Glycogen phosporylase b kinase (also called synthase-phosphorylase kinase (SPK) or simply phosphorylase b kinase) is part of the glycogen kinase cascade regulatory system (Figure 13.18). The enzyme is converted from the inactive form to active by phosphorylation catalyzed by cAMP-dependent protein kinase. Glycogen phosphorylase b kinase, when active, phosphorylates the glycogen phosphorylase b (less active form) to convert it to the more active form (glycogen phosphorylase a).

Calmodulin plays a special role as an integral subunit of the enzyme. This dependence is particularly important in muscle, where contraction is stimulated by calcium release. Thus, Ca²⁺ plays a dual role, in provision of the energy substrates needed to support muscle contraction and in contraction itself.