Glycogen phosphorylase a is the more active form of glycogen phosphorylase. It catalyzes the phosphorolysis of glycogen to form glucose-1-phosphate. Two features distinguish glycogen phosphorylase a from the b form:

1. The a form is derived from the b form by phosphorylation of the b form by the enzyme glycogen phosphorylase b kinase (Figure 13.18).

2. The b form requires AMP for allosteric activation and is thus active only when cells are at a low energy state.