Glycogenin

Glycogenin is an enzyme involved in glycogen biosynthesis. This enzyme was discovered by Dr. William J. Whelan, a fellow of the Royal Society of London. It is a homodimer of 37-kd subunits and is classified as a glycosyltransferase.

Function
The main enzyme involved in glycogen polymerisation, glycogen synthase, can only add to an existing chain of at least 8 glucose residues. Glycogenin acts as the primer, to which further glucose monomers may be added. It achieves this by catalyzing the addition of glucose to itself by first binding glucose from UDP-glucose to the hydroxyl group of Tyr-194. Seven more glucoses can be added, each derived from UDP-glucose, by glycogenin's glucosyltransferase activity. Once sufficient residues have been added, glycogen synthase takes over extending the chain. Glycogenin remains covalently attached to the reducing end of the glycogen molecule.