HYDROLYSIS OF ALPHA-D-GLUCOSYL AND BETA-D-GLUCOSYL FLUORIDE BY INDIVIDUAL GLUCOSIDASES - NEW EVIDENCE FOR SEPARATELY CONTROLLED PLASTIC ANDCONSERVED PHASES IN GLYCOSYLASE CATALYSIS

alpha-Glucosidases from sugar beet seed and ungerminated rice catalyze d the hydrolysis of beta-D-glucopyranosyl fluoride to form alpha-D-glu cose. The reactions were slow, with V/K = 11-15x10(-3) or similar to 1 -2% of that for hydrolysis of p-nitrophenyl a-D-glucopyranoside, but w ere not due to any impurity in the substrate or to contaminating beta- glucosidase or glucoamylase. Furthermore, almond beta-glucosidase prom oted hydrolysis of alpha-D-glucosyl fluoride to form beta-D-glucose at an exceedingly low rate, V/K = 4x10(-4). This weak reaction did not s tem from any impurity in the substrate or to contamination with alpha- glucosidase or glucoamylase, but it was partly (similar to 20%) attrib utable to a trace of accompanying trehalase. That all three glucosidas es acted upon both alpha- and beta-D-glucosyl fluoride, albeit at low efficiency with the disfavored anomer, reflects the previously demonst rated ability of each enzyme's catalytic groups to respond flexibly to substrates of different types. That the disfavored D-glucosyl fluorid e in each case was converted into a product of the same configuration as from enitols or favored D-glucosyl substrates provides additional e vidence for the two-step nature of the chemical mechanisms of glucosid ases, in which the stereochemistry of water attack on the enzyme-stabi lized oxocarbonium ion is strictly maintained, regardless of the initi al anomeric configuration of the substrate.