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 catabolism involves the cleavage of α-1,4-glucosidic bonds by glycogen phosphorylase Glyp from the non-reducing ends of the linear chain releasing glucose 1-phosphate. The final four glycosyl residues of each branch are removed in two steps by glycogen debranching enzyme Agl. Alternatively, glycogen can be degraded in the lysosomes via an autophagic process known as glycophagy. In D. melanogaster adults, glycogen is primarily catabolized in the fat body and flight muscles to produce glucose and trehalose. (Adapted from FBrf0238357, FBrf0223427 and PMID:22248338.)