Glycogen is the major storage form of carbohydrate in animals similar to starch in plants.
It is a homopolymer made up of repeated units of α- D glucose and each molecule is linked to another by 1→4 glycosidic bond which is a link connecting the 1st C atom of the active glucose residue to the 6th C atom of the approaching glucose molecule.
Once there is a chain consisting of 8 to 10 glycosidic residues in the glycogen fragment, branching begins by 1→6 linkages.
What is Glycogenolysis?
Glycogenolysis, the process by which glycogen, the primary carbohydrate stored in the liver and muscle cells of animals, is broken down into glucose to provide immediate energy or to maintain blood glucose levels during times of need.
Glycogenolysis is thus the breakdown of glycogen (n) to glucose-1-phosphate and glycogen (n-1).
Glycogen branches are catabolized by the sequential removal of glucose monomers via phosphorolysis, by the enzyme glycogen phosphorylase.
Glycogenolysis takes place in the cytoplasm of cells in the muscle, liver, and adipose tissue.
Result: Glucose-1-phosphate is released from the non-reducing ends of glycogen chains.
- Glycogen phosphorylase cleaves the bond linking a terminal glucose residue to a glycogen branch by substitution of a phosphoryl group for the α-[1→4] linkage.
- Glucose-1-phosphate is converted to glucose-6-phosphate by the enzyme phosphoglucomutase.
- Glucose residues are phosphorolysed from branches of glycogen until four residues before a glucose that is branched with a α-[1→6] linkage.
- Glycogen de-branching enzyme then transfers three of the remaining four glucose units to the end of another glycogen branch.
- This exposes the α-[1→6] branching point, which is hydrolyzed by α[1→6] glucosidase, removing the final glucose residue of the branch as a molecule of glucose and eliminating the branch.
- This is the only case in which a glycogen metabolite is not glucose-1-phosphate. The glucose is subsequently phosphorylated to glucose-6-phosphate by hexokinase.
- Glycogen phosphorylase breaks α-1,4 linkages and de-branching enzyme breaks α-1,6 linkages to release single units of glucose-1- phosphate.
- Phosphoglucomutase converts glucose- 1-phosphate to glucose-6-phosphate, which is then shuttled into the glycolytic pathway.
Glucagon (liver) and epinephrine (liver and muscle) stimulate glycogenolysis via the cAMP protein kinase A phosphorylation cascade, which results in the phosphorylation and thus activation of glycogen phosphorylase.
Insulin inhibits glycogenolysis via dephosphorylation and thus results in the inactivation of glycogen phosphorylase.
- Glycogenolysis plays an important role in the fight-or-flight response.
- It contributes to the regulation of glucose levels in the blood.
- The metabolism of glycogen polymers becomes important during fasting.
- In myocytes (muscle cells), glycogen degradation serves to provide an immediate source of glucose-6-phosphate for glycolysis, to provide energy for muscle contraction.
- In hepatocytes), the main purpose of the breakdown of glycogen is for the release of glucose into the bloodstream for uptake by other cells.
- David Hames and Nigel Hooper (2005). Biochemistry. Third ed. Taylor & Francis Group: New York.
- Lehninger, A. L., Nelson, D. L., & Cox, M. M. (2000). Lehninger principles of biochemistry. New York: Worth Publishers.