Crystallographic studies on N-azidoacetyl-β- d-glucopyranosylamine, an inhibitor of glycogen phosphorylase: Comparison with N-acetyl-β- d-glucopyranosylamine

Abstract

N-Acetyl-β- d-glucopyranosylamine (NAG) is a potent inhibitor ( K i = 32 μM) of glycogen phosphorylase b (GPb), and has been employed as a lead compound for the structure-based design of new analogues, in an effort to utilize its potential as a hypoglycaemic agent. Replacement of the acetamido group by azidoacetamido group resulted in an inhibitor, N-azidoacetyl-β- d-glucopyranosylamine (azido-NAG), with a K i value of 48.7 μM, in the direction of glycogen synthesis. In order to elucidate the mechanism of inhibition, we determined the ligand structure in complex with GPb at 2.03 Å resolution, and the structure of the fully acetylated derivative in the free form. The molecular packing of the latter is stabilized by a number of bifurcated hydrogen bonds of which the one involving a bifurcated C–H⋯N⋯H–C type hydrogen bonding is rather unique in organic azides. Azido-NAG can be accommodated in the catalytic site of T-state GPb at approximately the same position as that of NAG and stabilizes the T-state conformation of the 280s loop by making several favourable contacts to residues of this loop. The difference observed in the K i values of the two analogues can be interpreted in terms of desolvation effects, subtle structural changes of protein residues and changes in water structure.