Crystallographic studies on α- and β-D-glucopyranosyl formamide analogues, inhibitors of glycogen phosphorylase

The catalytic site of glycogen phosphorylase (GP) is currently under investigation as a target for inhibition of hepatic glycogenolysis under high glucose conditions. Three D-glucopyranosyl analogues, C-(1-azido-alpha-D-glucopyranosyl) formamide, C-(1-acetamido-alpha-D-glucopyranosyl) formamide, and C-(1-hydroxy-beta-D-glucopyranosyl) formamide, were recognised as moderate competitive inhibitors of muscle glycogen phosphorylase b (GPb) [with respect to alpha-D-glucose 1-phosphate (Glc-1-P)] with K i values of 1.80 (+/-0.2) mM, 0.31 (+/-0.01) mM, and 0.88 (+/-0.04) mM, respectively. In order to elucidate the structural basis of inhibition, we determined the structure of muscle GPb complexed with the three compounds at 2.1, 2.06 and 2.0 Angstrom resolution, respectively. The complex structures revealed that the inhibitors can be accommodated in the catalytic site of T-state GPb with very little change of the tertiary structure, and provide a rationalisation for understanding potency of the inhibitors. The glucopyranose moiety makes the standard hydrogen bonds and van der Waals contacts as observed in the GPb-glucose complex, while the substituent groups in the alpha- and beta-position of the C1 atom make additional hydrogen bonding and van der Waals interactions to the protein.