The recognition of a special ubiquinone functionally central in the ubiquinone-cytochrome b-c2 oxidoreductase.

Although the energy conserving membranes of the photosynthetic bacterium Rhodopseudomonas sphaeroides contain a 25 (+/- 3)-fold molar excess of ubiquinone over the photochemical reaction center, the activity of the ubiquinone-cytochrome b-c2 oxidoreductase is unaffected by quinone extraction until only 3, or at most 4, ubiquinones remain; only then does further extraction prevent the function of the oxidoreductase. Since 2 of these last ubiquinones are integral parts of the photochemical reaction center, we conclude that the ubiquinone-cytochrome b-c2 oxidoreductase requires only 1, or at most 2, molecules of ubiquinone-10 for its function. Earlier kinetic data identified a major electron donor to ferricytochrome c2 as a single molecule (known as Z) which requires 2 electrons and 2 protons for its equilibrium reduction. Hence, we identify a single molecule of quinone, probably ubiquinone-10 in a special environment, as a major electron donor to ferricytochrome c2 in the ubiquinone cytochrome b-c2 oxidoreductase.