glycogenolysis

Glycogen

Glucose

Glucose-6-phosphate

Glycogenolysis (also known as "Glycogenlysis") is the catabolism of glycogen by removal of a glucose monomer through cleavage with inorganic phosphate to produce glucose-1-phosphate.^[1] 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 produced in the Alpha cells of the Islets of Langerhans.

• 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 in humans do not possess glucose-6-phosphatase and hence 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. It can also be administered intramuscularly.

Reaction

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. (, 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 three remaining glucose units to another 1,4 end of glycogen. The final action of the debranching enzyme is the hydrolysis of the glucose attached as a 1,6-branched, giving one free glucose. This is the only case in which glucose-1-phosphate is not a glycogen metabolite.

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

References

External links

• The chemical logic of glycogen degradation at ufp.pt
• MeSH Glycogenolysis