glycogen
American Heritage Dictionary:
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gly·co·gen
(glī'kə-jən) 
n.
A polysaccharide, (C6H10O5)n, that is the main form of carbohydrate storage in animals and occurs primarily in the liver and muscle tissue. It is readily converted to glucose as needed by the body to satisfy its energy needs. Also called animal starch.
glycogenic gly'co·gen'ic (-jĕn'ĭk) adj.
n.
A polysaccharide, (C6H10O5)n, that is the main form of carbohydrate storage in animals and occurs primarily in the liver and muscle tissue. It is readily converted to glucose as needed by the body to satisfy its energy needs. Also called animal starch.
glycogenic gly'co·gen'ic (-jĕn'ĭk) adj.
Britannica Concise Encyclopedia:
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glycogen
Principal storage carbohydrate of animals, occurring primarily in the liver and resting muscles. It is also found in various bacteria, fungi, and yeasts. Glycogen is a branched polysaccharide, a long chain of glucose units, into which it is broken down when energy is needed.
For more information on glycogen, visit Britannica.com.
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The primary reserve polysaccharide of the animal kingdom. It is found in the muscles and livers of all higher animals, as well as in the cells of lower animals. Because of its close relationship to starch, it is often called animal starch, although glycogen is found in some lower plants, fungi, yeast, and bacteria. See also Starch.
Glycogen is a nonreducing, white, amorphous polysaccharide which dissolves readily in cold water, forming an opalescent, colloidal solution. The molecular weight of glycogen is usually very high, and it varies with the source and the method of preparation; molecular weights of the order of 1−20 × 106 have been reported. Chemical studies show glycogen to possess a branched structure similar to the amylopectin starch fraction.
In its biochemical reactions, glycogen is similar to starch. It is attacked by the same plant amylases that attack starch, and like starch, it is degraded to maltose and dextrins. Both glycogen and starch are broken down by animal or plant phosphorylase enzyme in the presence of inorganic phosphate with the production of α-D-glucose-1-phosphate. See also Carbohydrate metabolism.
The metabolic formation of glycogen from glucose in the liver is frequently termed glycogenesis. In fasted animals, glycogen formation can be induced by the feeding, not only of materials that can be hydrolyzed to glucose and other monosaccharides, such as fructose, but also of various other materials. A number of L-amino acids, such as alanine, serine, and glutamic acid, upon deamination in the liver give rise to substances, such as pyruvic acid and α-ketoglutaric acid, that can be converted in the liver to glucose units which are subsequently converted to glycogen. Furthermore, substances such as glycerol derived from fats, dihydroxyacetone, or lactic acid can all be utilized for glycogen synthesis in the liver. Such noncarbohydrate precursors are termed glycogenic compounds. The process of glycogen formation from these precursors is known as glyconeogenesis. The term glycogenolysis is used to connote glycogen breakdown. See also Polysaccharide.
Oxford Food & Nutrition Dictionary:
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glycogen
The storage carbohydrate in the liver and muscles, a branched polymer of glucose units. It has a similar structure to the amylopectin form of starch, but is more highly branched. In an adult there are about 250 g of glycogen in the muscles and 100 g in the liver in the fed state.
Since glycogen is rapidly broken down to glucose after an animal is killed, meat and animal liver do not contain glycogen.
Oxford Food & Fitness Dictionary:
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glycogen
animal starch
The main carbohydrate store in the body; the liver and muscles are the main storage sites. Glycogen is a complex carbohydrate (polysaccharide) made of glucose units that are released when it is digested or metabolized. It is the major energy source for muscles. Compared with other fuels, proportionately more glycogen is used during power activities (e.g. weight lifting, sprinting, and jumping) than during endurance activities. The muscle stores can be loaded above their normal capacity by carbohydrate loading (see separate entry), sometimes known as glycogen supercompensation or glycogen overshoot.
The first scientific evidence that low muscle glycogen levels cause fatigue were provided by studies of cyclists from the volunteer Stockholm firemen. They pedalled until they were exhausted, at which time their muscle glycogen levels had fallen to zero.
Oxford Companion to the Body:
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glycogen
In the 1840s and 50s, Claude Bernard was applying his great scientific mind to the problem of ‘sugars’ in the body, in particular how the liver could apparently make sugars and ‘squirt them into the blood … in a regulated manner’ when he had fed an animal only on protein. In 1855 he coined the term ‘matière glycogene’ — sugar-making material. He removed a liver, washed it out with water, and found that there was still sugar in a subsequent wash-out. He concluded that the sugar-forming substance was stored in the liver, and was not water soluble. Eventually he succeeded in isolating the ‘emulsive material of the liver’, found it to be similar to starch, and listed its properties in an account so complete as to be valid to this day. It was to be over 70 years before the medical significance of glycogen storage came to light, when defective storage in liver, kidney, and heart became recognized. Another 70 years on, and the several associated diseases are well understood, mainly as enzyme deficiencies, whilst deliberate boosting of glycogen storage in muscle before a marathon run is common knowledge.
