Phosphorylase kinase (EC 2.7.1.38) is a key enzyme involved in the regulation of the glycogenolysis pathway. It catalyzes the Ca²⁺-dependent phosphorylation and activation of the enzyme glycogen phosphorylase to make the active form glycogen phosphorylase. Phosphorylase kinase is composed of 4 subunits with a stoichiometry of (αβγδ)₄. The γ subunit is the catalytic subunit. The regulatory domain (residues 277-387) of γ contains a sequence resembling the sites phosphorylated in known γ substrates with the exception that a valine₃₃₂ occurs at the analogous position of the phosphorylated serine or threonine residue.