Interactions between ADP and the coupling factor of photophosphorylation.

The coupling factor of photophosphorylation, which carries out the terminal steps in the light-dependent synthesis of ATP in spinach chloroplasts, forms tight complexes with [(14)C]ADP in vitro. The bound [(14)C]ADP undergoes a transphosphorylation reaction to give [(14)C]AMP and [(14)C]ATP. The [(14)C]ATP remains tightly bound, and can be recovered conveniently only by denaturation of the enzyme nucleotide complex. If spinach membranes are illuminated in the presence of pyocyanine and [(3)H]AMP or [(32)P]P(i), the enzyme can be recovered as a tight complex with [(3)H]ADP or [(32)P]ADP. The evidence indicates that AMP is an earlier acceptor of phosphate than is ADP, in a light-driven phosphorylation reaction. It also suggests that AMP serves as a cofactor in photophosphorylation.