Glycogenolysis is 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.
The overall reaction for the breakdown of glycogen to glucose-1-phosphate is:
- glycogen(n residues) + Pi ⇌ glycogen(n-1 residues) + glucose-1-phosphate
Glucose residues are phosphorolysed from branches of glycogen until four residues before a glucose that is branched with a α[1→6] linkage. Glycogen debranching 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 hydrolysed 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.
Glycogenolysis takes place in the cells of the muscle and liver tissues in response to hormonal and neural signals. In particular, glycogenolysis plays an important role in the fight-or-flight response and the regulation of glucose levels in the blood.
In hepatocytes (liver cells), the main purpose of the breakdown of glycogen is for the release of glucose into the bloodstream for uptake by other cells. The phosphate group of glucose-6-phosphate is removed by the enzyme glucose-6-phosphatase, which is not present in myocytes, and the free glucose exits the cell via GLUT2 facilitated diffusion channels in the hepatocyte cell membrane.
Glycogenolysis is regulated hormonally in response to blood sugar levels by glucagon and insulin, and stimulated by epinephrine during the fight-or-flight response. Insulin potently inhibits glycogenolysis.
In myocytes, glycogen degradation may also be stimulated by neural signals.
- DL Nelson & MM Cox (2008). Lehninger principles of biochemistry (5th ed.). New York: W.H. Freeman. pp. 595-596. ISBN 071677108X. OCLC 191854286.
- Sargsyan A, Herman MA (2019). "Regulation of Glucose Production in the Pathogenesis of Type 2 Diabetes". Current Diabetes Reports. 19 (9): 77. doi:10.1007/s11892-019-1195-5. PMC 6834297. PMID 31377934.
- Lodish; et al. (2007). Molecular Cell Biology (6th ed.). W. H. Freeman and Company. p. 658. ISBN 1429203145.
|Wikimedia Commons has media related to Glycogenolysis.|
|Scholia has a topic profile for Glycogenolysis.|
- The chemical logic of glycogen degradation at ufp.pt
- Glycogenolysis at the US National Library of Medicine Medical Subject Headings (MeSH)