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Glycogenolysis is the process of converting the food storage carbohydrate polymer glycogen into glucose for the body to use as energy. Glycogen is a polysaccharide — a long sugar chain — of glucose molecules with side branches. It is a way for animal cells to store excess carbohydrates until needed. Glycogen is found in the liver and in muscle tissue.
This process can be triggered by low blood sugars, through the action of the hormone glucagon, which is released by the pancreas and travels to the liver. It can also be actuated as a stress response by the action of the adrenal gland hormone epinephrine, also known as adrenaline. Both hormones trigger the catabolism of glycogen, its breakdown to glucose, and the release of energy.
Glycogenolysis begins with the removal of individual glucose molecules from the glycogen chain. A molecule of inorganic phosphate is then added to the molecule to make glucose-1-phosphate. This reaction is carried out by the enzyme glycogen phosphorylase and is the key regulatory step of the process. If it adds a phosphate group, the reaction proceeds, but if it removes one, the process stops.
The next step is to move the phosphate group to another position on the molecule to make glucose-6-phosphate. This is a key metabolic step, since glucose-6-phosphate can go into several different pathways. If energy is needed immediately, it goes into glycolysis in liver or muscle cells. Glycolysis is part of aerobic respiration, and the glucose-6-phosphate is broken down to generate adenosine triphosphate (ATP). This is a source of fuel for the cells.
Alternatively, if there is no immediate need for the glucose, the liver cells can use the enzyme glucose-6-phosphatase to release the free glucose into the blood. It is then distributed so that it can be taken up by other cells. Glucose ends up in the brain in this manner.
Glycogen is found in microorganisms and in animals, while plants make starch as their storage polymer. Starch does not have the branched side chains that glycogen has. Glycogen is produced when there is an excess of energy in the form of glucose or ATP. It is synthesized by glycogen synthase, which is inhibited by glucagon and epinephrine.
Insulin acts in the opposite manner to glucagon, inducing the synthesis of glycogen to increase the storage of glucose. Glucagon has been used medically in cases of severe diabetes, when oral sugar could not be given.