Glycogen is the form in which carbohydrate is stored in the body. Each molecule of glycogen is formed by the linkage in branching chains of many thousands of glucose molecules. Thus, glycogen is a natural polymer, a polysaccharide, which has a similar structure to the starch which is found in plants.
Most tissues of the body are able to store small amounts of glycogen, but the main sites of glycogen storage are the liver and skeletal muscles. In both cases, glycogen is made from glucose within the cells in which it is stored, and the synthetic process is stimulated by the hormone insulin. When glycogen is stored within muscle and liver cells, it retains water along with it (approximately 3 g of water for each gram of glycogen), so changes in glycogen content can cause quite noticeable changes in total body weight. For example, in the first few days of starvation, glycogen is used by the liver to maintain blood sugar and by muscle metabolism, and the associated water is excreted from the body in the urine, accounting for a major part of 1-2 kg loss of weight.
There are important differences between the major functions of liver and muscle glycogen. The main role of liver glycogen is to provide a reserve supply of glucose in order that blood glucose concentration can be kept at an adequate level to supply the brain (which does not use other fuels) during periods of fasting, or when glucose use is increased during physical work and exercise. Thus, after meals some of the carbohydrate consumed is stored as liver glycogen, and during fasting (even just overnight) this glycogen is broken down and the glucose is released into the blood. In a healthy adult, the liver glycogen store is usually between 50 and 100 g, containing enough glucose to satisfy the brain's requirements for up to 24 hours.
A subject exercised one leg only to exhaustion. Specimens of the quadriceps muscle were taken from both legs immediately after the exercise (by 'needle biopsy'). The figure shows the microscopic appearance of the muscle, magnified × about 25 000; (a) from the leg which was not exercised; normal glycogen content is seen as granules (black dots) between the muscle fibres; (b) from the leg which was exercised; the glycogen granules are depleted. Reproduced with permission from E. Hultman
The main role of muscle glycogen is to provide fuel for the muscle's own contraction during exercise. In fact muscle glycogen cannot be broken down to glucose and so cannot be used to raise blood glucose concentration directly. However, in some circumstances, when their metabolism is partly anaerobic, skeletal muscles produce lactic acid from glycogen. When this lactic acid passes into the blood it is taken up by the liver, where it is converted into glucose; thus it can be used indirectly to raise blood glucose. The major stimulus causing the breakdown of muscle glycogen is contraction of the muscles. Thus, the onset of exercise is accompanied by the initiation of glycogen breakdown. The extent to which the muscles continue to use their glycogen store depends on the intensity of the exercise. With low intensity exercise (such as slow walking, cycling, or swimming) the muscles do not use much glycogen as they are able to take up fat from the blood as a source of energy for contraction. However, with higher intensity exercise (jogging, brisk uphill walking, running) the muscles need to use glycogen or glucose from the blood to support the higher rate of energy expenditure (see figure). A well-nourished person will have enough glycogen in their muscle to enable them to exercise for 1-2 hours at approximately two-thirds of their maximum capacity for aerobic exercise. However if people consume a very high carbohydrate diet, especially for at least three days after first depleting their muscle glycogen levels, it is possible to double this normal glycogen content, ensuring that a longer period of exercise can be sustained before it is used up. This is known as carbohydrate loading, or glycogen supercompensation, and is often used by distance — especially marathon — runners before an important race.
— I. A. Macdonald
See also blood sugar; exercise; metabolism; muscle.
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A highly branched polysaccharide consisting of alpha glucose units. The liver stores most of the glycogen in the body. Liver glycogen is readily hydrolysed to glucose to maintain blood sugar. See also muscle glycogen.
Columbia Encyclopedia:
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glycogen
glycogen (glī'kəjən), starchlike polysaccharide (see carbohydrate) that is found in the liver and muscles of humans and the higher animals and in the cells of the lower animals. Chemically it is a highly branched condensation polymer of glucose; it is readily hydrolyzed to glucose. Glycogen is formed by the liver from glucose in the bloodstream and is stored in the liver; conversion of glucose to glycogen (glycogenesis) and hydrolysis of glycogen to glucose (glycogenolysis) together are the usual mechanism for maintenance of normal levels of blood sugar. Glycogen is also produced by and stored in muscle cells; during short periods of strenuous activity, energy is released in the muscles by direct conversion of glycogen to lactic acid. During normal activity, energy is released by metabolic oxidation of glucose to lactic acid.
