Glycogen phosphorylase as a molecular target for type 2 diabetes therapy

The regulation of the hepatic glucose output through glycogenolysis is an important target for type 2 diabetes therapy. Glycogenolysis is catalyzed in liver, muscle and brain by tissue specific isoforms of glycogen phosphorylase (GP). Because of its central role in glycogen metabolism, GP has been exploited as a model for structure-assisted design of potent inhibitors, which may be relevant to the control of blood glucose concentrations in type 2 diabetes. Several regulatory binding sites have been identified in GP, such as the catalytic, the allosteric, and the inhibitor binding sites. Protein crystallography has contributed significant structural information on the specificity and interactions that distinguish the binding sites, and also revealed a new unexpected binding site (new allosteric site). In this review, the kinetic, crystallographic binding, and physiological studies of a number of compounds, inhibitors of GP, are described, and the essential inhibitory and binding properties of specific compounds are analyzed in an effort to provide rationalizations for the affinities of these compounds and to exploit the molecular interactions that might give rise to a better inhibitor. These studies have given new insights into fundamental structural aspects of the enzyme enhancing our understanding of how the enzyme recognizes and specifically binds ligands, that could be of potential therapeutic value in the treatment of type 2 diabetes.