Reaction mechanism of chitobiose phosphorylase from Vibrio proteolyticus: identification of family 36 glycosyltransferase in Vibrio.

A family 36 glycosyltransferase gene was cloned from Vibrio proteolyticus. The deduced amino acid sequence showed a high degree of identity with ChBP (chitobiose phosphorylase) from another species, Vibrio furnissii. The recombinant enzyme catalysed the reversible phosphorolysis of (GlcNAc)2 (chitobiose) to form 2-acetamide-2-deoxy-alpha-D-glucose 1-phosphate [GlcNAc-1-P] and GlcNAc, but showed no activity on cellobiose, indicating that the enzyme was ChBP, not cellobiose phosphorylase. In the synthetic reaction, the ChBP was active with alpha-D-glucose 1-phosphate as the donor substrate as well as GlcNAc-1-P to produce beta-D-glucosyl-(1-->4)-2-acetamide-2-deoxy-D-glucose with GlcNAc as the acceptor substrate. The enzyme allowed aryl-beta-glycosides of GlcNAc as the acceptor substrate with 10-20% activities of GlcNAc. Kinetic parameters of (GlcNAc)2 in the phosphorolysis and GlcNAc-1-P in the synthetic reaction were determined as follows: phosphorolysis, k(0)=5.5 s(-1), K(m)=2.0 mM; synthetic reaction, k(0)=10 s(-1), K(m)=14 mM, respectively. The mechanism of the phosphorolytic reaction followed a sequential Bi Bi mechanism, as frequently observed with cellobiose phosphorylases. Substrate inhibition by GlcNAc was observed in the synthetic reaction. The enzyme was considered a unique biocatalyst for glycosidation.