Glycogenesis

 Glycogenesis

Definition

Glycogenesis is the process of the formation of Glycogen from glucose.

Formation and mobilization of glycogen

Glycogen is formed in the liver and muscles. When blood sugar tends to fall, Glycogen is converted into glucose and mobilized in the blood stream. Thus blood glucose level is maintained. When extra energy is needed, like in starvation, muscular exercise, cold conditions, liver glycogen is mobilized.

This action is helped by certain hormones such as adrenaline (epinephrine), glucagon, thyroxin, growth or somatotrophic hormone (STH) of anterior pituitary, etc. Stimulation of the sympathetic has same function. It is antagonized by insulin. Insulin helps Glycogenesis in liver and prevents Glycogenolysis.

Steps of Glycogenesis

1. Glucose is converted into glucose 6-phosphate by the action of glucokinase or hexokinase with conversion of ATP to ADP.
2. Glucose-6-phosphate is converted into glucose-1-phosphate by the action of phosphoglucomutase, passing through the obligatory intermediateglucose-1,6-bisphosphate.
3. Glucose-1-phosphate is converted into UDP-glucose by the action of the enzyme UDP-glucose pyrophosphorylase. Pyrophosphate is formed, which is later hydrolyzed by pyrophosphatase into two phosphate molecules.
4. The enzyme glycogenin  creates initial short glycogen chains, which are then lengthened and branched by the other enzymes of Glycogenesis. Glycogenin, a homodimer, has a tyrosine residue on each subunit that serves as the anchor for the reducing end of glycogen. Initially, about seven UDP-glucose molecules are added to each tyrosine residue by glycogenin, forming α(1→4) bonds.
5. Once a chain of seven glucose monomers is formed, glycogen synthase binds to the growing glycogen chain and adds UDP-glucose to the 4-hydroxyl group of the glucosyl residue on the non-reducing end of the glycogen chain, forming more α (1→4) bonds in the process.
6. Branches are made by glycogen branching enzyme (also known as amylo-α(1:4)→α(1:6)transglycosylase), which transfers the end of the chain onto an earlier part via α-1:6 glycosidic bond, forming branches, which further grow by addition of more α-1:4 glycosidic units.