CGMP is required to activate this enzyme. The enzyme occurs as a dimer in higher eukaryotes. The C-terminal region of each polypeptide chain contains the catalytic domain that includes the ATP and protein substrate binding sites. This domain catalyses the phosphorylation by ATP to specific serine or threonine residues in protein substrates . The enzyme also has two allosteric cGMP-binding sites (sites A and B). Binding of cGMP causes a conformational change that is associated with activation of the kinase .