Glycogen phosphorylase revisited: extending the resolution of the R- and T-state structures of the free enzyme and in complex with allosteric activators

The crystal structures of free T-state and R-state glycogen phosphorylase (GP) and of R-state GP in complex with the allosteric activators IMP and AMP are reported at improved resolution. GP is a validated pharmaceutical target for the development of antihyperglycaemic agents, and the reported structures may have a significant impact on structure-based drug-design efforts. Comparisons with previously reported structures at lower resolution reveal the detailed conformation of important structural features in the allosteric transition of GP from the T-state to the R-state. The conformation of the N-terminal segment (residues 7-17), the position of which was not located in previous T-state structures, was revealed to form an alpha-helix (now termed alpha 0). The conformation of this segment (which contains Ser14, phosphorylation of which leads to the activation of GP) is significantly different between the T-state and the R-state, pointing in opposite directions. In the T-state it is packed between helices alpha 4 and alpha 16 (residues 104-115 and 497-508, respectively), while in the R-state it is packed against helix alpha 1 (residues 22'-38') and towards the loop connecting helices alpha 4' and alpha 5' of the neighbouring subunit. The allosteric binding site where AMP and IMP bind is formed by the ordering of a loop (residues 313-326) which is disordered in the free structure, and adopts a conformation dictated mainly by the type of nucleotide that binds at this site.

Automated summary

The crystal structures of glycogen phosphorylase in free T-state and R-state, as well as in R-state complexed with allosteric activators IMP and AMP, were reported at improved resolution. This study provides detailed insight into the conformational changes of glycogen phosphorylase during the transition from T-state to R-state.