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Renin Angiotensin Mechanism.

Glycogenolysis

 

Definition:

Breakdown of Glycogen to Glucose is called Glycogenolysis.

  • Glycogen stored in the liver and muscle cells of animals, is broken down into glucose to provide immediate energy or to maintain blood glucose levels during times of need.

  • Glycogenolysis is thus the breakdown of glycogen (n) to glucose-1-phosphate and glycogen (n-1).

  • Glycogen branches are catabolized by the sequential removal of glucose monomers via phosphorolysis, by the enzyme glycogen phosphorylase.

Location:

  • Takes place in the cytoplasm of cells in the muscle, liver, and adipose tissue.

  • Result: Glucose-1-phosphate is released from the non-reducing ends of glycogen chains.

Steps:

  1. Glycogen phosphorylase cleaves the bond linking a terminal glucose residue to a glycogen branch by substitution of a phosphoryl group for the α-[1→4] linkage.

  2. Glucose-1-phosphate is converted to glucose-6-phosphate by the enzyme phosphoglucomutase.

  3. Glucose residues are phosphorylated from branches of glycogen until four residues before a glucose that is branched with a α-[1→6] linkage. 

  4. Glycogen debranching enzyme then transfers three of the remaining four glucose units to the end of another glycogen branch.

  5. This exposes the α-[1→6] branching point, which is hydrolyzed by α[1→6] glucosidase, removing the final glucose residue of the branch as a molecule of glucose and eliminating the branch.

  6. This is the only case in which a glycogen metabolite is not glucose-1-phosphate. The glucose is subsequently phosphorylated to glucose-6-phosphate by hexokinase.

Important Enzymes:

  • Glycogen phosphorylase breaks α-1,4 linkages and debranching enzyme breaks α-1,6 linkages to release single units of glucose-1- phosphate.

  • Phosphoglucomutase converts glucose- 1-phosphate to glucose-6-phosphate, which is then shuttled into the glycolytic pathway.

Stimulation:

  • Glucagon (liver) and epinephrine (liver and muscle) stimulate glycogenolysis via the cAMP protein kinase. 

Inhibition:

  • Insulin inhibits glycogenolysis via dephosphorylation and thus results in the inactivation of glycogen phosphorylase.

Commonly Asked Questions.

  1. Write in short about “Glycogenolysis.”.
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