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). The granules contain both glycogen and the enzymes of glycogen synthesis and degradation. Two types of compartments within the cell (organelles)—a peroxisome and three mitochondria—are also labeled.

Glycogen, a large, branched polymer of linked glucose (Glc) residues, is the principal storage form of glucose in animal cells, though it is also found in various species of microorganisms, such as bacteria and fungi. A readily mobilized energy store, glycogen increases the amount of glucose immediately available to the organism (1) between meals and (2) during muscular activity. The ability to maintain a steady supply of glucose, which is the major sugar circulating in the blood of higher animals, is crucial to survival since the brain relies on glucose as its preferred fuel.

Glycogen is found in the form of granules in the cytosol, the internal fluid of the cell. About three-fourths of the body’s glycogen supply is stored in muscle cells. However, liver cells (hepatocytes), have the highest concentration of glucose (a maximum of 8% in liver versus 1% of the muscle mass of an adult male human). Small amounts of glycogen are also found in the kidneys, and even smaller amounts in certain glial cells in the brain and in white blood cells.

Glycogen is utilized differently depending on the tissue in which it is stored:

• In liver, the breakdown of glycogen (glycogenolysis) and its synthesis (glycogenesis) help to regulate the blood glucose level. Glucose is not a major fuel for the liver; instead, it primarily exports glucose for the benefit of other tissues.
• In skeletal muscle, glycogen is an energy reserve that can be tapped during exercise. Muscle cells lack the ability to pass glucose into the blood, so the glycogen store is destined for internal use, powering muscle contraction during strenuous activity.

Glycogen-storage disorders are a type of inherited metabolic disease related to deficiencies of enzymes that participate in glycogen metabolism.

Glycogen's branched structure makes it an accessible energy source

The structure of glycogen. Most glucose residues are linked by α-1,4 glycosidic bonds (labeled at top). Approximately 1 in 10 Glucose residues form α-1,6 glycosidic bonds, creating a branched structure (green). The non-reducing end-branches (red) faciliate glycogen's interaction with enzymes involved in its synthesis and breakdown.

Glycogen is a highly-branched polymer of about 30,000 glucose residues. It has a molecular weight between 10⁶ and 10⁷ daltons. Most of the glucose units are linked by α-1,4 glycosidic bonds, which are formed between the hemiacetal group of a saccharide and the hydroxyl group of an alcohol. Specifically, the carbon-1 of one sugar molecule is linked to the carbon-4 of the other. In the alpha configuration, the oxygen atom is located below the plane of the sugar ring.

However, approximately 1 in 10 glucose residues also forms an α-1,6 glycosidic bond with an adjacent glucose, which results in the creation of a branch. Glycogen has only one reducing end and a large number of non-reducing ends with a free hydroxyl group at carbon-4.

The alpha linkages contribute to glycogen's open helical structure. The branches increase the solubility of glycogen and make its sugar units accessible. The enzymes involved in the breakdown and synthesis of glycogen 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, making it a readily mobilized source of energy.

Starch, which plays a similar energy-storage role in plants, can also exist in a branched form called amylopectin, though it has a lower degree of branching (about 1 in 30 glucose residues have α-1,6 bonds). In contrast, cellulose, the other major polysaccharide in plants, is an unbranched polymer of glucose, in which β-1,4 linkages form very long, straight chains. This closed structure is suited to the structural role of cellulose, in contrast to the open helices of glycogen and starch, which are nutritional molecules that provide an accessible store of glucose.

Glycogen in liver functions to maintain blood sugar levels

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 is an energy reserve for strenuous exercise

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.

References

ISBN links support NWE through referral fees

Stryer, Lubert. 1995. Biochemistry, 4th edition. New York, NY: W.H. Freeman.

External links

• Glycogen detection using Periodic Acid Schiff Staining

Template:ChemicalSources

See also

• metabolism
• glycolysis
• Peptidoglycan
• Glycosidic bond
• Glycogenesis

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