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Glycogen

Glucose

Glucose-6-phosphate

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]

Mechanism

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

glycogen_{(n residues)} + P_i 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).

References

^ "Glycogenesis".
^ Lodish; et al. (2007). Molecular Cell Biology (6th ed.). W. H. Freeman and Company. p. 658. ISBN 1429203145.

External links

The chemical logic of glycogen degradation at ufp.pt
Glycogenolysis at the US National Library of Medicine Medical Subject Headings (MeSH)

UpToDate Contents

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1. 健常被験者および糖尿病患者における低血糖に対する生理的応答 physiologic response to hypoglycemia in normal subjects and patients with diabetes mellitus
2. 幼児および小児における低血糖症の病因 etiology of hypoglycemia in infants and children
3. インスリン作用 insulin action
4. 先天性代謝異常：個々の障害の発見 inborn errors of metabolism identifying the specific disorder
5. 代謝性ミオパチーへのアプローチ approach to the metabolic myopathies

English Journal

Metabolic functions of glucocorticoid receptor in skeletal muscle.

Kuo T, Harris CA, Wang JC.SourceDepartment of Nutritional Science & Toxicology, University of California at Berkeley, Berkeley, CA 94720, United States; Graduate Program of Endocrinology, University of California at Berkeley, Berkeley, CA 94720, United States.
Molecular and cellular endocrinology.Mol Cell Endocrinol.2013 Nov 5;380(1-2):79-88. doi: 10.1016/j.mce.2013.03.003. Epub 2013 Mar 21.
Glucocorticoids (GCs) exert key metabolic influences on skeletal muscle. GCs increase protein degradation and decrease protein synthesis. The released amino acids are mobilized from skeletal muscle to liver, where they serve as substrates for hepatic gluconeogenesis. This metabolic response is criti
PMID 23523565

Effects of food-deprivation and refeeding on the regulation and sources of blood glucose appearance in European seabass (Dicentrarchus labrax L.).

Viegas I, Rito J, González JD, Jarak I, Carvalho RA, Metón I, Pardal MA, Baanante IV, Jones JG.SourceCNC-Center for Neuroscience and Cell Biology, University of Coimbra, 3004-517 Coimbra, Portugal; CFE-Center for Functional Ecology, University of Coimbra, Apartado 3046, 3001-401 Coimbra, Portugal; Department of Life Sciences, Faculty of Sciences and Technology, University of Coimbra, 3001-401 Coimbra, Portugal. Electronic address: iviegas@ci.uc.pt.
Comparative biochemistry and physiology. Part A, Molecular & integrative physiology.Comp Biochem Physiol A Mol Integr Physiol.2013 Nov;166(3):399-405. doi: 10.1016/j.cbpa.2013.07.013. Epub 2013 Jul 17.
Sources of blood glucose in European seabass (initial weight 218.0±43.0g; mean±S.D., n=18) were quantified by supplementing seawater with deuterated water (5%-(2)H2O) for 72h and analyzing blood glucose (2)H-enrichments by (2)H NMR. Three different nutritional states were studied: continuously fed
PMID 23871878

Ghrelin: much more than a hunger hormone.

Pradhan G, Samson SL, Sun Y.SourceaUSDA/ARS Children's Nutrition Research Center, Department of Pediatrics bDivision of Diabetes, Endocrinology, and Metabolism, Department of Medicine cHuffington Center on Aging, Department of Molecular and Cellular Biology, Baylor College of Medicine, Houston, Texas, USA.
Current opinion in clinical nutrition and metabolic care.Curr Opin Clin Nutr Metab Care.2013 Nov;16(6):619-24. doi: 10.1097/MCO.0b013e328365b9be.
PURPOSE OF REVIEW: Ghrelin is a multifaceted gut hormone that activates its receptor, growth hormone secretagogue receptor (GHS-R). Ghrelin's hallmark functions are its stimulatory effects on growth hormone release, food intake and fat deposition. Ghrelin is famously known as the 'hunger hormone'. H
PMID 24100676

Japanese Journal

Uterine Glycogen Metabolism in Mink during Estrus, Embryonic Diapause and Pregnancy

DEAN Matthew,HUNT Jason,MCDOUGALL Lisa,ROSE Jack
Journal of Reproduction and Development, 2014
… While glycogen phosphorylase protein was detected during estrus and diapause, glycogen synthase was almost undetectable after estrus, which probably contributed to a higher glycogenolysis/glycogenesis ratio during diapause. …
NAID 130004687240

福山型先天性筋ジストロフィーにおける糖代謝

小峯聡,石井かやの,池谷紀代子 [他],齋藤加代子,大澤眞木子
東京女子医科大学雑誌 83(E1), E25-E35, 2013-01-31
福山型先天性筋ジストロフィー (FCMD)は、日本では、小児期の筋ジストロフィーの中で2番目に最も多く見られる。FCMDには、長年の診療の中で本症患児の病状を脅かす問題点が多々認められている。我々は、FCMDの患者にケトン性低血糖にしばしば遭遇する。ケトン性低血糖は一般的に幼児期の健常児に認められるが、その機序は、FCMDでは明確にされていない。この研究は、FCMDにおける低血糖の機序を明らかにす …
NAID 110009559429

Effects of celiac superior mesenteric ganglionectomy on glucose homeostasis and hormonal changes during oral glucose tolerance testing in rats

Kumakura Atsushi,Shikuma Junpei,Ogihara Norikazu,Eiki Jun-ichi,Kanazawa Masao,Notoya Yoko,Kikuchi Masatoshi,Odawara Masato
Endocrine Journal 60(4), 525-531, 2013
… In contrast, the liver produces glucose by glycogenolysis and gluconeogenesis while fasting. … Glycogenesis and glycogenolysis are regulated by various mechanisms including hormones, the sympathetic and parasympathetic nervous systems and the hepatic glucose content. …
NAID 130004443868