Here is a term paper on ‘Glycogenesis in Animal Tissues’ especially written for school and college students.
The process of biosynthesis of glycogen from glucose or other sugars is known as glycogenesis. Glycogenesis is synthesized in practically all the tissues of the body but the major sites are liver and muscles. The aim of storing glycogen is to provide glycosyl units for energy purpose in the muscles, and to maintain blood sugar level within the normal range at the time of fasting so that delicate organs in the body may be protected against harmful effects of hypoglycemia.
Immediately after taking carbohydrate rich diet, the liver tissue may store glycogen approximately 5-6% of its weight. In adult man weighing 70 kg, liver constitutes about 1.8 kg. Total glycogen stored in the liver tissue in well fed subjects may vary form 90 of 110g. After fast of about 12 to 18 hrs, liver may become depleted to glycogen.
Muscles contain 0.7 to 1.0 percent glycogen on wet weight basis. Muscles constitute approximately 35 kg in an adult individual. Thus, 245 to 350 g glycogen may be stored in the muscles. Muscle glycogen is reduced after severe exercise, or when liver glycogen is almost totally depleted.
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It has been shown that there actions leading to synthesis of glycogen in liver, muscles and brain tissues are similar. Glucose is activated by an enzyme hexodinase in the presence of ATP and Mg+ + ions with the formation of glucose-6-PO4. The backward reaction is not possible by the same enzyme under physiological conditions. Another enzyme phosphoglucomutase changes glucose- 6-PO4 to glucose- 1-PO4.
This reaction requires the presence of glucose-1, 6-diphosphate as cofactor. In the next stage, glucose- 1-PO4 reacts with uridine triphosphate (UTP) under the influence of the enzyme uridine diphosphate glucose pyrophosp-horylase (UDPC-pyrophosphorylase) to form uridine diphosphate-glucose (UDPG).
In this reaction, pyrophosphate is liberated which is finally converted to orthophosphoric acid by another enzyme pyro-phosphatase making UDPG formation irreversible. UDPG serves as the glucosyl-unit donor in the biosynthesis of glycogen. The actual synthesis of glycogen from glucosyl units of UDPG requires the presence of a small amount of glycogen nucleus in the preformed state (primer) to which the glucosyl units form UDPG are attached forming 1, 4-glucosidic linkages (straight chains).
Such are action is catalysed by the enzyme UDPG-glycogen-transglycosylase, also known as glycogen synthetase. The reaction is stimulated by glucose-6-PO4 which is believed to bind the enzyme glycogensynthetase and stabilize it in the active form. As soon as the polysaccharide chain acquires a length of eight glucosyl-units, it is subjected to the action of another enzyme amylo-1, 6- glucosidase, also known as branching enzyme.
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This glucosidase cleaves the straight chain fragments and transfers them to the neighbouring chain. Attachments of these fragments occurs forming 1, 6-glucosidic linkages (branched chains). UDPG further attaches more glucosyl units on these branches. Straight chains are elongated and broken down forming new branches in the same manner.
The same process continues till a tree like structure of glycogen molecule is synthesized. The molecular weight of glycogen thus synthesized may vary from one to four millions, or even more. Other sugars such as galactose or fructose may also be converted into glycogen by way of first forming glucose, glucose-6-PO4, or glucose-1-PO4.