Glycogenolysis - Mechanism

Mechanism

The overall reaction for the breakdown of glycogen to glucose-1-phosphate is:

glycogen_{(n residues)} + P_i glycogen_{(n-1 residues)} + glucose-1-phosphate

Here, glycogen phosphorylase cleaves the bond linking a terminal glucose residue to a glycogen branch by substitution of a phosphoryl group for the α linkage. Glucose residues are phosphorolysed from branches of glycogen until four residues before a glucose that is branched with a α linkage. Glycogen debranching enzyme then transfers three of the remaining four glucose units to the end of another glycogen branch. This exposes the α branching point, which is hydrolysed by α glucosidase, removing the final glucose residue of the branch as a molecule of glucose and eliminating the branch. This is the only case in which a glycogen metabolite is not glucose-1-phosphate. The glucose is concomitantly phosphorylated to glucose-1-phosphate by hexokinase. Glucose-1-phosphate is converted to glucose-6-phosphate by the enzyme phosphoglucomutase.