New perspectives on the storage and organization of muscle glycogen

Due to its large mass, skeletal muscle contains the largest depot of stored carbohydrate in the body in the form of muscle glycogen. Readily visualized by the electron microscope, glycogen granules appear as bead-like structures localized to specific subcellular locales. Each glycogen granule is a functional unit, not only containing carbohydrate, but also enzymes and other proteins needed for its metabolism. These proteins are not static, but rather associate and dissociate depending on the carbohydrate balance in the muscle. This review examines glycogen-associated proteins, their interactions, and roles in regulating glycogen metabolism. While certain enzymes such as glycogen synthase and glycogen phosphorylase have been extensively studied, other proteins such as the glycogen initiating and targeting proteins are just beginning to be understood. Two metabolically distinct forms of glycogen, pro- and marcoglycogen have been identified that vary in their carbohydrate complement per molecule and have different sensitivities to glycogen synthesis and degradation. Glycogen regulation takes place not only by allosteric regulation of enzymes, but also due to other factors such as subcellular location, granule size, and association with various glycogen-related proteins.