Glycogenesis is the process by which glucose is stored in the form of glycogen, mainly in the liver and muscle. The glycogen stored in the liver is used for glucose homeostasis, while that stored in muscle is mainly for its own use. The process of glycogenesis occurs in the cytosol and requires ATP. It is induced by insulin and inhibited by glucagon and adrenaline/epinephrine.
The process of glycogenesis involves four stages:
- synthesis of UDP-glucose,
- synthesis of a primer,
- chain elongation,
- and branching.
Synthesis of UDP-glucose
UDP-glucose is the "activated" form of glucose that can be used for glycogenesis.
Primer Synthesis
Glycogen synthase cannot initiate chain synthesis, like DNA polymerase. It needs a primer that is synthesized by glycogenin. Glycogenin is a protein that can covalently bind to the anomeric carbon of a UDP-glucose molecule (UDP is released). The bond is formed with a tyrosine residue of glycogenin. Glycogenin exists as a dimer, and each can glucosylate the other (autoglucosylation). Glycogenin can add about four glucosyl units in this way to form a primer. An existing chain of glycogen can also act as a primer.
Chain Elongation
Glycogen synthase adds UDP-glucose to the non-reducing end of the chain. Glucosyl units are joined by α (1→4) linkages. UDP is released.
Branching
There is branching, on average, every eight glucosyl units apart. The action is done by the branching enzyme. The enzyme breaks off six to eight units from the non-reducing end and adds it to the middle of the chain via an α (1→6) linkage. There are now two chains available for elongation.
In summary, glycogenesis is a complex process that occurs in the liver and muscle and is responsible for the storage of glucose as glycogen. It involves four stages: synthesis of UDP-glucose, synthesis of a primer, chain elongation, and branching. Understanding the process of glycogenesis can provide insight into the regulation of glucose homeostasis in the body.
