A hypothesis concerning the structure of cAMP-and cGMP-dependent protein kinases.

cAMP-and cGMP-dependent protein kinases have been purified. Each enzyme demonstrates high specificity and affinity for the cyclic nucleotide with binding of two moles of nucleotide per holoenzyme and each enzyme is an ATP: phosphotransferase. The holoenzymes have similar molecular weights and demonstrate similar molecular asymmetry. A structural model relating the two enzymes is proposed. cGMP-dependent protein kinase is proposed to be a dimer composed of two identical protomers in isologous association with the chains arranged in anti-parallel fashion. cAMP-dependent protein kinase is proposed to have a similar structure with a dyad axis of symmetry but with a discontinuity in each chain. These structures account for the differing mechanisms of cyclic nucleotide activation of the two enzymes.