Involvement of an essential arginyl residue in the coupling activity of Rhodospirillum rubrum chromatophores.

The arginine reagents phenylglyoxal and 2,3-butanedione in borate buffer completely inhibited photophosphorylation and Mg-ATPase of Rhodospirillum rubrum chromatophores. The inactivation rates followed apparent first order kinetics. Oxidative phospho-rylation and the light-dependent ATP-P_i exchange reactions ofR. rubrum chromatophores and the Ca-ATPase activity of the soluble coupling factor were similarly inhibited by 2,3-butanedione in borate buffer. The apparent order of reaction with respect to inhibitor concentrations for all these reactions gave values of near 1 suggesting that inactivation was the consequence of modifying one arginine per active site. ATP synthesis and hydrolysis by R. rubrum chromatophores were strongly protected against inactivation by ADP and ATP, respectively, and by other nucleotides that are substrates of the reactions but not by the products. Similarly, the Ca-ATPase of the soluble coupling factor was protected by ATP but not by ADP. Inactivation of chromatophores reactions by butanedione in borate buffer was more rapid in the light than in the dark. The results suggest that the catalytic sites for ATP synthesis and hydrolysis on the chromatophore coupling factor are different and both contain an essential arginine.