A new allosteric site in glycogen phosphorylase b as a target for drug interactions

Background: In muscle and liver, glycogen concentrations are regulated by t he coordinated activities of glycogen phosphorylase (GP) and glycogen synth ase. GP exists in two forms: the dephosphorylated low-activity form GPb and the phosphorylated high-activity form GPa. In both forms, allosteric effec ters can promote equilibrium between a less active T state and a more activ e R state. GP is a possible target for drugs that aim to prevent unwanted g lycogen breakdown and to stimulate glycogen synthesis in non-insulin-depend ent diabetes. As a result of a data bank search, 5-chloro-1H-indole-2-carbo xylic acid (1 -(4-fluorobenzyl)-2-(4-hydroxypiperidin-1-yl)-2-oxoethyl)amid e, CP320626, was identified as a potent inhibitor of human liver GP. Struct ural studies have been carried out in order to establish the mechanism of t his unusual inhibitor. Results: The structure of the cocrystallised GPb-CP320626 complex has been determined to 2.3 Angstrom resolution. CP32026 binds ata site located at th e subunit interface in the region of the central cavity of the dimeric stru cture. The site has not previously been observed to bind ligands and is som e 15 Angstrom from the AMP allosteric site and 33 a from the catalytic site . The contacts between GPb and CP320626 comprise six hydrogen bonds and ext ensive van der Waals interactions that create a tight binding site in the T -state conformation of GPb. In the R-state conformation of GPa these intera ctions are significantly diminished. Conclusions: CP320626 inhibits GPb by binding at a new allosteric site. Alt hough over 30 Angstrom from the catalytic site, the inhibitor exerts its ef fects by stabilising the T state at the expense of the R state and thereby shifting the allosteric equilibrium between the two states, The new alloste ric binding site offers a further recognition site in the search for improv ed GP inhibitors.