Effect of added nucleophiles and pH on alpha-D-mannosidase-catalyzed reactions

The effect of (1) added nucleophiles, (2) the concentration of water, and (3) pH on the alpha-D-mannosidase-catalyzed hydrolysis of substituted phenyl alpha-D-mannopyranosides has been investigated.. A two-step mechanism, with formation of a mannosyl-enzyme complex, is proposed. With the phenyl alpha-D-mannopyranosides, the formation of this intermediate complex is the rate-limiting step. The activity of the enzyme is controlled by two dissociable groups having pK approximately 3 and pK approximately 6, respectively. It is proposed that one of the groups functions as a proton-donor, whereas the other group stabilizes the mannosyl-enzyme complex by ion-pair formation or covalent binding.