title: Characterization of the interchain and intrachain interactions between the binding sites of the free regulatory moiety of protein kinase I.
abstract: The interaction between the four binding sites (two A sites and two B sites) of the regulatory subunit dimer of protein kinase I (RI2) was studied. The rate of association of c[3H]AMP to site B was slower when site A had already been occupied. Occupation of site A also retarded the rate of dissociation of c[3H]AMP from site B. This site A-B interaction was intrachain since it was observed also for a monomeric fragment of RI2. Thus, each monomer of RI2 must have one A site and one B site. Quantitative analysis of the rate constants for cAMP binding to variously liganded RI2 suggested little or no thermodynamic coupling between site A and B. This conclusion was supported by equilibrium binding data. Occupation of one A site retarded the dissociation of c[3H]AMP from the A site of the other subunit (interchain interaction). The rate kinetic constants as well as equilibrium binding data indicated a positively cooperative site A-A interaction. The interaction between cAMP and either site was enthalphy-driven (25 degrees C), the process being accompanied by a loss of entropy. The thermodynamic parameters did not support the occurrence of an abrupt conformational change at a certain level of ligandation of RI2. Half-maximal saturation of either site occurred at 1-2 nM cAMP (37 degrees C, pH 7.0, 0.15 M KCl). The concentration of RI2 did not detectably influence any binding parameters. Aging of RI2 produced a form with minimally, if at all, altered Mr, but which showed a more rapid release of c[3H]AMP bound to site B.
mesh_terms: D015124:8-Bromo Cyclic Adenosine Monophosphate / Q000378:metabolism; D000818:Animals; D001665:Binding Sites; D000242:Cyclic AMP / Q000378:metabolism; D007700:Kinetics; D046911:Macromolecular Substances; D008970:Molecular Weight; D009132:Muscles / Q000201:enzymology*; D011485:Protein Binding; D011487:Protein Conformation; D011494:Protein Kinases / Q000302:isolation & purification / Q000378:metabolism*; D011817:Rabbits
chemical_list: D046911:Macromolecular Substances; D015124:8-Bromo Cyclic Adenosine Monophosphate; D000242:Cyclic AMP; D011494:Protein Kinases