Glycogen synthase

Glycogen Synthase (GS) is an enzyme that catalyses the production of glycogen. It catalyses a condensation reaction between UDP-glucose and Glycogen (n-residues) to form glycogen (n+1 residues) and UDP, elongating the glycogen polymer. 

Regulation of the enzyme is achieved through the multiple phosphorylation of Glycogen Synthase at different sites by multiples kinases, including Protein Kinase A (PKA) and Glycogen Synthase Kinase 3 (GSK3). The enzyme is phosphorylated in it's inactive state and must interact with glucose-6-phosphate to be activated. Binding results in a conformational change, increasing the affinity of the enzyme for it's substrate.

In the presence of insulin, glycogen synthesis is increased. This is due to the inhibition of GSK3 by Insulin, preventing Glycogen Synthase being phosphorylated and inactivated. Insulin also activates a phosphatase called Protein Phosphatase-1 (PP1), which dephosphorylates GS. The pathway as to how this occurs is unclear.