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Glycogen

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Glycogen is a polysaccharidethat is the principal storage form of glucosein animal cells. Glycogen is found in the form of granules in the cytosolin 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 glialcells in the brain and white blood cells.

Inhaltsverzeichnis

1 Structure and biochemistry
2 Function and regulation of liver glycogen
3 Glycogen in muscle and other cells
4 Glycogen and marathon running
5 Disorders of glycogen metabolism
6 See also

Structure and biochemistry

Glycogen is a highly branched polymerof 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.

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Function and regulation of liver glycogen

As a carbohydratemeal is eaten and digested, blood glucose levels rise, and the pancreassecretes insulin. Glucose from the 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 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.

Glucagonis 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.

Glycogen and marathon running

Due to the body's ability to hold around 2,000 kcal of glycogen, marathonrunners 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.

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 glycogenolysisnecessary to maintain normal blood sugar levels. Glucagonis a common treatment for this type of hypoglycemia.

Various inborn errors of metabolismare caused by deficiencies of enzymes necessary for glycogen synthesis or breakdown. These are collectively referred to as glycogen storage diseases.