Stereochemical studies of D-glucal hydration by alpha-glucosidases and exo-alpha-glucanases: indications of plastic and conserved phases in catalysis by glycosylases.
Alpha-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 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 exo-glucanases showed no detectable substrate inhibition.
Chiba
S
S
Department of Microbiology and Immunology, Albert Einstein College of Medicine, Bronx, New York 10461.
Brewer
C F
CF
Okada
G
G
Matsui
H
H
Hehre
E J
EJ
D000818:Animals; D001173:Arthrobacter / Q000201:enzymology; D001234:Aspergillus niger / Q000201:enzymology; D003837:Deoxy Sugars; D003847:Deoxyglucose / Q000031:analogs & derivatives* / Q000378:metabolism; D006026:Glycoside Hydrolases / Q000378:metabolism*; D007700:Kinetics; D010143:Paecilomyces / Q000201:enzymology; D012233:Rhizopus / Q000201:enzymology; D012639:Seeds / Q000201:enzymology; D013552:Swine; D000520:alpha-Glucosidases / Q000097:blood / Q000378:metabolism*
D003837:Deoxy Sugars; C001289:d-glucal; D003847:Deoxyglucose; D006026:Glycoside Hydrolases; D000520:alpha-Glucosidases; C021780:exo-1,3-alpha-glucanase