The four protein kinase A (PKA) regulatory subunit isoforms differ in their tissue specificity and functional characteristics. The specific isoform activated in response to glucagon signalling is not known. The PKA kinase is a tetramer of two regulatory and two catalytic. The regulatory subunits block the catalytic subunits. Binding of cAMP to the regulatory subunit leads to the dissociation of the tetramer into two active dimers made up of a regulatory and a catalytic subunit.