studies on New Reactions in Glycosylase Catalysis

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Aspergillus niger alpha-glucosidase catalyzed the slow hydroysis of beta-Dglucopyranosyl fluoride to form alpha-D-glucose. Maltotriose competitively inhibited the hydrolysis, and beta-D-glucosyl fluoride in turn competitively inhibited the hydrolysis of p-nitrophenyl alpha-D-glucopyranoside, indicating that beta-D-glucosyl fluoride is bound at the same site as known substrates for the alpha-glucosidase.Also alpha-glucosidases from sugar beet and rice catalyzed the hydrolysis of beta-D-glucosyl fluoride to form alpha-D-glucose. The reaction was slow, with V/K 1-2% of that for PNP alpha-D-glucoside hydrolysis, but was not due to any impurity in the substrate or to contaminating beta-glucosidase or glucoamylase. Maltopentaose was a linear competitive inhibitor of PNP alpha-glucoside hydrolysis in each case. Almond beta-glucosidase, promoted alpha-D-glucosyl fluoride hydrolysis to form beta-glucose at an exceedingly low rate. This weak reaction did not stem from any impurity in substance, … More or to contamination with alpha-glucosidase or glucoamylase, but it was partly(ca. 20%)attributable to a trace of accompanying trehalase.That the disfavored D-glucosyl fluoride in each case was converted to a product of the same configuration as from enitols or favored D-glucosyl substrates provides ndw evidence for a separate stage of catalysis in which the steric outcome of reactions of a glucosidase is strictly conserved by protein structure regardless of substrate configuration.Coffee bean alpha-galactosidase was found to catalyze the hydration of D-galactal and D-galacto-octenitol, each a known substrate for beta-galactosidase. The enzyme protonated each substrate from beneath the plane of the ring, as assumed for alpha-D-galactosides. These results provide an unequivocal assignment of the orientation of an acidic catalytic group to the alpha-galactosidase reaction center. The findings are discussed in light of the concept that catalysis by glycosidases involves a "plastic" protonation phase and a "conserved" product configuration phase. Less