UDP glycogen is a activated form of glucose. At the time of glycogen synthesis, glucose is converted into its activate form, UDP-glucose. This conversion involves three reactions. First, free glucose is phosphorylated at the expense of an ATP molecule in the subsequent reaction. It is isomerised to glucose-1 phosphate which is then converted into UDP-glucose. The energetic form, UDP glucose is the donor of glucose in glycogen synthesis. In the following step, UDP glucose molecule do not polymerise with each other instead, they are sequentially added to a fragment of glycogen which is a polyglucose or a branch monomer consisting of at least four monomeric units.
This fragment of glycogen is called a primer. The glucose molecule can get attached to the protein only by the amino acids containing hydroxyl group. The addition of the initial molecules of glucose from UDP glucose to the hydroxyl group of amino acid of the protein is called the initiation reaction. So UDP glucose play a important role in glycogen synthesis. UDP glucose is Also used up in the elongation of the chain. More and more molecules of glucose are sequentially transferred from UDP — glucose → of the first glucose molecule.