N-acetyl-β-D-glucopyranosylamine: A potent T-state inhibitor of glycogen phosphorylase. A comparison with α-D-glucose

Structure-based drug design has led to the discovery of a number of glucose analogue inhibitors of glycogen phosphorylase that have an increased affinity compared to α-D-glucose (K_i = 1.7 mM). The best inhibitor in the class of N-acyl derivatives of β-D-glucopyranosylamine, N-acetyl-β-D-glucopyranosylamine (1-GlcNAc), has been characterized by kinetic, ultracentrifugation, and crystallographic studies. 1-GlcNAc acts as a competitive inhibitor for both the b (K_i = 32 μM) and the α (K_i = 35 μM) forms of the enzyme with respect to glucose 1-phosphate and in synergism with caffeine, mimicking the binding of glucose. Sedimentation velocity experiments demonstrated that 1-GlcNAc was able to induce dissociation of tetrameric phosphorylase α and stabilization of the dimeric T-state conformation. Co-crystals of the phosphorylase b-1-GlcNAc-IMP complex were grown in space group P4₃2₁2, with native-like unit cell dimensions, and the complex structure has been refined to give a crystallographic R factor of 18.1%, for data between 8 and 2.3 Å resolution. 1-GlcNAc binds tightly at the catalytic site of T-state phosphorylase b at approximately the same position as that of α-D-glucose. The ligand can be accommodated in the catalytic site with very little change in the protein structure and stabilizes the T-state conformation of the 280s loop by making several favorable contacts to Asn 284 of this loop. Structural comparisons show that the T-state phosphorylase b-1-GlcNAc-IMP complex structure is overall similar to the T-state phosphorylase b-α-D-glucose complex structure. The structure of the 1-GlcNAc complex provides a rational for the biochemical properties of the inhibitor.