Glycogen is a
polysaccharideof glucose(Glc) which functions as the secondary short term energy storage in animalcells. It is made primarily by the liverand the muscles, but can also be made by the brain, uterus, and the vagina. [Anatomy and Physiology. Saladin, Kenneth S. McGraw-Hill, 2007.] Glycogen is the analogue of starch, a less branched glucose polymer in plants, and is commonly referred to as animal starch, having a similar structure to amylopectin. Glycogen is found in the form of granules in the cytosolin many cell types, and plays an important role in the glucose cycle. Glycogen forms an energyreserve that can be quickly mobilized to meet a sudden need for glucose, but one that is less compact than the energy reserves of triglycerides(fat). In the liver hepatocytes, glycogen can compose up to 8% of the fresh weight (100–120 g in an adult) soon after a meal. Fact|date=May 2007 Only the glycogen stored in the liver can be made accessible to other organs. In the muscles, glycogen is found in a much lower concentration (1% of the muscle mass), but the total amount exceeds that in liver. Small amounts of glycogen are found in the kidneys, and even smaller amounts in certain glialcells in the brainFact|date=June 2008 and white blood cellsFact|date=June 2008. The uterus also stores glycogen during pregnancy to nourish the embryo.
tructure and biochemistry
Glycogen is a highly branched
polymerthat is better described as a dendrimerof about 60,000 glucose residues and has a molecular weight between 106 and 107 daltons (~4.8 million).Fact|date=May 2007 Most of Glc units are linked by α-1,4 glycosidic bonds, approximately 1 in 12 Glc residues also makes alpha-1,6 glycosidic bond with a second Glc, which results in the creation of a branch. Glycogen does not possess a reducing end: the 'reducing end' glucose residue is not free but is covalently bound to a proteintermed glycogeninas a beta-linkageto a surface tyrosine residue. Glycogeninis a glycosyltransferase and occurs as a dimer in the core of glycogen. The glycogen granules contain both glycogen and the enzymes of glycogen synthesis ( glycogenesis) and degradation ( glycogenolysis). The enzymes are nested between the outer branches of the glycogen molecules and act on the non-reducing ends. Therefore, the many non-reducing end-branches of glycogen facilitate its rapid synthesis and catabolism.
Function and regulation of liver glycogen
As a meal containing
carbohydrates is eaten and digested, blood glucoselevels rise, and the pancreassecretes insulin. Glucose from the hepatic portal veinenters the livercells ( hepatocytes). Insulin acts on the hepatocytes to stimulate the action of several enzymes, including glycogen synthase. Glucose molecules are added to the chains of glycogen as long as both insulin and glucose remain plentiful. In this postprandialor "fed" state, the liver takes in more glucose from the blood than it releases.
After a meal has been digested and glucose levels begin to fall, insulin secretion is reduced, and glycogen synthesis stops. About four hours after a mealFact|date=May 2007, glycogen begins to be broken down and converted again to glucose.
Glycogen phosphorylaseis the primary enzyme of glycogen breakdown. For the next 8–12 hours, glucose derived from liver glycogen will be the primary source of blood glucose to be used by the rest of the body for fuel. Glucagonis another hormone produced by the pancreas, which in many respects serves as a counter-signal to insulin. When the blood sugarbegins to fall below normal, glucagon is secreted in increasing amounts. It stimulates glycogen breakdown into glucose even when insulin levels are abnormally high.
In muscle and other cells
Muscle cell glycogen appears to function as an immediate reserve source of available glucose for muscle cells. Other cells that contain small amounts use it locally as well. Muscle cells lack glucose-6-phosphatase enzyme, so they lack the ability to pass glucose into the blood, so the glycogen they store internally is destined for internal use and is not shared with other cells, unlike liver cells.
Glycogen debt and endurance exercise
Due to the body's inability to hold more than around 2,000 kcal of glycogen,Fact|date=May 2007 long-distance athletes such as marathon runners, cross-country skiers, and
cyclists go into glycogen debt, where almost all of the athlete's glycogen stores are depleted after long periods of exertion without enough energy consumption. This phenomenon is referred to as "hitting the wall". In marathon runners it normally happens around the 20 mile (32 km) point of a marathon, where around 100 kcal are spent per mile,Fact|date=May 2007 depending on the size of the runner and the race course. However, it can be delayed by a carbohydrate loadingbefore the task.
When experiencing glycogen debt, athletes often experience extreme fatigue to the point that it is difficult to move.
A study published in the Journal of Applied Physiology (online
May 8, 2008) suggests that when athletes ingest both carbohydrate and caffeinefollowing exhaustive exercise, their glycogen is replenished more rapidly. [cite journal |Pedersen DJ, Lessard SJ, Coffey VG, Churchley EG, Wootton AM, Ng T, Watt MJ, Hawley JA|year=2008|month=May|title=High rates of muscle glycogen resynthesis after exhaustive exercise when carbohydrate is coingested with caffeine.|journal=J Appl Physiol|year=2008 |date=May 8 |pmid=18467543 |PubMed - as supplied by publisher] [ [http://newswise.com/articles/view/542216/ Post-exercise Caffeine Helps Muscles Refuel] Newswise, Retrieved on July 6, 2008.]
