hydrolase Family 3 (GH3) beta gl, we showed that the pK(a) of the catalytic acid/base residue, E441, is low (similar to 2) during either reaction due to E441-R125-E128 and E441-R125-E166 hydrogen bond networks. The low basicity of E441 would reduce its ability to deprotonate the acceptor. This may be less critical for transglycosylation because sugars have a lower deprotonation enthalpy than water. Moreover, their acidity would be increased by hydrogen bonding with R169 at the acceptor binding site. In contrast, no such interaction was observed for catalytic water. The results are likely applicable to other GH3 beta gls because R125, E128, E166, and R169 are conserved. As these enzymes are commonly used in biomass degradation, there is interest in developing variants with enhanced hydrolytic activity. This may be accomplished by elevating the acid/base residue pK(a) by disrupting its hydrogen bond networks and reducing the affinity and reactivity of a sugar acceptor by mutating R169.

Published in

Organic and Biomolecular Chemistry
2018, Volume: 16, number: 2, pages: 316-324 Publisher: ROYAL SOC CHEMISTRY