Fig. 5. GS and GN cooperate to synthesise glycogen.

Glucose is converted into glycogen through the action of glycogenin (GN), glycogen synthase (GS) and glycogen branching enzyme (GBE). GN interacts with GS to feed the initial glucose chain into the GS active site for elongation. GS is regulated by allosteric activation and inhibitory phosphorylation. Phospho-S641 (pS641) from one C- terminal tail interacts with the regulatory helices α22 to cause enzyme inhibition. This can be relieved by glucose-6-phoshate (G6P), with or without phosphatases, to reach a high activity state. Kinases can phosphorylate GS to inhibit the enzyme.