On the interactions of adenosine 3',5'-monophosphate with the components of protein kinase I. A theoretical equilibrium analysis.

Cyclic-AMP-dependent protein kinase is activated through the dissociation of active catalytic subunits from a regulatory dimer. The regulatory subunit consists of four cyclic-AMP-binding sites and two binding sites for catalytic subunits. Under well defined experimental conditions, protein kinase activation obeys apparent positive cooperativity and is linearly coupled to cyclic AMP binding. The simulation of theoretical models is used for testing working hypotheses. Here we demonstrate that the proposed stoichiometry of protein kinase activation may account for the experimentally observed properties of the system. The restrictive conditions under which theory and experimental observations are compatible are: (1) functional dependence between the two monomers of the regulatory dimer, (2) the only complexes which can accumulate at equilibrium in the considered conditions are R2C2, (cyclic AMP)2R2C and (cyclic AMP)4R2 (where R and C are the regulatory and catalytic subunits of protein kinase). An experimental procedure is proposed in order to check the validity of the theoretical predictions. The determination of the sequence of events leading to activation or inactivation of protein kinase is discussed.