Oxford Dictionary of Biochemistry:
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glycogenic
- of, relating to, or involving glycogen.
- of or relating to the formation of sugar (i.e. glucose), especially in animals; glucogenic. (Note: the term is often used confusingly in the sense of glycogenetic.)
| glycogenetic, glycogenesis, glycogenase | |
| glycogenin, glycogenolysis, glycogenosis |
Saunders Veterinary Dictionary:
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glycogenic
Pertaining to, characterized by, or promoting glycogenesis; pertaining to glycogen.
Mosby's Dental Dictionary:
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glycogen
(glī′kōjen)
n
n
A branched, homopolysaccharide of glucose held by α 1-4 and α 1-6 glucosidic bonds. Liver glycogen provides a ready source of blood glucose through glycogenosis.
Random House Word Menu:
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categories related to 'glycogen'
- Physiology - glycogen: white polysaccharide sugar, derived from glucose, that is principal form in which carbohydrate is stored in tissue
Wikipedia on Answers.com:
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Glycogen
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This article needs additional citations for verification. Please help improve this article by adding citations to reliable sources. Unsourced material may be challenged and removed. (December 2008) |
Glycogen is a molecule that serves as the secondary long-term energy storage in animal and fungal cells, with the primary energy stores being held in adipose tissue. Glycogen is made primarily by the liver and the muscles, but can also be made by glycogenesis within the brain and stomach.[2]
Glycogen is the analogue of starch, a glucose polymer in plants, and is sometimes referred to as animal starch, having a similar structure to amylopectin but more extensively branched and compact than starch. Glycogen is a polymer of α(1→4) glycosidic bonds linked, with α(1→6)-linked branches. Glycogen is found in the form of granules in the cytosol/cytoplasm in many cell types, and plays an important role in the glucose cycle. Glycogen forms an energy reserve that can be quickly mobilized to meet a sudden need for glucose, but one that is less compact than the energy reserves of triglycerides (lipids).
In the liver hepatocytes, glycogen can compose up to eight percent of the fresh weight (100–120 g in an adult) soon after a meal.[3] Only the glycogen stored in the liver can be made accessible to other organs. In the muscles, glycogen is found in a low concentration (one to two percent of the muscle mass). The amount of glycogen stored in the body—especially within the muscles, liver, and red blood cells[4][5][6]—mostly depends on physical training, basal metabolic rate, and eating habits such as intermittent fasting. Small amounts of glycogen are found in the kidneys, and even smaller amounts in certain glial cells in the brain and white blood cells. The uterus also stores glycogen during pregnancy to nourish the embryo.[7]
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Function and regulation of liver glycogen
As a meal containing carbohydrates is eaten and digested, blood glucose levels rise, and the pancreas secretes insulin. Glucose from the portal vein enters liver cells (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 postprandial or "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. When it is needed for energy, glycogen is broken down and converted again to glucose. Glycogen phosphorylase is 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.
Glucagon is another hormone produced by the pancreas, which in many respects serves as a counter-signal to insulin. In response to insulin level below normal (when blood levels of glucose begin to fall below the normal range), glucagon is secreted in increasing amounts to stimulate glycogenolysis and gluconeogenesis pathways.
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 the enzyme glucose-6-phosphatase, which is required to pass glucose into the blood, so the glycogen they store is destined for internal use and is not shared with other cells. (This is in contrast to liver cells, which, on demand, readily do break down their stored glycogen into glucose and send it through the blood stream as fuel for the brain or muscles). Glycogen is also a suitable storage substance due to its insolubility in water, which means it does not affect the osmotistic levels and pressure of a cell.
Glycogen depletion and endurance exercise
Long-distance athletes such as marathon runners, cross-country skiers, and cyclists often experience glycogen depletion, 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, depending on the size of the runner and the race course.[citation needed]
Glycogen depletion can be forestalled in four possible ways. First, during exercise carbohydrates with the highest possible rate of conversion to blood glucose per time (high glycemic Index) are ingested continuously. The best possible outcome of this strategy replaces about 35% of glucose consumed at heart rates above about 80% of maximum. Second, through training, the body can be conditioned to burn fat earlier, faster, and more efficiently[citation needed], sparing carbohydrate use from all sources. Third, by consuming foods low on the glycemic Index for 12–18 hours before the event, the liver and muscles will store the resulting slow but steady stream of glucose as glycogen, instead of fat. This process is known as carbohydrate loading.
