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

Abstract

The Rieske iron-sulfur center in the photosynthetic bacterium Rhodopseudomonas sphaeroides appears to be the direct electron donor to ferricytochrome c 2, reducing the cytochrome on a submillisecond timescale which is slower than the rapid phase of cytochrome oxidation ( t 1 2 3–5 μ s ). 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 (1–2 ms) antimycin-sensitive phase of ferricytochrome c 2 reduction, attributed to a specific ubiquinone-10 molecule (Q z), 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 Q z (Q zH 2) reduces the Rieske center in an antimycin-sensitive reaction. From the extent of the UHDBT-sensitive ferricytochrome c 2 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 g y 1.90 to g y 1.89, and shifts the E m,7 from 280 to 350 mV. While this latter shift may account for the subsequent failure of the iron-sulfur center to reduce ferricytochrome c 2, 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 H + II; these may reflect additional sites of action of the inhibitor.