Glycogenolysis
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Glycogenolysis is the catabolism of glycogen by removal of a glucose monomer through reaction with phosphate to produce glucose-1-phosphate. This derivative of glucose is then converted to glucose 6-phosphate, a key intermediate in glycolysis. The hormones glucagon and epinephrine stimulate glycogenolysis.
Glycogenolysis requires three enzymes :
- Glycogen phosphorylase (breaks down glucose polymer at α-1-4 linkages, yielding glucose-1-phosphate and a shorter glycogen molecule)
- Debranching enzyme transferase / α-1,6-Glucosidase (bifunctional enzyme) (transfers α-1-6-linked glucose to end of glycogen chain for glycogen phosphorylase, and removes the glucose present at the branching point as free glucose)
- Phosphoglucomutase (converts glucose-1-phosphate to glucose-6-phosphate)
Glycogenolysis occurs in the muscle and liver tissue, where glycogen is stored, as a hormonal response to epinephrine (e.g., adrenergic stimulation) and/or glucagon, a pancreatic peptide triggered by low blood glucose concentrations. Liver (hepatic) cells can consume the glucose 6-phosphate in glycolysis, or remove the phosphate group using the enzyme glucose 6-phosphatase and release the free glucose into the bloodstream for uptake by other cells. Since muscle cells lack glucose 6-phosphatase, they cannot convert glucose-6-P into glucose, and therefore use the glucose-6-phosphate for their own energy demands. However, since the glucose residues present as the branching points of the glycogen molecule are removed as free glucose (rather than as glucose-6-phosphate), even muscle cells are able to release a small amount of glucose into the bloodstream.
Perenteral administration of glucagon is a common human medical intervention in diabetic emergencies when sugar cannot be given orally.