Glycogen is a highly branched polymer composed of chains of D-glucose linked linearly by α-1,4-glucosidic bonds and joined together by α-1,6-glucosidic bonds between branch points. It is stored in cytosolic granules and serves as an energy source when food supplies are low. Glycogen synthesis involves the auto-catalyzed synthesis of a glucose oligomer on glycogenin Gyg, followed by linear extension of the glucose oligomer by glycogen synthase Glys, and the formation of branches by glycogen branching enzyme Agbe. In D. melanogaster adults, it is primarily synthesized and stored in the fat body, flight muscles and in the oocyte. (Adapted from FBrf0238357 and PMID:22248338.)