The effect of phosphate on light-induced exchange of ADP at the tight nucleotide binding site of CF1.

The effects of phosphate on the energy-dependent exchange of the tightly bound adenine nucleotides in chloroplast thylakoids has been studied. 1. Phosphate was found to have a dual effect on ADP binding; at low light intensities Pi enhanced the rate of binding and the total amount of tightly bound ADP, while at saturating light intensities, it decreased the level of bound nucleotides. Half-maximal stimulation or inhibition of ADP binding occurred at about 30 microM phosphate. 2. Phosphate inhibition of ADP binding at saturating light intensities is prevented in the presence of an ADP-regenerating system. Pi had no effect on the amount of ATP bound to chloroplast coupling factor 1 (CF1), which was lower than the amount of ADP bound. 3. Arsenate acted similarly to phosphate at low light intensities but not at saturating light intensities. 4. At low light intensities the interaction of phosphate with the membrane-bound CF1 increased the binding affinity for ADP about 8-fold. 5. Kinetic analysis of the ADP binding showed that phosphate increased the rate constant for ADP binding to the adenine nucleotide-depleted form of CF1.