Thioglycoside hydrolysis catalyzed by β-glucosidase

Sweet almond beta-glucosidase (EC 3.2.1.21) has been shown to have significant thioglycohydrolase activity. While the K 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 (pK(a) 4.5) and a protonated group (pK(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 beta-glucosidase, thioglucoside hydrolysis does not involve general acid catalysis. There is no solvent kinetic isotope effect on the enzyme-catalyzed hydrolysis of pNPSG. (c) 2007 Elsevier Inc. All rights reserved.