Evidence for the Quercetin Binding Site of Glycogen\nPhosphorylase as a Target for Liver-Isoform-Selective Inhibitors against\nGlioblastoma: Investigation of Flavanols Epigallocatechin Gallate\nand Epigallocatechin

Glycogen phosphorylase (GP)\nis the rate-determining enzyme in glycogenolysis, and its druggability\nhas been extensively studied over the years for the development of\ntherapeutics against type 2 diabetes (T2D) and, more recently, cancer.\nHowever, the conservation of binding sites between the liver and muscle\nisoforms makes the inhibitor selectivity challenging. Using a combination\nof kinetic, crystallographic, modeling, and cellular studies, we have\nprobed the binding of dietary flavonoids epigallocatechin gallate\n(EGCG) and epigallocatechin (EGC) to GP isoforms. The structures of\nrmGPb-EGCG and rmGPb-EGC complexes were determined by X-ray crystallography,\nshowing binding at the quercetin binding site (QBS) in agreement with\nkinetic studies that revealed both compounds as noncompetitive inhibitors\nof GP, with EGCG also causing a significant reduction in cell viability\nand migration of U87-MG glioblastoma cells. Interestingly, EGCG exhibits\ndifferent binding modes to GP isoforms, revealing QBS as a promising\nsite for GP targeting, offering new opportunities for the design of\nliver-selective GP inhibitors.