Two distinct quinone-modulated modes of antimycin-sensitive cytochrome b reduction in the cytochrome bc1 complex

Reduction of cytochrome b-560 (analogous to cyt b-562 of mitochondria) via an antimycin-sensitive route has been revealed in chromatophores of the photosynthetic bacterium, Rhodopseudomonas sphaeroides Ga. Indeed, the results suggest that two reductive mechanisms can be operative. One is consistent with the idea that the quinol generated at the reaction center Q B site enters the Q pool and, via the Q c site, equilibrates with cytochrome b-560. The other reductive mode circumvents redox equilibrium with the pool; we consider that this could result from a direct encounter of the reaction center with the bc 1 complex perhaps involving a direct Q B-Q C site interaction. This latter reaction is suppressed by occupancy of the Q C site, not only by antimycin but by ubiquinol and ubiquinone.