Aryl thioglycoside-based affinity purification of exo-acting cellulases

Isolation of cellulolytic enzymes from moldy silage by new culture-independent strategy

A culture-independent strategy has been developed for investigation of cellulases in moldy silage. By the qualitative differences in the adsorption of cellulases on lingo-cellulosics, a new cellobiohydrolase (CBH) with apparent molecular mass of 194 kDa was isolated and characterized. The entire extracellular proteins of silage were separated by two-dimensional gel electrophoresis, and five potential endoglucanases were identified by activity staining. These results demonstrate the feasibility of direct screening cellulases from environment without microorganism cultivation and this strategy could be expected to facilitate the research of uncultured microorganisms.

O-GlcNAcase catalyzes cleavage of thioglycosides without general acid catalysis

O-GlcNAcase catalyzes the removal of N-acetylglucosamine residues from serine and threonine residues of post-translationally modified proteins using a catalytic mechanism involving substrate-assisted catalysis and general acid/base catalysis. Since thioglycosides are widely perceived as resistant to hydrolysis by glycosidases, it was surprising to find that O-GlcNAcase also catalyzes the efficient hydrolysis of S-glycosides. Brønsted analyses and pH-activity studies of the O-GlcNAcase-catalyzed hydrolysis of a series of aryl S- and O-glycosides reveal that O-GlcNAcase effects hydrolysis of thioglycosides without the assistance of general acid catalysis. alpha-Deuterium kinetic isotope effects for O- and S-glycosides, as well as Taft-like analyses using N-fluoroacetyl-beta-glycosides, suggest that O-GlcNAcase accomplishes hydrolysis of thioglycosides by stabilizing late transition states. For S-glycosides this transition state shows greater nucleophilic participation from the 2-acetamido group than for O-glycosides. The rate constants governing the O-GlcNAcase-catalyzed hydrolysis of O- and S-glycosides as compared to those previously determined for the spontaneous hydrolysis of structurally similar O,O- and O,S-acetals show a similar ratio. O-GlcNAcase therefore demonstrates similar catalytic proficiency toward both O- and S-glycosides. We conclude that O-GlcNAcase is a bifunctional catalyst capable of efficiently cleaving thioglycosides without general acid catalysis, an observation that may have biological implications.