UDP-(5F)-GlcNAc acts as a slow-binding inhibitor of MshA, a retaining glycosyltransferase

Glycosyltransferase enzymes play important roles in numerous cellular pathways. Despite their participation in many therapeutically relevant pathways, there is a paucity of information on how to effectively inhibit this class of enzymes. Here we report that UDP-(5F)-GlcNAc acts as a slow-binding, competitive inhibitor of the retaining glycosyltransferase MshA from Corynebacterium glutamicum (K_i ≈ 1.6 μM). The kinetic data are consistent with a single-step inhibition mechanism whose equilibration is slow relative to catalysis. We believe that this is the first slow-onset inhibitor to be reported for the glycosyltransferase family of enzymes. The potent inhibition of the enzyme by the fluoro-substituted substrate is consistent with the involvement of an oxocarbenium transition-state structure, which has been previously proposed for this family of enzymes. Additionally, although several members of the GT-B enzyme family, including MshA, have been shown to undergo a conformational change upon UDP-GlcNAc binding, the kinetic data are inconsistent with a two-step inhibition mechanism. This suggests that there may be other conformations of the enzyme that are useful for the design of inhibitors against the large family of GT-B glycosyltransferase enzymes.