The Conformationally Controlled Reactivity of Carbasugars Uncovers the Choreography of Glycoside Hydrolase Catalysis

Glycoside hydrolases (GHs) catalyze hydrolyses of glycoconjugates in which the enzyme choreographs a series of conformational changes during the catalytic cycle. As a result, some glycoside hydrolase families, including the α-amylases (GH13), have their chemical steps concealed kinetically. To address this issue for a GH13 enzyme we made seven cyclohexenyl-based carbasugars of α-D-glucopyranoside that we show are good covalent inhibitors of a GH13 yeast α-glucosidase. The linear free energy relationships between rate constants and pKa of the leaving group is curved upwards, which is indicative of a change in mechanism, with the better leaving groups reacting by a SN1 mechanism, while reaction rates for the worse leaving groups are limited by a conformational change of the Michaelis complex prior to a rapid SN2 reaction with the enzymatic nucleophile. Five bicyclo[4.1.0]heptyl-based carbaglucoses were tested with this enzyme, and our results are consistent with pseudo-glycosidic bond cleavage occurs via SN...