glycogen

n.
A polysaccharide, (C₆H₁₀O₅)_n, that is the main form of carbohydrate storage in animals and occurs primarily in the liver and muscle tissue. It is readily converted to glucose as needed by the body to satisfy its energy needs. Also called animal starch.

glycogenic gly'co·gen'ic (-jĕn'ĭk) adj.

For more information on glycogen, visit Britannica.com.

The primary reserve polysaccharide of the animal kingdom. It is found in the muscles and livers of all higher animals, as well as in the cells of lower animals. Because of its close relationship to starch, it is often called animal starch, although glycogen is found in some lower plants, fungi, yeast, and bacteria. See also Starch.

Glycogen is a nonreducing, white, amorphous polysaccharide which dissolves readily in cold water, forming an opalescent, colloidal solution. The molecular weight of glycogen is usually very high, and it varies with the source and the method of preparation; molecular weights of the order of 1−20 × 10⁶ have been reported. Chemical studies show glycogen to possess a branched structure similar to the amylopectin starch fraction.

In its biochemical reactions, glycogen is similar to starch. It is attacked by the same plant amylases that attack starch, and like starch, it is degraded to maltose and dextrins. Both glycogen and starch are broken down by animal or plant phosphorylase enzyme in the presence of inorganic phosphate with the production of α-D-glucose-1-phosphate. See also Carbohydrate metabolism.

The metabolic formation of glycogen from glucose in the liver is frequently termed glycogenesis. In fasted animals, glycogen formation can be induced by the feeding, not only of materials that can be hydrolyzed to glucose and other monosaccharides, such as fructose, but also of various other materials. A number of L-amino acids, such as alanine, serine, and glutamic acid, upon deamination in the liver give rise to substances, such as pyruvic acid and α-ketoglutaric acid, that can be converted in the liver to glucose units which are subsequently converted to glycogen. Furthermore, substances such as glycerol derived from fats, dihydroxyacetone, or lactic acid can all be utilized for glycogen synthesis in the liver. Such noncarbohydrate precursors are termed glycogenic compounds. The process of glycogen formation from these precursors is known as glyconeogenesis. The term glycogenolysis is used to connote glycogen breakdown. See also Polysaccharide.

---

In the 1840s and 50s, Claude Bernard was applying his great scientific mind to the problem of ‘sugars’ in the body, in particular how the liver could apparently make sugars and ‘squirt them into the blood … in a regulated manner’ when he had fed an animal only on protein. In 1855 he coined the term ‘matière glycogene’ — sugar-making material. He removed a liver, washed it out with water, and found that there was still sugar in a subsequent wash-out. He concluded that the sugar-forming substance was stored in the liver, and was not water soluble. Eventually he succeeded in isolating the ‘emulsive material of the liver’, found it to be similar to starch, and listed its properties in an account so complete as to be valid to this day. It was to be over 70 years before the medical significance of glycogen storage came to light, when defective storage in liver, kidney, and heart became recognized. Another 70 years on, and the several associated diseases are well understood, mainly as enzyme deficiencies, whilst deliberate boosting of glycogen storage in muscle before a marathon run is common knowledge.

Glycogen is the form in which carbohydrate is stored in the body. Each molecule of glycogen is formed by the linkage in branching chains of many thousands of glucose molecules. Thus, glycogen is a natural polymer, a polysaccharide, which has a similar structure to the starch which is found in plants.

Most tissues of the body are able to store small amounts of glycogen, but the main sites of glycogen storage are the liver and skeletal muscles. In both cases, glycogen is made from glucose within the cells in which it is stored, and the synthetic process is stimulated by the hormone insulin. When glycogen is stored within muscle and liver cells, it retains water along with it (approximately 3 g of water for each gram of glycogen), so changes in glycogen content can cause quite noticeable changes in total body weight. For example, in the first few days of starvation, glycogen is used by the liver to maintain blood sugar and by muscle metabolism, and the associated water is excreted from the body in the urine, accounting for a major part of 1-2 kg loss of weight.

