Glycogenin

Glycogenin is an enzyme involved in the initiation of glycogen biosynthesis. Because glycogen synthase (the main enzyme involved in glycogen synthesis) can only add to an existing chain, glycogenin acts as the "primer" to begin this mechanism. Glycogenin can catalyze the addition of glucose to itself by binding the first UDP-Glucose to a Tyr-194 residue in its active site. It then continues to add additional glucose residues until it reaches its limit (~8) at which point glycogen synthase takes over. Glycogenin remains bound to the glucose chain.