Glycogenesis pathway is the process by which glycogen is synthesized and glucose molecules are added to the glycogen chains for storage. The liver is typically involved in the process. When blood glucose levels are relatively high, the peptide hormone insulin can trigger glycogenesis.
Glycogenesis metabolism primarily occurs in the liver and muscle tissue. In the liver, glycogen is stored and acts as a reserve for glucose, which can be released into the bloodstream as needed to maintain blood sugar levels. In muscle tissue, glycogen is stored and used as a source of energy for muscle contractions during physical activity.
Figure 1: Glycogenesis Location
There are several important enzymes involved in the process of glycogenesis pathway. These include: Glucokinase, Phosphoglucomutase, UDP-glucose, pyrophosphorylase, Glycogen synthase, Glycogen branching enzyme. All the above enzymes work in concert to convert glucose into glycogen, a storage form of glucose in the body.
1. Synthesis of UDP-glucose: The enzymes hexokinase (in muscle) and glucokinase (in liver) convert glucose to glucose 6-phosphate. Phosphoglucomutase catalyses the conversion of glucose 6-phosphate to glucose 1-phosphate. Uridine diphosphate glucose (UDPG) is synthesized from glucose 1-phosphate and UTP by UDP-glucose pyrophosphorylase.
Figure 2: Glycogenesis Pathway
2. Primer to initiate glycogenesis: A small fragment of pre-existing glycogen must act as a ‘primer’ to initiate glycogen synthesis. In the absence of a glycogen primer, a specific protein known as ‘glycogenin’ can accept glucose from UDPG. The hydroxyl group of the amino acid tyrosine of glycogenin is the site at which the initial glucose unit is attached. The glycogen initiator synthase enzyme converts the first molecule of glucose to glycogenin. The glycogenin then takes up a few glucose residues to form a primer fragment that acts as an acceptor for the remaining glucose molecules.
3. Glycogen by glycogen synthase: The production of 1,4-glycosidic bonds is mediated by glycogen synthase. This enzyme transfers glucose from UDP-glucose to the non-reducible end of glycogen in order to generate D1,4 links.
4. Formation of branches in glycogen: Glycogen synthase can catalyse the synthesis of a linear unbranched molecule with 1,4 α- glycosidic linkages. Glycogen, however, is a branched tree-like structure. The action of a branching enzyme, glucosyl D-4-6 transferase, causes the formation of branches. This enzyme transfers a fragment of five to eight glucose residues from the non-reducing end of the glycogen chain (by breaking D-1,4 links) to another glucose residue where it is connected by a D-1,6 bond. This makes a new non-reducing end along with the one that was already there. The enzymes glycogen synthase and glucosyl 4-6 transferase make glycogen even longer and more branched.
Glycogenesis pathway is controlled by hormones. The varied phosphorylation of glycogen synthase and glycogen phosphorylase is one of the primary forms of regulation. This is controlled by enzymes, which are controlled by hormonal activity, which is controlled by a variety of factors. As a result, when compared to allosteric regulatory systems, there are numerous possible effectors.
FAQs on Glycogenesis Pathway
Glycogenesis metabolism is the mechanism by which the body stores glucose in the liver and muscle tissue as glycogen.
Glycogenesis metabolism primarily occurs in the liver and muscle tissue.
Glycogenesis metabolism happens when there is too much glucose in the blood. Insulin makes it easier for liver and muscle cells to take in glucose and turns on the enzyme glycogen.
Insulin hormone stimulates glycogenesis. The pancreas secretes insulin in response to increased blood glucose levels.