There are important differences between the major functions of liver and muscle glycogen. The main role of liver glycogen is to provide a reserve supply of glucose in order that blood glucose concentration can be kept at an adequate level to supply the brain (which does not use other fuels) during periods of fasting, or when glucose use is increased during physical work and exercise. Thus, after meals some of the carbohydrate consumed is stored as liver glycogen, and during fasting (even just overnight) this glycogen is broken down and the glucose is released into the blood. In a healthy adult, the liver glycogen store is usually between 50 and 100 g, containing enough glucose to satisfy the brain's requirements for up to 24 hours.

A subject exercised one leg only to exhaustion. Specimens of the quadriceps muscle were taken from both legs immediately after the exercise (by 'needle biopsy'). The figure shows the microscopic appearance of the muscle, magnified × about 25 000; (a) from the leg which was not exercised; normal glycogen content is seen as granules (black dots) between the muscle fibres; (b) from the leg which was exercised; the glycogen granules are depleted. Reproduced with permission from E. Hultman


The main role of muscle glycogen is to provide fuel for the muscle's own contraction during exercise. In fact muscle glycogen cannot be broken down to glucose and so cannot be used to raise blood glucose concentration directly. However, in some circumstances, when their metabolism is partly anaerobic, skeletal muscles produce lactic acid from glycogen. When this lactic acid passes into the blood it is taken up by the liver, where it is converted into glucose; thus it can be used indirectly to raise blood glucose. The major stimulus causing the breakdown of muscle glycogen is contraction of the muscles. Thus, the onset of exercise is accompanied by the initiation of glycogen breakdown. The extent to which the muscles continue to use their glycogen store depends on the intensity of the exercise. With low intensity exercise (such as slow walking, cycling, or swimming) the muscles do not use much glycogen as they are able to take up fat from the blood as a source of energy for contraction. However, with higher intensity exercise (jogging, brisk uphill walking, running) the muscles need to use glycogen or glucose from the blood to support the higher rate of energy expenditure (see figure). A well-nourished person will have enough glycogen in their muscle to enable them to exercise for 1-2 hours at approximately two-thirds of their maximum capacity for aerobic exercise. However if people consume a very high carbohydrate diet, especially for at least three days after first depleting their muscle glycogen levels, it is possible to double this normal glycogen content, ensuring that a longer period of exercise can be sustained before it is used up. This is known as carbohydrate loading, or glycogen supercompensation, and is often used by distance — especially marathon — runners before an important race.

— I. A. Macdonald

See also blood sugar; exercise; metabolism; muscle.

The storage carbohydrate in the liver and muscles, a branched polymer of glucose units. It has a similar structure to the amylopectin form of starch, but is more highly branched. In an adult there are about 250 g of glycogen in the muscles and 100 g in the liver in the fed state.

Since glycogen is rapidly broken down to glucose after an animal is killed, meat and animal liver do not contain glycogen.

animal starch

The main carbohydrate store in the body; the liver and muscles are the main storage sites. Glycogen is a complex carbohydrate (polysaccharide) made of glucose units that are released when it is digested or metabolized. It is the major energy source for muscles. Compared with other fuels, proportionately more glycogen is used during power activities (e.g. weight lifting, sprinting, and jumping) than during endurance activities. The muscle stores can be loaded above their normal capacity by carbohydrate loading (see separate entry), sometimes known as glycogen supercompensation or glycogen overshoot.

The first scientific evidence that low muscle glycogen levels cause fatigue were provided by studies of cyclists from the volunteer Stockholm firemen. They pedalled until they were exhausted, at which time their muscle glycogen levels had fallen to zero.

(glī′kōjen)
n

A branched, homopolysaccharide of glucose held by α 1-4 and α 1-6 glucosidic bonds. Liver glycogen provides a ready source of blood glucose through glycogenosis.

A highly branched polysaccharide consisting of alpha glucose units. The liver stores most of the glycogen in the body. Liver glycogen is readily hydrolysed to glucose to maintain blood sugar. See also muscle glycogen.

Pertaining to, characterized by, or promoting glycogenesis; pertaining to glycogen.

This entry is from Wikipedia, the leading user-contributed encyclopedia. It may not have been reviewed by professional editors (see full disclaimer)