Glycogenolysis

Glycogenolysis is the catabolism of glycogen by removal of a glucose monomer through cleavage with inorganic 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.

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.

Clinical significance
Parenteral (intravenous) administration of glucagon is a common human medical intervention in diabetic emergencies when sugar cannot be given orally.

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 &alpha;-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. )

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)

Third step
The 3rd and last stage converts G1P (glucose-1-phosphate) to G6P (glucose-6-phosphate) through the enzyme phosphoglucomutase.

Regulation
The key regulatory enzyme of the process of glycogenolysis is Glycogen phosphorylase:
 * Phosphorylation --> activation
 * Dephosphorylation --> inhibition