The role of the Rieske iron-sulfur center as the electron donor to ferricytochrome c2 in Rhodopseudomonas sphaeroides.

The Rieske iron-sulfur center in the photosynthetic bacterium Rhodopseudomonas sphaeroides appears to be the direct electron donor to ferricytochrome c2, reducing the cytochrome on a submillisecond timescale which is slower than the rapid phase of cytochrome oxidation (t 1/2 3-5 microseconds). The reduction of the ferricytochrome by the Rieske center is inhibited by 5-n-undecyl-6-hydroxy-4,7-dioxobenzothiazole (UHDBT) but not by antimycin. The slower (102 ms) antimycin-sensitive phase of ferricytochrome c2 reduction, attributed to a specific ubiquinone-10 molecule (Qz), and the associated carotenoid spectral response to membrane potential formation are also inhibited by UHDBT. Since the light-induced oxidation of the Rieske center is only observed in the presence of antimycin, it seems likely that the reduced form of Qz (QzH2) reduces the Rieske Center in an antimycin-sensitive reaction. From the extent of the UHDBT-sensitive ferricytochrome c2 reduction we estimate that there are 0.7 Rieske iron-sulfur centers per reaction center. UHDBT shifts the EPR derivative absorption spectrum of the Rieske center from gy 1.90 to gy 1.89, and shifts the Em,7 from 280 to 350 mV. While this latter shift may account for the subsequent failure of the iron-sulfur center to reduce ferricytochrome c2, it is not clear how this can explain the other effects of the inhibitor, such as the prevention of cytochrome b reduction and the elimination of the uptake of HII(+); these may reflect additional sites of action of the inhibitor.