Disorders of glycogen metabolism
The most common disease in which glycogen
metabolismbecomes abnormal is diabetes, in which, because of abnormal amounts of insulin, liver glycogen can be abnormally accumulated or depleted. Restoration of normal glucose metabolism usually normalizes glycogen metabolism as well.
hypoglycemiacaused by excessive insulin, liver glycogen levels are high, but the high insulin level prevents the glycogenolysisnecessary to maintain normal blood sugar levels. Glucagonis a common treatment for this type of hypoglycemia.
Various inborn errors of metabolism are caused by deficiencies of enzymes necessary for glycogen synthesis or breakdown. These are collectively referred to as
glycogen storage diseases.
Glycogen synthesis differs from glycogen breakdown. Unlike breakdown, synthesis is
endergonic, meaning that glycogen is not synthesized without the input of energy. Energy for glycogen synthesis comes from UTP, which reacts with glucose-1-phosphate, forming UDP-glucose, in reaction catalysed by UDP-glucose pyrophosphorylase. Glycogen is synthesized from monomers of UDP-glucoseby the enzyme Glycogen synthase, which progressively lengthens the glycogen chain. As glycogen synthase can only lengthen an existing chain, the protein glycogeninis needed to initiate the synthesis of glycogen.
Glycogen is cleaved from the nonreducing ends of the chain by the enzyme
glycogen phosphorylaseto produce monomers of glucose-1-phosphate that is then converted to Glucose 6-phosphate. A special debranching enzymeis needed to remove the alpha(1-6) branches in branched glycogen and reshape the chain into linear polymer. The G6P monomers produced have three possible fates:
*G6P can continue on the
glycolysispathway and be used as fuel.
*G6P can enter the
pentose phosphate pathwayvia the enzyme Glucose-6-phosphate dehydrogenaseto produce NADPH and 5-carbon sugars.
*In the liver and kidney, G6P can be dephosphorylated back to Glucose by the enzyme
Glucose 6-phosphatase. This is the final step in the gluconeogenesispathway.
it is not gluconeogenesis it is glucogenesis in the breakdown section of glycogen. No amino acids or proteins are being broken down to create this glucose, therefore it is glucogenesis.
* [http://www.histochem.net/protocol%20periodic%20acid%20schiff.htm Glycogen detection using Periodic Acid Schiff Staining]
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Look at other dictionaries:
Glycogen — Gly co*gen, n. [Gr. ? sweet + gen: cf. F. glycog[ e]ne.] (Physiol. Chem.) A white, amorphous, tasteless substance resembling starch, soluble in water to an opalescent fluid. It is found abundantly in the liver of most animals, and in small… … The Collaborative International Dictionary of English
glycogen — glycogen. См. гликоген. (Источник: «Англо русский толковый словарь генетических терминов». Арефьев В.А., Лисовенко Л.А., Москва: Изд во ВНИРО, 1995 г.) … Молекулярная биология и генетика. Толковый словарь.
glycogen — starch like substance found in the liver and animal tissue, 1860, from Fr. glycogène, sugar producer, from Gk. glykys вЂњsweetвЂќ (see GLUCOSE (Cf. glucose)) + Fr. gène (see GEN (Cf. gen)). Coined in 1848 by French physiologist Claude Bernard… … Etymology dictionary
glycogen — [glī′kə jən, glī′kəjen΄] n. [ GLYCO + GEN] a polysaccharide, (C6H10O5) x, produced and stored in animal tissues, esp. in the liver and muscles, and changed into glucose as the body needs it … English World dictionary
Glycogen — Ausschnitt aus einem Glykogenpolymer Das Glykogen (auch Glycogen, tierische Stärke oder Leberstärke) ist ein verzweigtes Polysaccharid (Vielfachzucker), das aus Glucose Einheiten aufgebaut ist. Glykogen dient der kurz bis mittelfristigen… … Deutsch Wikipedia
glycogen — /gluy keuh jeuhn, jen /, n. Biochem. a white, tasteless polysaccharide, (C6H10O5)n, molecularly similar to starch, constituting the principal carbohydrate storage material in animals and occurring chiefly in the liver, in muscle, and in fungi and … Universalium
glycogen — A glucosan of high molecular weight, resembling amylopectin in structure [with α(1,4) linkages] but with even more highly branched [α(1,6) linkages, as well as a small number of α(1,3) linkages], found in most of the tissues of the body,… … Medical dictionary
glycogen — n. a carbohydrate consisting of branched chains of glucose units. Glycogen is the principal form in which carbohydrate is stored in the body: it is the counterpart of starch in plants. Glycogen is stored in the liver and muscles and may be… … The new mediacal dictionary
glycogen — Branched polymer of D glucose (mostly a (1 4) linked, but some a (1 6) at branch points). Size range very variable, up to 105 glucose units. Major short term storage polymer of animal cells, and is particularly abundant in the liver and to a… … Dictionary of molecular biology
glycogen — n. form of carbohydrate stored in body, found especially in liver. ♦ glycogenesis, n. formation of sugar from glycogen. ♦ glycogenic, ♦ glycogenous, a … Dictionary of difficult words