When experiencing glycogen debt, athletes often experience extreme fatigue to the point that it is difficult to move. As a reference, the very best professional cyclists in the world will usually finish a 4-5hr stage race right at the limit of glycogen depletion using the first 3 strategies.
A study published in the Journal of Applied Physiology (online May 8, 2008) suggests that, when athletes ingest both carbohydrate and caffeine following exhaustive exercise, their glycogen is replenished more rapidly.[8][9]
Disorders of glycogen metabolism
The most common disease in which glycogen metabolism becomes 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.
In hypoglycemia caused by excessive insulin, liver glycogen levels are high, but the high insulin level prevents the glycogenolysis necessary to maintain normal blood sugar levels. Glucagon is 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.
Synthesis
Main article: Glycogenesis
Glycogen synthesis is, unlike its breakdown, endergonic. This means that glycogen synthesis requires the input of energy. Energy for glycogen synthesis comes from UTP, which reacts with glucose-1-phosphate, forming UDP-glucose, in a reaction catalysed by UDP-glucose pyrophosphorylase. Glycogen is synthesized from monomers of UDP-glucose by the enzyme glycogen synthase, which progressively lengthens the glycogen chain with (α1→4) bonded glucose. As glycogen synthase can lengthen only an existing chain, the protein glycogenin is needed to initiate the synthesis of glycogen. The glycogen-branching enzyme, amylo (α1→4) to (α1→6) transglycosylase, catalyzes the transfer of a terminal fragment of 6-7 glucose residues from a nonreducing end to the C-6 hydroxyl group of a glucose residue deeper into the interior of the glycogen molecule. The branching enzyme can act upon only a branch having at least 11 residues, and the enzyme may transfer to the same glucose chain or adjacent glucose chains.
Breakdown
Main article: Glycogenolysis
Glycogen is cleaved from the nonreducing ends of the chain by the enzyme glycogen phosphorylase to produce monomers of glucose-1-phosphate, which is then converted to glucose 6-phosphate by phosphoglucomutase. A special debranching enzyme is 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 glycolysis pathway and be used as fuel.
- G6P can enter the pentose phosphate pathway via the enzyme Glucose-6-phosphate dehydrogenase to 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 gluconeogenesis pathway.
See also
References
- ^ Page 12 in: Exercise physiology: energy, nutrition, and human performance By William D. McArdle, Frank I. Katch, Victor L. Katch Edition: 6, illustrated Published by Lippincott Williams & Wilkins, 2006 ISBN 0-7817-4990-5, ISBN 978-0-7817-4990-9, 1068 pages
- ^ Anatomy and Physiology. Saladin, Kenneth S. McGraw-Hill, 2007.
- ^ Campbell, Neil A.; Brad Williamson; Robin J. Heyden (2006). Biology: Exploring Life. Boston, Massachusetts: Pearson Prentice Hall. ISBN 0-13-250882-6. http://www.phschool.com/el_marketing.html.
- ^ Moses SW, Bashan N, Gutman A (December 1972). "Glycogen metabolism in the normal red blood cell". Blood 40 (6): 836–43. PMID 5083874. http://www.bloodjournal.org/cgi/pmidlookup?view=long&pmid=5083874.
- ^ http://jeb.biologists.org/cgi/reprint/129/1/141.pdf
- ^ Miwa I, Suzuki S (November 2002). "An improved quantitative assay of glycogen in erythrocytes". Annals of Clinical Biochemistry 39 (Pt 6): 612–3. doi:10.1258/000456302760413432. PMID 12564847.
- ^ Campbell, Neil A.; Brad Williamson; Robin J. Heyden (2006). Biology: Exploring Life. Boston, Massachusetts: Pearson Prentice Hall. ISBN 0-13-250882-6. http://www.phschool.com/el_marketing.html.
- ^ Pedersen DJ, Lessard SJ, Coffey VG, et al. (July 2008). "High rates of muscle glycogen resynthesis after exhaustive exercise when carbohydrate is coingested with caffeine". Journal of Applied Physiology 105 (1): 7–13. doi:10.1152/japplphysiol.01121.2007. PMID 18467543.
- ^ Post-exercise Caffeine Helps Muscles Refuel Newswise, Retrieved on July 6, 2008.
External links
- Glycogen detection using Periodic Acid Schiff Staining
- Glycogen storage disease - McArdle's Disease Website
- MeSH Glycogen
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