Glycogen Biosynthesis

A major fate of glucose in animals is the synthesis of glycogen. Figure 16.8 schematically depicts the steps by which glucose is incorporated into a glycogen chain. In addition to the steps shown here, a process that introduces branches in the chain is an important part of glycogen synthesis and is shown in Figure 16.10. Apart from hexokinase, which is a glycolytic enzyme, glycogen biosynthesis employs only four other enzymes. These are summarized as follows:

> Phosphoglucomutase - catalyzes the interconversion of glucose-1-phosphate (G1P) and glucose-6-phosphate (G6P).

> > G1P <=> G6P

> This enzyme is the common link between glycogen biosynthesis and glycogen breakdown.

> UDP-glucose pyrophosphorylase - catalyzes the transfer of glucose (from G1P) to uridine triphosphate, forming UDP-glucose and pyrophosphate.

> > G1P + UTP <=> UDP-Glucose + PPi

> This has the effect of "activating" glucose for incorporation into the growing glycogen chain in the next step of the process. Furthermore, this reaction is made essentially irreversible by hydrolysis of the pyrophosphate to 2Pi.

> Glycogen Synthase - catalyzes the formation of an (1->4) glycosidic bond between carbon 4 of an existing glycogen chain and carbon 1 of a glucose (from UDP-glucose). UDP is released in the process. Glycogen synthase is the major point of regulation of glycogen biosynthesis. The reaction is depicted in Figure 16.9.

> Amylo-(1,4->1,6)-transglycosylase (branching enzyme) - This enzyme, most commonly called branching enzyme, introduces the abundant (1->6) branches, which are characteristic of glycogen molecules. The reaction transfers a terminal fragment, some 6 or 7 residues long, from a branch terminus at least 11 residues in length to a hydroxyl group at the 6-position of a glucose residue in the interior of the polymer (Figure 16.10). As a result of the transfer, two termini recognized by glycogen synthase are created where only one existed before. This means that the synthesis and the mobilization of glycogen can proced quickly and efficiently, depending on the needs of the cell.

See also: Reciprocal Regulation of Glycogen Biosynthesis and Mobilization, Glycogen Metabolism Diseases, Glycogen Catabolism (from chapter 13), Glycogen, Glucose