glycogen metabolism-1

GLYCOGEN METABOLISM
Glycogen is a branched homopolysaccharides made exclusively from a-D-glucose.

The primary glycosidic bond is an alpha-1,4 glycosidic linkage. After an average of eight to ten glucosyl residues (glucose), there is a branch containing an alpha-1,6 glycosidic linkage .

Glycogen is the storage form of carbohydrates in the human body. The major sites of storage are liver and muscle. The major function of liver glycogen is to provide glucose during fasting. when blood glucose level falls, liver glycogen is broken down and helps to maintain blood glucose level. After taking food, blood glucose tends to rise, which causes glycogen deposition in liver. About 5 hours after taking food, the blood glucose tends to fall, but, glycogen is lysed to glucose so that the energy needs are met. After about 18 hours fasting, most of the liver glycogen is depleted, at this time depot fats are hydrolysed and energy requirement is met by fatty acid oxidation.The function of muscle glycogen is to act as reserve fuel for muscle contraction.

DEGRADATION OF GLYCOGEN (GLYCOGENOLYSIS)

1. Glycogen Phosphorylase
Glycogen phosphorylase removes glucose as glucose-1-phosphate from glycogen. It removes glucose units one at a time. Enzyme sequentially hydrolyses alpha-1,4 glycosidic linkages, till it reaches a glucose residue, 3-4 glucose units away from a branch point. It cannot attack the 1,6 linkage at branch point. If glycogen phosphorylase alone acts on a glycogen molecule, the final product is a highly branched molecule; it is called limit dextrin. PLP (pyridoxal phosphate) is an essential cofactor in the glycogen phosphorylase reaction.



2. Debranching by bifunctional two Enzymes
Then a block of 3 glucose residues (trisaccharide unit) are transferred from the branching point to another branch by enzyme alpha-1,4 ? alpha-1,4 glucan transferase. Now the branch point is free. Then alpha-
1,6- glucosidase (debranching enzyme) can hydrolyse the remaining glucosyl unit held in alpha-1,6 linkage at the branch point. This glucose residue is released as free glucose. At this stage, the ratio of glucose-1-
phosphate to free glucose is about 8:1.The transferase and alpha-1,6-glucosidase will together convert the branch point to a linear one. With the removal of the branch point, then phosphorylase enzyme can proceed with its action.

3. Phosphoglucomutase
Phosphorylase reaction produces glucose-1- phosphate while debranching enzyme releases glucose. The glucose-1-phosphate is converted to glucose-6-phosphate by phosphoglucomutase

4. Glucose-6-phosphatase in Liver
Next, hepatic glucose-6-phosphatase hydrolyses glucose-6-phosphate to glucose. The free glucose is released to the blood stream.
5. Muscle Lacks Glucose-6-phosphatase
Muscle will not release glucose to the blood stream, because muscle tissue does not contain glucose-6-phosphatase. Instead, in muscle, glucose-6-phosphate undergoes glycolysis to produce ATP for muscle contraction.