Not “from above”, but “from the side”: Configuration-retaining β-glycosidases protonate their substrate either anti or syn to the endocyclic C1−O bond as the first step in the enzymic cleavage of the glycosidic bond (see schematic drawing). Insights into the mechanism of action of glycosidases have been gained by a combination of the synthesis of inhibitors, the study of the kinetics of their inhibition, and the analysis of the crystal structures of glycosidases and glycosidase–ligand complexes.
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Heightman et al. (1999) studied this question.