The electric field generated by photosynthetic reaction center induces rapid reversed electron transfer in the bc(1) complex

The cytochrome be, complex is the central enzyme of respiratory and photosy nthetic electron-transfer chains. It couples the redox work of quinol oxida tion and cytochrome reduction to the generation of a proton gradient needed for ATP synthesis. When the quinone processing Q(j)- and Q(o)-sites of the complex are inhibited by both antimycin and myxothiazol, the flash-induced kinetics of the b-heme chain, which transfers electrons between these site s, are also expected to be inhibited. However, we have observed in Rhodobac ter sphaeroides chromatophores, that when a fraction of heme bit is reduced , flash excitation induces fast (half-time similar to0.1 ms) oxidation of h eme b(H), even in the presence of antimycin and myxothiazol. The sensitivit y of this oxidation to ionophores and uncouplers, and the absence of any de lay in the onset of this reaction, indicates that it is due to a reversal o f electron transfer between b(L) and b(H) hemes, driven by the electrical f ield generated by the photosynthetic reaction center. In the presence of an timycin A, but absence of myxothiazol, the second and following flashes ind uce a similar (similar to0.1 ms) transient oxidation of similar to 10% of t he cytochrome bH reduced on the first flash. From the observed amplitude of the field-induced oxidation of heme bH, we estimate that the equilibrium c onstant for sharing one electron between hemes b(L) and b(H) is 10-15 at pH 7. The small value of this equilibrium constant modifies our understanding of the thermodynamics of the Q-cycle, especially in the context of a dimer ic structure of be, complex.