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

Background: Glycogen concentrations in muscle and liver are regulated by the coordinated actions of glycogen phosphorylase (GP) and glycogen synthase. GP exists in two forms: the dephosphorylated, low-activity form (GPb) and the phosphorylated, high-activity form (GPa). Both forms are subject to regulation by allosteric effectors, which promote a dynamic equilibrium between a less active T-state and a more active R-state. GP has been proposed as a potential therapeutic target for drugs aimed at preventing excessive glycogen breakdown and stimulating glycogen synthesis, particularly in conditions such as non-insulin-dependent diabetes. Through a database search, 5-chloro-1H-indole-2-carboxylic acid (1-(4-fluorobenzyl)-2-(4-hydroxypiperidin-1-yl)-2-oxoethyl)amide (CP320626) was identified as a potent inhibitor of human liver GP. Structural studies were conducted to elucidate the mechanism of action of this novel inhibitor.

Results: The structure of the cocrystallized GPb-CP320626 complex was determined at a resolution of 2.3 Å. CP320626 binds at a previously uncharacterized site located at the subunit interface within the central cavity CP-91149 of the dimeric structure. This site is approximately 15 Å from the AMP allosteric site and 33 Å from the catalytic site. The binding interactions between GPb and CP320626 involve six hydrogen bonds and extensive van der Waals contacts, forming a tight binding pocket in the T-state conformation of GPb. In contrast, these interactions are significantly weakened in the R-state conformation of GPa.

Conclusions: CP320626 inhibits GPb by binding at a novel allosteric site. Despite being over 30 Å away from the catalytic site, the inhibitor stabilizes the T-state conformation at the expense of the R-state, thus shifting the allosteric equilibrium between the two states. This new allosteric binding site represents a potential target for the development of improved GP inhibitors.