GH47 and Other Glycoside Hydrolases Catalyze Glycosidic Bond Cleavage with the Assistance of Substrate Super-arming at the Transition State.

Super-armed glycosyl donors, whose substituents are predominantly held in pseudoaxial positions, exhibit strongly increased reactivity in glycosylation through significant stabilization of oxocarbenium-like transition states. Examination of X-ray crystal structures reveals that the GH47 family of glycoside hydrolases have evolved so as to distort their substrates away from the ground state conformation in such a manner as to present multiple C-O bonds in pseudoaxial positions and so benefit from conformational super-arming of their substrates, thereby enhancing catalysis. Through analysis of literature mutagenic studies, we show that a suitably placed aromatic residue in GHs 6 and 47 sterically enforces super-armed conformations on their substrates. GH families 45, 81, and 134 on the other hand impose conformational super-arming on their substrates, by maintaining the more active ring conformation through hydrogen bonding rather than steric interactions. The recognition of substrate super-arming by select GH families provides a further parallel with synthetic carbohydrate chemistry and nature and opens further avenues for the design of improved glycosidase inhibitors.