The breakdown of glycogen (glycogenolysis) requires two enzymes, glycogen phosphorylase and (1,4 ->1,4) glucantransferase (a "Debranching Enzyme"). Glycogen phosphorylase catalyzes the phosphorolytic cleavage of (1->4) bonds, generating glucose-1-phosphate in the process. The branch points of glycogen are comprised of (1->6) bonds, however and glycogen phosphorylase does not cleave within four glucose units of an (1->6) bond. Instead, the debranching enzyme, transfers three of the four glucoses to another branch and hydrolytically cleaves the remaining glucose as free glucose (Figure 13.17).

Glycogen phosphorylase is present in two forms, glycogen phosphorylase a (the active form) and glycogen phosphorylase b (the relatively inactive form). Phosphorylase a is phosphorylated at a serine residue whereas phosphorylase b is not The two forms are interconverted by phosporylase b kinase (which puts phosphates on) or a phosphatase (which takes phosphates off).