Role of Bound ADP in Photosynthetic ATP Formation by Chromatophores from Rhodospirillum rubrum

Abstract

1) Chromatophores from Rhodospirillum rubrum cells were bound with 0.14 nmole of ADP and 1–4 nmoles of P₁ per A_880nm unit (7.1 nmoles of bacteriochlorophyll). These values correspond to approximately 16 molecules and 110–440 molecules in each chromatophore of the average size, respectively. The binding of ADP was tight, but that of P₁ was loose; the exchange of the bound ADP for added ADP did not occur, whereas the exchange between the bound and added P₁ proceeded at a slow rate. 2) All the bound ADP was phosphorylated into ATP in the light (approximately 1,000 foot-candles); the ATP thus formed was also at the bound state. The phosphorylation was completed within 0.2 sec at 0°C. The phosphorylation of added ADP occurred after completion of the phosphorylation of the bound ADP. 3) The phosphorylation of the bound ADP was not inhibited either by oligomycin or by Triton X-100, both of which were potent inhibitors for the phosphorylation of added ADP. 4) Besides the bound ADP, approximately 0.022 nmole/A_880nm of added ADP was weakly adsorbed by chromatophores. This value corresponds to approximately 3 molecules in each chromatophore of the average size. The adsorption was prevented in the presence of oligomycin. 5) Possibly, the phosphorylation of the bound ADP is the primary step of the phosphorylation of added ADP, and the phosphoryl transfer from the resulting bound ATP to added ADP is inhibited by the oligomycin such bound on the chromatophore membrane as to prevent the access of added ADPs to the bound ATP.