Glycogen is synthesized in vivo by a pathway different from that of its degradation. Since the direct conversion of G1P to glycogen and Pi is thermodynamically unfavorable under all Pi concentrations, glycogen biosynthesis requires and additional exergonic step. This is accomplisehd by combining G1P with UTP to form uridine diphosphate glucose (UDP-glucose).

Glucose-1-P + UTP + H2O UDP-glucose + 2Pi (catalyzed by UDP-glucose pyrophosphorylase)

UDP-glucose's high energy status permits it to spontaneously donate glucosyl units to the growing glycogen chain.

Two other enzymes are involved in the glycogen synthesis pathway:

-Glycogen synthase - catalyzes the transfer of glucose from UDP-glucose to the growing chain. New glucosyl units are added to the nonreducing terminal residues of glycogen. The activated glucosyl unit of UDP-glucose is transferred to th hydroxyl group at a C-4 terminus of glycogen to form an a-1,4 glycosydic linkage.

Glycogen synthase can add glucosyl residues only if the polysacchride chain already contains more than four residues. Thus, glycogen synthesis requires a primer. This primer function is carried out by glycogenin, that autocatalyzes the addition of some eight glucose units (UDP-glucose is the donor in this autoglycosylation). At this point, glycogen synthase, wich is tightly bond to glycogenin, takes over.

Mos significant, glycogen synthase is catalytically efficient only when is bound to glycogenin. This dependence has two important consequences:

-The number of glycogen granules is determined by the number of molecules of glycogenin.

-Elongation stops when the synthase is no longer in contact with glycogenin, wich forms the core of the particle. The synthase glycogenin interaction limits the size of glycogen granules.

-Glycogen branching enzyme - forms the a-1,6-linkages that make glycogen a branched polymer. Branching is important beacuse it increases the solubility of glycogen. Furthermore, branching creates a large number of terminal residues, the sites of action of glycogen phosphorylase and synthase. Thus, branching increases the rate of glycogen synthesis and degradation.