Glycogenolysis
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Glycogenolysis is the catabolism of glycogen by removal of a glucose monomer and addition of phosphate to produce glucose-1-phosphate. This derivative of glucose is then converted to glucose-6-phosphate, an intermediate in glycolysis.
The hormones glucagon and epinephrine stimulate glycogenolysis.
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[edit] Function
Glycogenolysis transpires 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.
- Muscle cells will not release glucose, but instead use the glucose-6-phosphate in glycolysis.
[edit] Clinical significance
Parenteral (intravenous) administration of glucagon is a common human medical intervention in diabetic emergencies when sugar cannot be given orally.
[edit] Reaction
[edit] First step
The overall reaction for the 1st step is:
Glycogen (n residues) + Pi <-----> Glycogen (n-1 residues)+ G1P
Here, glycogen phosphorylase cleaves the bond at the 1 position by substitution of a phosphoryl group. It breaks down glucose polymer at α-1-4 linkages until 4 linked glucoses are left on the branch. (Furthermore, glycogen phosphorylase (EC 2.4.1.1) can be used as a marker enzyme to determine glycogen breakdown. )
[edit] Second step
The 2nd step involves the debranching enzyme that moves the remaining glucose units to another non-reducing end. This results in more glucose units available to glycogen phosphorylase (step 1)
[edit] Third step
The 3rd and last stage converts G1P (glucose-1-phosphate) to G6P (glucose-6-phosphat) through the enzyme phosphoglucomutase.
[edit] External links
| Metabolism: carbohydrate metabolism |
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| Fermentation (Ethanol, Lactic acid) - Glycolysis/Gluconeogenesis - Glycogenesis/Glycogenolysis - Pentose phosphate pathway - Photosynthesis (Carbon fixation) |
