Stereochemical studies of D-glucal hydration by .alpha.-glucosidases and exo-.alpha.-glucanases: indications of plastic and conserved phases in catalysis by glycosylases

Publisher Website
Google Scholar
Abstract
.alpha.-Glucosidases from Aspergillus niger, pig serum, ungerminated rice, buckwheat, and sugar beet seeds (but not from brewer''s yeast or honeybee) were found to catalyze the hydration of D-glucal. Each reactive .alpha.-glucosidase, incubated with D-glucal in D2O, was shown to protonate (deuteriate) this prochiral substrate from above its re face, i.e., from a direction opposite that assumed for protonating .alpha.-D-glucosidic substrates. At the same time, D-glucal hydration catalyzed by three of the .alpha.-glucosidases that acted rapidly enough in D2O to determine product configuration was found to yield 2-deoxy-D-glucose of the same specific (.alpha.-) configuration as the D-glucose produced from .alpha.-D-glucosidic substrates. These findings substantially extend those reported earlier for the hydration of D-glucal by one (Candida tropicalis) .alpha.-glucosidase preparation. Together with other recent results, they suggest that the process of catalysis by .alpha.-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 .alpha.-glucosidases, three "inverting" exo-.alpha.-glucanases (Arthrobacter globiformis glucodextranase; Rhizopus niveus and Paecilomyces varioti glucoamylase) were found to protonate D-glucal from below its si face. Further, whereas the catalysis of D-glucal hydration by the .alpha.-glucosidases was intensively inhibited by excess substrate, that promoted by the exoglucanases showed no detectable substrate inhibition.