Dependency on Environmental Redox Potential of Photophosphorylation in Rhodopseudomonas spheroides

An all-glass apparatus was constructed which allowed measurement of photophosphorylation at different redox potentials. The luciferin-luciferase assay was adapted so that ATP could be measured within a few minutes of its formation. Optimal photophosphorylation activity in chromatophores from Rhodopseudomonas spheroides under saturating white light was 150 μmol of ATP/hr per mg of bacteriochlorophyll and was found at an environmental redox potential near 0 V at pH 7.9. Activity decreased at lower potentials, with a midpoint for the transition occurring at –0.09 V at pH 7.9 and –0.03 V at pH 7.0. The latter value is close to that for decreased phototrap activity measured previously by following light-induced absorbance changes (E₀′ = –0.02 V). Photophosphorylation activity also decreased at higher potential with an apparent midpoint at 0.30 V. This is the first measurement of such a redox-linked component in photophosphorylation.