Glycogen is the principal storage form of glucose in animal cells. In humans, the most glycogen is found in the liver (10% of the liver mass), whereas muscles only contain a relatively low amount of glycogen (1% of the muscle mass). In addition, small amounts of glycogen are found in certain glial cells in the brain.
Sometimes called "animal starch" for its resemblance with starch found in plants, it is stored in liver and muscle cells and can be converted to glucose if needed. In the liver this conversion is regulated by the hormoneglucagon. Under certain conditions, between meals for instance, liver glycogen is an important source of blood glucose. Muscle cell glycogen appears to be only for local use. Glycogen is the primary glucose (energy) storage mechanism. It is stored in the form of granules in the cytosol which is where glycolysis takes place. These granules contain both glycogen and the necessary enzymes for its conversion into glucose.
Glycogen is a highly branched glucose polymer. It is formed of small chains of 8 to 12 glucose molecules linked together with &alpha (1→4) bonds. These small chains are in turn linked together with &alpha (1→6) bonds. A single molecule of glycogen can be made of up to 120,000 molecules of glucose. It is generated from glucose by the enzyme glycogen synthase. This process is called glycogenesis. The addition of a glucose molecule to glycogen takes two high energy bonds: one from ATP and one from UTP. Its breakdown into glucose, called glycogenolysis, is mediated by the enzyme glycogen phosphorylase. Its highly branched nature allows for the quick retrieval of glucose molecules when needed.
