Sucrose 6-alpha-D-glucosyltransferase from Streptococcus sobrinus: characterization of a glucosyl-enzyme complex

A covalent glucosyl-enzyme was isolated from a quenched reaction of Streptococcus sobrinus sucrose 6-alpha-D-glucosyltransferase and radiolabeled sucrose. No complex was observed with heat-inactivated enzyme or when sucrose was replaced with radiolabeled maltose or glucose. The complex was stable at pH 2 in 1% sodium dodecyl sulfate, 6.0 M urea, and 4.0 M guanidine hydrochloride, but became increasingly labile with increased pH (32-min half-life at pH 7.0). D-Glucose was the exclusive radiolabeled compound identified when all radioactivity was released under mild alkaline conditions. Glucosyl-enzyme hydrolysis rates were linearly dependent on hydroxide ion concentration, giving a second-order rate constant of 2.15 x 10(5) M-1 min-1. When compared to the base lability of known glycosyl amino acid derivatives, the pH dependency of the glucosyl-enzyme most closely paralleled a glucosyl linkage to a carboxyl group. A novel application of a carbohydrate high-performance liquid chromatography column in aqueous solution was used to identify the anomeric form of D-glucose released on (i) alkaline hydrolysis of denatured glucosyl-enzyme and (ii) native enzyme hydrolysis of sucrose. The beta-anomer was identified in the former case and the alpha-anomer in the latter. The results with the denatured glucosyl-enzyme are consistent with a beta-glucosyl ester linkage to an aspartic or glutamic acid that hydrolyzes at the ester carbon with retention of anomeric configuration; for native glucosyltransferase catalysis, the data are consistent with a beta-glucosyl covalent intermediate as well, where deglucosylation occurs by attack at the acetal carbon with anomeric inversion.(ABSTRACT TRUNCATED AT 250 WORDS)