Reactive thioglucoside substrates for β-glucosidase

A new, very efficient, class of thioglycoside substrates has been found for beta-glucosidase. While thioglycosides are usually resistant to hydrolysis, even in the presence of acids or most glycohydrolases, the beta-D-glucopyranosides of 2-mercaptobenzimidazole (GlcSBiz) and 2-mercaptobenzoxazole (GlcSBox) have been found to be excellent substrates for beta-glucosidase from both sweet almond (a family 1 glycohydrolase) and Aspergillus niger (a family 3 glycohydrolase), reacting nearly as well as beta-nitrophenyl beta-D-glucoside. The enzyme-catalyzed hydrolysis of GlcSBiz proceeds with retention of configuration. As with the (1000-fold slower) hydrolysis of phenyl thioglucosides catalyzed by the almond enzyme, the pL (pH/pD)-independent k(cat)/K-M does not show a detectable solvent deuterium kinetic isotope effect (SKIE), but unlike the hydrolysis of phenyl thioglucosides, a modest SKIE is seen on k(cat) [(D2O)k(cat) = 1.28 (+/-0.06)] at the pL optimum (5.5 <= pL <= 6.6). A solvent isotope effect is also seen on the K-M for the N-methyl analog of GlcSBiz. These results suggest that the mechanism for the hydrolysis of the beta-thioglucoside of 2-mercaptobenzimidazole and of 2-mercaptobenzoxazole involves remote site protonation (at the ring nitrogen) followed by cleavage of the thioglucosidic bond resulting in the thione product. (C) 2013 Elsevier Inc. All rights reserved.