Glycogen phosphorylase catalyzes phosphorolysis of glycogen to glucose-1-phosphate (Figure 13.18).

Two forms of the enzyme exist. The relatively "inactive" form 'b' has no phosphate, but can be converted to the more active form 'a' by action of the enzyme glycogen phosphorylase b kinase.

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 phosphorylase b kinase.

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