Glycogen is a polysaccharide, an animal starch that is the principal storage form of glucose in animal cells, particulalry in the liver and muscles. Glycogen is found in the form of granules in the cytosol in many cell types. Hepatocytes have the highest concentration of it - up to 8% of the fresh weight in well fed state, or 100–120 g in an adult - giving liver a distinctive, "starchy" taste. 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 glial cells in the brain and white blood cells.
Structure and biochemistry
Glycogen is a highly-branched polymer of 10,000 to 120,000 Glc residues and molecular weight between 106 and 107 daltons. Most of Glc units are linked by a α-1,4 glycosidic bonds, approximately 1 in 12 Glc residues also makes a α-1,6 glycosidic bond with a second Glc which results in creating of a branch. Glycogen has only one reducing end and a large number of nonreducing ends with a free hydroxy group at carbon 4. 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 nonreducing ends. Therefore, the many nonreducing end-branches of glycogen facilitate its rapid synthesis and breakdown.
Function and regulation of liver glycogen
As a carbohydrate meal is eaten and digested, blood glucose levels rise, and the pancreas secretes insulin. Glucose from the portal vein enters the 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. About four hours after a meal, glycogen begins to be broken down to be 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.
Glycogen 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 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.
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Glycogen and marathon running
Due to the body's ability to hold around 2,000 kcal of glycogen, marathon runners commonly experience a phenomenon referred to as "hitting the wall" around the 20 mile (32 km) point of a marathon. (Approximately 100 kcal are utilized per mile, depending on the size of the runner and the race course.) When experiencing glycogen debt, runners many times undergo intense muscle cramping.
- 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.
- 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.
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.
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".
Glycogen depletion can be forestalled in three 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, 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.Template:MEDRS
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