Glycogen 168986 224383196 2008-07-08T16:43:08Z 75.92.225.19 /* Glycogen debt and endurance exercise */ [[Image:Glykogen.svg|thumb|right|400px|Glycogen Structure Segment]] '''Glycogen''' is a [[polysaccharide]] of [[glucose]] (Glc) which functions as the primary short term energy storage in [[animal]] cells. It is made primarily by the [[liver]] and the [[muscle]]s, but can also be made by the [[brain]], [[uterus]], and the [[vagina]].<ref>Anatomy and Physiology. Saladin, Kenneth S. McGraw-Hill, 2007.</ref> Glycogen is the analogue of [[starch]], a less branched glucose polymer in [[plant]]s, and is commonly referred to as '''animal starch''', having a similar structure to [[amylopectin]]. Glycogen is found in the form of granules in the [[cytosol]] in many [[cell (biology)|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]] (fat). In the liver [[hepatocyte]]s, glycogen can compose up to 8% of the fresh weight (100–120&nbsp;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 [[muscle]]s, 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 [[kidney]]s, and even smaller amounts in certain [[glial]] cells in the [[brain]]{{Fact|date=June 2008}} and [[white blood cells]]{{Fact|date=June 2008}}. The uterus also stores glycogen during pregnancy to nourish the embryo. ==Structure and biochemistry== [[Image:Glycogen.png|thumb|right|350px|Glycogen structure]] Glycogen is a highly branched [[polymer]] that is better described as a [[dendrimer]] of about 60,000 glucose residues and has a molecular weight between 10<sup>6</sup> and 10<sup>7</sup> [[atomic mass unit|daltons]] (~4.8 million).{{Fact|date=May 2007}} Most of Glc units are linked by α-1,4 [[glycosidic bond]]s, approximately 1 in 12 Glc residues also makes alpha-1,6 glycosidic bond with a <span style="color: green; font-weight: bold;">second Glc</span>, which results in the creation of a branch. Glycogen does not possess a [[redox|reducing]] end: the '[[reducing sugar|reducing end]]' glucose residue is not free but is covalently bound to a [[protein]] termed [[glycogenin]] as a [[beta-linkage]] to a surface tyrosine residue. [[Glycogenin]] is 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 [[carbohydrate]]s is eaten and digested, [[blood glucose]] levels rise, and the [[pancreas]] secretes [[insulin]]. Glucose from the [[portal vein]] enters the [[liver]] cells ([[hepatocyte]]s). Insulin acts on the hepatocytes to stimulate the action of several [[enzyme]]s, 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 [[digestion|digested]] and glucose levels begin to fall, insulin secretion is reduced, and glycogen synthesis stops. About four hours after a meal{{Fact|date=May 2007}}, glycogen begins to be 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. When the [[blood sugar]] begins 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 [[Calorie|kcal]] <!-- Let's use metric units (kJ)!!! -->of glycogen,{{Fact|date=May 2007}} long-distance athletes such as [[Marathon (sport)|marathon]] runners, [[Cross-country skiing|cross-country skiers]], and [[cyclist]]s 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" or "[[bonk (condition)|bonking]]". 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 loading]] before the task. When experiencing glycogen debt, athletes often experience extreme [[fatigue (physical)|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 [[caffeine]] following exhaustive exercise, their glycogen is replenished more rapidly.<ref>{{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 CO-INGESTED WITH CAFFEINE.|journal=J Appl Physiol|year=2008 |date=May 8 |pmid=18467543 |PubMed - as supplied by publisher}}</ref><ref>[http://newswise.com/articles/view/542216/ Post-exercise Caffeine Helps Muscles Refuel] Newswise, Retrieved on July 6, 2008.</ref> ==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 error of metabolism|inborn errors of metabolism]] are caused by deficiencies of enzymes necessary for glycogen synthesis or breakdown. These are collectively referred to as [[glycogen storage disease]]s. ==Synthesis== {{Main article|Glycogenesis}} 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 [[Uridine triphosphate|UTP]], which reacts with [[glucose-1-phosphate]], forming [[UDP-glucose]], in 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. As glycogen synthase can only lengthen an existing chain, the protein [[glycogenin]] is needed to initiate the synthesis of glycogen. ==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 that is then converted to [[G6P|Glucose 6-phosphate]]. 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. ==References== <references /> == See also == *[[Peptidoglycan]] ==External links== * [http://www.histochem.net/protocol%20periodic%20acid%20schiff.htm Glycogen detection using Periodic Acid Schiff Staining] * {{MeshName|Glycogen}} {{carbohydrates}} [[Category:Nutrition]] [[Category:Polysaccharides]] [[Category:Exercise physiology]] [[Category:Hepatology]] [[ar:جليكوجين]] [[bg:Гликоген]] [[cs:Glykogen]] [[da:Glykogen]] [[de:Glykogen]] [[es:Glucógeno]] [[eo:Glikogeno]] [[fr:Glycogène]] [[id:Glikogen]] [[it:Glicogeno]] [[he:גליקוגן]] [[lt:Glikogenas]] [[hu:Glikogén]] [[mk:Гликоген]] [[ms:Glikogen]] [[nl:Glycogeen]] [[ja:グリコーゲン]] [[no:Glykogen]] [[oc:Glicogèn]] [[pl:Glikogen]] [[pt:Glicogénio]] [[ro:Glicogen]] [[ru:Гликоген]] [[sq:Glikogjeni]] [[simple:Glycogen]] [[sk:Glykogén]] [[sl:Glikogen]] [[sr:Гликоген]] [[fi:Glykogeeni]] [[sv:Glykogen]] [[uk:Глікоген]] [[zh:糖原]]