Glycogen

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Glycogen:-:

It is the reserve carbohydrate found in liver [6-8%] and muscles [1-2%] of animals and humans. The glycogen content of liver is more than that of muscles. It is also found in plants which have no chlorophyll content e.g. fungi and yeast but not in green plants. Glycogen:-

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Molecular weight of glycogen obtained from different sources may range from 10⁵ - 10⁸ and each molecule contains from 5000 -10000 glucose units. It is a branched polysaccharide and every branch arises after every 8-12 glucose units, In straight chain, the glucose units are linked by α 1-4 linkages and in branched chain the sugar units are linked with each other with the help of α 1-6 linkage . Glycogen is stored as granules in the cytosol where most of the enzymes of the glycogen synthesis and breakdown are present.

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Glycogen is very similar in structure to amylopectin but shows more branching.it forms tiny granules inside cells which are usually associated with SER.

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Structure of amylopectin and glycogen

Isolation of Glycogen:-:

Isolation of Glycogen:- The isolation of Glycogen is based on the principle that glycogen is released from the tissues by heating strong alkali (KOH) and precipitated by the addition of ethanol(95%), sodiumsulphate is used as a co precipitant to give a quantative yield of glycogen. Glycogen when subjected to acid hydrolysis yields only D-glucose . Also the hydrolysis of fully methylated glycogen yields 2,3,5,6-tetremethyl glucose in such an amount which indicates one end group for every 12 to 18 glucose units. But molecular weight of glycogen is very high(10⁵-10⁸). This shows that glycogen is highly branched molecule.

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G lycogen on hydrolysis with β -amylase yields nearly 30% maltose indicating that glycogen has much more branched structure than the amylopectin. Glycogen when hydrolyzed enzymatically yields isomaltose indicating 1,6-linkages . Thus the structural principle of glycogen resembles that of amylopectin but the no. of branched chains appear to be much higher and of smaller lengths in a linear section. The average chain length in glycogen is about 10-14 glucose units while it is 24 in amylopectin.

Synthesis of Glycogen:-:

Synthesis of Glycogen:- Synthesis of glycogen from glucose is called glycogenesis . Glycogenesis takes part in cytosol and requires ATP and UTP besides glucose. Glycogenesis mainly occur in liver and muscles . Initially phosphorylation occurs to form glucose- 6- phosphate which is converted to glucose-1-phosphate catalyzed by the enzyme phosphoglucomutase . Next the active glucose units are formed with uridine triphosphate .The C 1 of active glucose unit forms a glycosidic bond with each C₄ of a terminal glucose residue on glycogen(primer).

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A second enzyme , branching enzyme transfers a part of 1 4 chain (minimum length 6 glucose resduces ) to a neighboring chain to form 1 6 linkage . The reactions are :

Glycogen as a fuel reserve:-:

Glycogen as a fuel reserve:- As such fat is the fuel reserve of the body; however fat is not preferred. Instead of fat glycogen is preferred for routine day to day use of energy for following reasons : Glycogen can be rapidly mobilised . Glycogen can generate energy in the absence of oxygen Brain depends upon glucose supply which mostly comes from glycogen.

Functions of Glycogen:-:

Functions of Glycogen:- Primary function of liver glycogen is to maintain blood glucose levels particularly between meals. Muscle glycogen serves as a fuel reserve for the supply of ATP during muscle contraction. Liver glycogen is totally depleted after 12-18 hours of fasting whereas muscle glycogen is exhausted after prolonged vigorous exercise. Liver glycogen acts as storage for emergency & supplies hexose sugars for the maintenance of blood glucose between meals.

Glycogen Storage Diseases:-:

Glycogen Storage Diseases:- It is a group of inherited disorders characterized by the deposition of an abnormal type or quantity of glycogen in tissues. Glycogen storage disease type Ӏ (Von Giake’s disease): In this disease the renal tubular cells and liver cells are characteristically loaded with glycogen but these stores are not available, thus resulting hypoglycemia . Lack of glucose results into mobilization of fatty acids and increased rate of oxidation resulting into ketosis. The biochemical defect is usually due to defect in enzyme ,or complete absence of glucose-6-phosphatase.

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Glycogen storage disease type ӀӀ ( Pompe’s disease) : In this disease glycogen accumulates in lysosomes due to deficiency of lysosomal 1 4 , 1 6 , glycosidase which degrades glycogens . Glycogen storage disease type ӀӀӀ (Limit dextronosis ): In this disease the polysaccharides of the limit dextrin type accumulates due to absence of debranching enzyme.

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Glycogen storage disease type Ӏ V( Amylopectinosis ): It is characterized by the accumulation of polysaccharides with fewer branches due to absence of branching enzyme. Glycogen storage disease type V( McArdle’s Syndrome): In this case muscle passes abnormal amount of glycogen . There is diminished tolerance to exercise . The fundamental cause is the absence of the muscle phosphorylase.

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