World Library  
Flag as Inappropriate
Email this Article

Glycogenolysis

Article Id: WHEBN0000353805
Reproduction Date:

Title: Glycogenolysis  
Author: World Heritage Encyclopedia
Language: English
Subject: Carbohydrate metabolism, G protein, Beta-2 adrenergic receptor, Glucose, Adrenergic receptor
Collection: Diabetes, Hepatology, Metabolic Pathways
Publisher: World Heritage Encyclopedia
Publication
Date:
 

Glycogenolysis

Glycogenolysis is the breakdown of glycogen (n) to glucose-6-phosphate and glycogen (n-1). Glycogen branches are catabolized by the sequential removal of glucose monomers via phosphorolysis, by the enzyme glycogen phosphorylase.[1]

Contents

  • Mechanism 1
  • Function 2
  • Regulation 3
  • Clinical significance 4
  • Pathology 5
  • See also 6
  • References 7
  • External links 8

Mechanism

The overall reaction for the breakdown of glycogen to glucose-1-phosphate is:

glycogen(n residues) + Pi glycogen(n-1 residues) + glucose-1-phosphate

Here, 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 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.

Function

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 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 (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.

Regulation

Glycogenolysis is regulated hormonally in response to blood sugar levels by glucagon and insulin, and stimulated by epinephrine during the fight-or-flight response. In myocytes, glycogen degradation may also be stimulated by neural signals.[2]

Clinical significance

Parenteral (intravenous) administration of glucagon is a common human medical intervention in diabetic emergencies when sugar cannot be given orally. It can also be administered intramuscularly.

Pathology

Mutations at the liver enzyme glycogen phosphorylase (GPLL, 847 aa) lead to glycogen storage disease VI (Hers disease). Mutations at the muscle enzyme glycogen phosphorylase (GPMM, 842 aa) lead to McArdle's disease. Deficiencies at the glycogen debranching enzyme lead to glycogen storage disease III (Oxford Dictionary of BioMedicine, 2010).

See also

References

  1. ^ "Glycogenesis". 
  2. ^ Lodish; et al. (2007). Molecular Cell Biology (6th ed.). W. H. Freeman and Company. p. 658.  

External links

  • The chemical logic of glycogen degradation at ufp.pt
  • Glycogenolysis at the US National Library of Medicine Medical Subject Headings (MeSH)
This article was sourced from Creative Commons Attribution-ShareAlike License; additional terms may apply. World Heritage Encyclopedia content is assembled from numerous content providers, Open Access Publishing, and in compliance with The Fair Access to Science and Technology Research Act (FASTR), Wikimedia Foundation, Inc., Public Library of Science, The Encyclopedia of Life, Open Book Publishers (OBP), PubMed, U.S. National Library of Medicine, National Center for Biotechnology Information, U.S. National Library of Medicine, National Institutes of Health (NIH), U.S. Department of Health & Human Services, and USA.gov, which sources content from all federal, state, local, tribal, and territorial government publication portals (.gov, .mil, .edu). Funding for USA.gov and content contributors is made possible from the U.S. Congress, E-Government Act of 2002.
 
Crowd sourced content that is contributed to World Heritage Encyclopedia is peer reviewed and edited by our editorial staff to ensure quality scholarly research articles.
 
By using this site, you agree to the Terms of Use and Privacy Policy. World Heritage Encyclopedia™ is a registered trademark of the World Public Library Association, a non-profit organization.
 


Copyright © World Library Foundation. All rights reserved. eBooks from Project Gutenberg are sponsored by the World Library Foundation,
a 501c(4) Member's Support Non-Profit Organization, and is NOT affiliated with any governmental agency or department.