QM/MM analysis of cellulase active sites and actions of the enzymes on substrates

Abstract

Biodegradation of cellulosic biomass requires the actions of three types of secreted enzymes; endoglucanase (EC 3.2.1.4), cellobiohydrolase or exoglucanase (EC 3.2.1.91), and β-glucosidase (EC 4.2.1.21). These enzymes act synergistically to hydrolyse the β-1,4 bonds of cellulose and converts it into simple sugar. Hydrolysis of the glycosidic bond can occur either by net retention or by inversion of anomeric configuration at the anomeric center. QM/MM simulations are useful tools to study the energetics of the reactions and analyze the active-site structures at different states of the catalysis, including the formation of unstable transition states. Here, a brief description of previous work on glycoside hydrolases is first given. The results of the QM/MM potential energy and free energy simulations corresponding to glycosylation and deglycosylation processes are then provided for two retaining endoglucanases, Cel12A and Cel5A. The active-site structural features are analyzed based on the QM/MM results. The role of different residues and hydrogen bonding interactions during the catalysis and the importance of the sugar ring distortion are discussed for these two enzymes. © 2010 American Chemical Society.