Chemical mechanism of beta-glucosidase from Trichoderma reesei QM 9414. pH-dependence of kinetic parameters.

The variation of kinetic parameters of beta-glucosidase from Trichoderma reesei QM 9414 with pH was used to gain information about the chemical mechanism of the reaction catalysed by this enzyme. The pH-dependence of Vmax. and Vmax./Km for p-nitrophenyl beta-D-glucopyranoside showed that a group with a pK value of 4.3 must be unprotonated and a group with a pK value of 5.9 must be protonated for activity. Temperature and solvent-perturbation studies indicated that these groups are a histidine residue and a carboxy group respectively. Profiles of pKi for maltose as competitive inhibitor showed that binding is prevented when a group on the enzyme with a pK value of 4.5 becomes protonated.