Glycogen

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An electron micrograph of a section of a liver cell. Glycogen deposits are shown as accumulations of electron dense particles (arrows). Two types of compartments within the cell—a peroxisome and three mitochondria—are also labeled.

Glycogen is a polysaccharide that is the principal storage form of glucose (Glc) in animal cells. A readily mobilized energy store, glycogen increases the amount of glucose, which is the major sugar circulating in the blood of higher animals, immediately available to the organism (1) between meals and (2) during muscular activity.

Glycogen is found in the form of granules in the cytosol, the internal fluid of the cell, in many cell types. Lliver cells (hepatocytes) have the highest concentration of glucose- up to 8%, or 100–120 g in an adult). In skeletal muscle, glycogen is found in a much lower concentration (as high as 1% of the muscle mass), but, given its greater mass, the total amount exceeds that in liver, so about three-fourths of glycogen is stored in muscle. Small amounts of glycogen are found in the kidneys, and even smaller amounts in certain glial cells in the brain and white blood cells.

glycogen is utilized differently depending on the tissue. two important functions: regulates blood glucose level, especially important as ; energy reserve for skeletal muscle during strenuous exercise

storage disorders and other glycogen-related disease

Structure and biochemistry

Glycogen is a highly-branched polymer of about 30,000 glucose residues and has a molecular weight between 10⁶ and 10⁷ daltons (4.8 million approx.). Most of Glc units are linked by a α-1,4 glycosidic bonds, approximately 1 in 12 Glc residues also makes -1,6 glycosidic bond with a second Glc which results in the creation of a branch. Glycogen only has one reducing end and a large number of non-reducing ends with a free hydroxyl 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 non-reducing ends. Therefore, the many non-reducing end-branches of glycogen facilitate its rapid synthesis and breakdown.

Glycogen structure.

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.

Glycogen and marathon running

Due to the body's ability to hold no more than 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 experience fatigue.

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.

External links

• Glycogen detection using Periodic Acid Schiff Staining

Template:ChemicalSources

See also

• metabolism
• glycolysis
• Peptidoglycan
• Glycosidic bond
• Glycogenesis

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