Liver and muscle are the two major storage tissues for glycogen. In muscle, the need for ATP results in the conversion of glycogen to G6P for entry into glycolysis. In liver, low blood glucose concentration triggers glycogen breakdown to G6P, wich in this case is hydrolyzed to glucose and released into the bloodstream to reverse the situation.

Glycogen breakdown requires the action of three enzymes:

1- Glycogen phosphorylase - catalyzes the sequential removal of glycosyl residues (as glucose-1-P) from the nonreducing end of the glycogen molecule by phosphorolysis (bond cleavage by the substitution of a phosphate group). This enzyme will only release a glucose unit that is at least five units from a branch point. The phosphorolytic cleavage of glycogen is energetically advantageous because the released sugar is phosphorylated. In contrast, a hydrolytic cleavage would yield glucose, wich would have to be phosphorylated at the expense of an ATP to enter the glycolytic pathway. Also, glucose-1-P, ionized under physiologic conditions, cannot diffuse out of the cell.

2- Glycogen debranching enzyme - removes glycogen's branches, thereby permiting the glycogen phosphorylase reaction to go to completation. It also hydrolyzes a(1-->6) linked glucosyl units to yield glucose.

3- Phosphoglucomutase - converts Glucose-1-P to Glucose-6-P.