Hydrolysis of alpha- and beta-D-glucosyl fluoride by individual glucosidases: new evidence for separately controlled "plastic" and "conserved" phases in glycosylase catalysis
1993
Matsui, H. | Tanaka, Y. | Brewer, C.F. | Blanchard, J.S. | Hehre, E.J.
alpha-Glucosidases from sugar beet seed and ungerminated rice catalyzed the hydrolysis of pyranosyl fluoride to form alpha-D-glucose. The reactions were slow, with V/K = 11-15 x 10(-3) or approximaterly 1-2% of that for hydrolysis of p-nitrophenyl alpha-D-glucopyranoside, but were not due to any impurity in the substrate or to contaminating beta-glucosidase or glucoamylase. Furthermore, almond P-glucosidase promoted hydrolysis of alpha-D-glucosyl fluoride to form beta-D-glucose at an exceedingly low rate, V/K = 4 This weak reaction did not stem from any impurity in the substrate or to contamination with alpha-glucosidase or glucoamylase, but it was partly (approximately equal 20%) attributable to a trace of accompanying trehalase. That all three glucosidases acted upon both alpha and beta-D-glucosyl fluoride, albeit at low efficiency with the disfavored anomer, reflects the previously demonstrated ability of each enzyme's catalytic groups to respond flexibly to substrates of different types. That the disfavored fluoride in each case was converted into a product of the same configuration as from enitols or favored D-glucosyl substrates provides additional evidence for the two-step nature of the chemical mechanisms of glucosidases, in which the stereochemistry of water attack on the enzyme-stabilized oxocarbonium ion is strictly maintained, regardless of the initial anomeric configuration of the substrate.
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