Thioglycoside hydrolysis catalyzed by β-glucosidase

Sweet almond β-glucosidase (EC 3.2.1.21) has been shown to have significant thioglycohydrolase activity. While the K m values for the S- and O-glycosides are similar, the k cat values are about 1000-times lower for the S-glycosides. Remarkably, the pH-profile for k cat/ K m for hydrolysis of p-nitrophenyl thioglucoside (pNPSG) shows the identical dependence on a deprotonated carboxylate (p K a 4.5) and a protonated group (p K a 6.7) as does the pH-profile for hydrolysis of the corresponding O-glycoside. Not surprisingly, in spite of the requirement for the presence of this protonated group in catalytically active β-glucosidase, thioglucoside hydrolysis does not involve general acid catalysis. There is no solvent kinetic isotope effect on the enzyme-catalyzed hydrolysis of pNPSG.