Inorganic phosphate-dependent ADP binding on the chloroplast coupling factor and its participation in ATP synthesis.

ADP binding brought about by inorganic phosphate addition (Pi-dependent ADP binding) on membrane-bound chloroplast coupling factor was studied and the following results were obtained. Under energization by illumination or by acid-base transition, Pi brought about the binding of ADP with an apparent Km value of 0.22 mM. This effect of Pi was lost rapidly after turning the light off or after acid to base transition, concomitant with the loss of ATP synthesizing activity. Pi-dependent ADP binding was inhibited by phlorizin to nearly the same extent as was ATP synthesis. The inhibitory effects of phlorizin on both the Pi-dependent ADP binding and ATP synthesis increased with the decrease of Pi concentration. These results suggest that the Pi-dependent ADP binding reaction participates in the ATP synthesis reaction and that phlorizin inhibits the P1 binding process.