Stereochemical Studies of D-Glucal Hydration by α-Glucosidases and Exo-α-glucanases: Indications of Plastic and Conserved Phases in Catalysis by Glycosylases

a-Glucosidases from Aspergillus niger, pig serum, ungerminated rice, buckwheat, and sugar beet seeds (but not from brewers' yeast or honeybee) were found to catalyze the hydration of D-glucal. Each reactive a-glucosidase, incubated with D-glucal in D₂0, was shown to protonate (deuteriate) this prochiral substrate from above its re face, i.e., from a direction opposite that assumed for protonating a-D-glucosidic substrates. At the same time, D-glucal hydration catalyzed by three of the a-glucosidases that acted rapidly enough in D₂0 to determine product configuration was found to yield 2-deoxy-D-glucose of the same specific (a-) configuration as the D-glucose produced from a-D-glucosidic substrates. These findings substantially extend those reported earlier for the hydration of D-glucal by one (Candida tropicalis) a-glucosidase preparation. Together with other recent results, they suggest that the process of catalysis by a-glucosidases (and perhaps glycosylases in general) may comprise two separate and separately controlled parts, namely, a “plastic” phase concerned with substrate protonation and a substrate-unrelated “conserved” phase concerned with the creation of product configuration. In contrast to the a-glucosidases, three “inverting” exo-α-glucanases (Arthrobacter globtformis glucodextranase; Rhizopus niveus and Paecilomyces varioti gluco-amylase) were found to protonate D-glucal from below its si face. Further, whereas the catalysis of D-glucal hydration by the a-glucosidases was intensively inhibited by excess substrate, that promoted by the exo-glucanases showed no detectable substrate inhibition.