The electric field generated by photosynthetic reaction center induces rapid reversed electron transfer in the bc1 complex

The cytochrome bc(1) complex is the central enzyme of respiratory and photosynthetic electron-transfer chains. It couples the redox work of quinol oxidation and cytochrome reduction to the generation of a proton gradient needed for ATP synthesis. When the quinone processing Q(i)- 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 sites, are also expected to be inhibited. However, we have observed in Rhodobacter sphaeroides chromatophores, that when a fraction of heme b(H) is reduced, flash excitation induces fast (half-time approximately 0.1 ms) oxidation of heme b(H), even in the presence of antimycin and myxothiazol. The sensitivity of this oxidation to ionophores and uncouplers, and the absence of any delay in the onset of this reaction, indicates that it is due to a reversal of electron transfer between b(L) and b(H) hemes, driven by the electrical field generated by the photosynthetic reaction center. In the presence of antimycin A, but absence of myxothiazol, the second and following flashes induce a similar ( approximately 0.1 ms) transient oxidation of approximately 10% of the cytochrome b(H) reduced on the first flash. From the observed amplitude of the field-induced oxidation of heme b(H), we estimate that the equilibrium constant 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 dimeric structure of bc(1) complex.