Glycogen

 Glycogen

Definition

Glycogen is a multi branched polysaccharide of glucose that serves as a form of energy storage in humans.

Chemistry and properties

It is called animal starch, because it is in this form that glucose remains stored in the liver and muscles. It is branched polysaccharides (amylopectin type) consisting of hundreds of glucose units linked together by glucosidic linkages, i.e., a-1, 4 linkage and 1, 6" linkage which are formed by specific enzymes-uridine diphosphate glucose (UDPG)-pyrophosphorylase, glycogen synthetase and amylo-(1.4-1, 6") transglucosidase respectively. It is soluble in water and makes an opalescent solution and gives red color with iodine. Glycogen liberates more energy than the corresponding weight of glucose. It does not diffuse into the intracellular fluid, as it exerts no osmotic pressure. It may be easily broken down into glucose by enzymes present in the liver.

Amount and distribution

In a normal adult about 500 gm of glycogen is present in the body, about 100-120 gm in liver and 400 gm in muscles, Liver and muscles are the chief storehouses. All growing tissues can store glycogen therefore they are present in large amounts in the placenta in its early stage and the fetal muscles etc. In the fetal muscles it may be as much as 40% of the total dried solids.

Glycogen in any tissue is not a static quantity. It is being constantly used up and re synthesized. So that at any time the glycogen of the tissue should be considered as a balance between the constant production and loss. Liver glycogen is most mobile. It is the first to be formed and is also the first to be mobilized. Muscle glycogen is much slower to move. There are remarkable differences between the metabolism of liver glycogen and muscle glycogen.

Functions of liver glycogen

1.  Liver glycogen is a ready source of glucose supply in the blood.

2.  It helps in the detoxicating mechanism in the liver.

3. If the liver glycogen level is high, ketone body formation and rate of deamination of amino acids are depressed.

4.  It protects the liver from the toxic effects of arsenic, carbon tetra chloride, etc.

Functions of muscle glycogen

Muscle cell glycogen is an immediate reserve source of available glucose for muscle cells. As muscle cells lack glucose-6-phosphatase, which is required to pass glucose into the blood, the glycogen they store is available solely for internal use and is not shared with other cells. This is in contrast to liver cells, which, on demand, readily do break down their stored glycogen into glucose and send it through the blood stream as fuel for other organs