Glycogen is synthesized as energy storage in liver during well fed state. Glycogenesis occurs in cytosol. It synthesizes the glycogen from α-D-glucose. Glycogenesis goes through the following phases:
1. Synthesis of UDP-glucose
Glucose needs to be attached to uridine diphosphate (UDP) initially and then added into glycogen growing chain. UDP-glucose pyrophosphorylase synthesizes UDP from glucose 1-phosphate and UTP.
2. Synthesis of primer to begin the elongation
Synthesis of glycogen chain cannot be started with glucose as adaptor. The elongation can only be done in the existing glycogen chain. Thus, a protein, glycogenin acts as adaptor for glucose residue from UDP-glucose. The side chain hydroxyl group of a specific tyrosine serves as the site at which the initial glucosyl unit is attached. This reaction is catalyzed by glcogenin. Glycogenin transfers few glucose residue to form (1→4)-linked short chain of glycogen. This short chain glycogen serves as primer for the elongation of glycogen chain. Then, enzyme glycogen synthase starts to elongate the chain.
Glycogen synthase adds the glucose residue to nonreducing end glycogen chain, forming glycosidic bond between the anomeric hydroxyl of carbon 1 of the activated glucose and carbon 4 of the accepting glucosyl residue. Glycogen synthase only make liner α(1→4) glycosidic bond. While, another enzyme branching enzyme (amylo-α(1→4) → α(1→6)-transglucosidase) responsible to α(1→6) glycosidic bond. Branching increases the number of nonreducing ends to which new glucosyl residues can be added. Eventually, it increases the glycogen synthesis and degradation. It also increases the size of glycogen molecule.