Glycogen synthesis occurs in the cytoplasm and can be divided into the following four steps:
(1) Glycogenesis begins with the phosphorylation of glucose by glucokinase in the liver and by hexokinase in muscle and other tissues.
(2) Glucose-6-phosphate is then converted into glucose-1-phosphate by phosphoglucomutase, a reaction similar to that catalysed by phosphoglycerol mutase.
(3) Glucose-1-phosphate is converted into uridine diphosphate (UTP)-glucose, the immediate precursor of glycogen synthesis, by reaction with uridine triphosphate (UTP). This reaction is catalysed by glucose 1-phosphate uridylyltransferase (or UDP-glucose pyrophosphorylase).
(4) Finally, the glucosyl residue at the C-1 position of UDP-glucose is added to the tyrosyl group at position 194 of glycogen protease in an Mg2+-dependent autocatalytic reaction. Glycosidases similarly extend the glucan chain by α(1→4) glycosidic bonds by adding 6 to 7 glucosyl units, using UDP-glucose as the substrate, in an autocatalytic reaction. The glucan primer of glycogen proteins is elongated by glycogen synthase using UDP glucose. Initially, the glycogenin primer and glycogen synthase are strongly bound using a 1:1 complex. As the dextran chain grows, glycogen synthase separates from glycogen.
