Suicide Substrate-based Inactivation of Endodextranase by ω-Epoxyalkyl α-D-Glucopyranosides

Abstract

Three kinds of ω-epoxyalkyl α-glucopyranosides (3′,4′-epoxybutyl α-D-glucopyranoside (E4G), 4′,5′-epoxypentyl α-D-glucopyranoside (E5G) and 5′,6′-epoxyhexyl α-D-glucopyranoside (E6G)), having alkyl chains of different lengths at their aglycone moieties, inactivated the endodextranase from Streptococcus mutans ATCC 25175 (SmDex) irreversibly with the pseudo-first order kinetics. Alkyl chain length-dependent inactivation was observed and the degree of activity loss was E5G, E6G and E4G, in that order, implying that the distance between epoxide group and glucosyl residue of ω-epoxyalkyl α-glucopyranoside was important in the modification of endodextranase. Inactivation by E5G followed the model of reversible intermediate-complex formation mechanism (suicide inhibitor-based mechanism). The rate constant of irreversible inactivation (k) and the dissociation constant of intermediate-complex (KR) of SmDex and E5G were 0.44 min^-1 and 1.45 mM, respectively. Hydrolytic reaction product (isomaltose) protected SmDex from E5G-inactivation, suggesting that E5G bound to the catalytic site of SmDex. This is the first report that ω-epoxyalkyl α-glucopyranoside becomes a suicide substrate for endodextranase.