glycogen phosphorylase

gly·co·gen phos·phor·y·lase (gliґko-jən fos-forґə-lās) [EC 2.4.1.1] an enzyme of the transferase class that catalyzes the phosphorolysis of a terminal α-1,4-glycosidic bond at the non-reducing end of a glycogen molecule, releasing a glucose 1-phosphate residue. The inactive form of the enzyme (phosphorylase b) is converted to the active, phosphorylated form (phosphorylase a) by phosphorylase kinase, and phosphorylase a is deactivated by phosphorylase phosphatase; phosphorylase b can also be activated by AMP without being phosphorylated. Two isozymes exist: the liver isozyme replenishes blood glucose, while the muscle isozyme mobilizes glycogen as fuel. Deficiency of glycogen phosphorylase, an autosomal recessive trait, causes glycogen storage disease. The muscle isozyme is absent in glycogen storage disease, type V; the liver isozyme is deficient in type VI. Called also phosphorylase.