Glycogen phosphorylase b is the less active form of glycogen phosphorylase. It differs from glycogen phosphorylase a in that it is not phosphorylated and that it requires AMP for activity. Glycogen phosporylase b is a substrate for the enzyme glycogen phosphorylase b kinase, which converts the b form to the a form by adding two phosphates. The reaction is stimulated in the presence of calcium via interaction of calmodulin with glycogen phosphorylase b kinase

Two features distinguish glycogen phosphorylase b from the a form:

1. The a form is derived from the b form by phosphorylation of the b form by the enzyme 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.