Binding and reaction studies with adenine nucleotides on purified coupling factor from Rhodospirillum rubrum

Equilibrium binding studies with ATPase isolated from Rhodospirillum rubum chromotophores have been carried out using gel filtration. Binding experiments with variable concentrations of [14C]ADP show a biphasic saturation curve. With a parameter fitting computer program the dissociation constants for two distinct binding sites are determined as 7 x 10(-6) and 9 x 10(-5) M, respectively. The enzyme-bound radioactivity is recovered as ADP (80-90%), and the rest is converted to AMP and ATP. In the free nucleotides a large amount of AMP (about 70%) is found in addition to ADP. Analogous binding experiments with [14C]ATP are monophasic. Most of the bound radioactivity can be identified as ADP showing a dissociation constant corresponding to the high affinity site. The pattern of the free nucleotides is the same as in the experiments with ADP. These results indicate three separate binding sites on the enzyme: a low and a high affinity site for ADP, and a site at which ATP hydrolysis takes place. The analysis of the nucleotides suggests for the ADP sites a phosphoryl group transfer to produce ATP and AMP. Various experiments exclude the contamination of the enzyme preparation with adenylate kinase.