Cytochrome b50 as a proton carrier in the photosynthetic redox chain of purple bacteria.

Recent data on the proton-translocating activity of b cytochromes in chromatophores of purple bacteria and their arrangement in the photosynthetic redox chain are discussed. These data appear to support the concept of the b50 and b-90 cytochrome doublet spanning the membrane. Current schemes of H+ transport by b cytochromes are considered, and the scheme of H+ translocation by cytochrome b50 taking up H+ at the outer side of the membrane and a quinone delivering them from this cytochrome to the inner space of the chromatophore is favored as the most probable in the light of recent findings. This scheme is applicable both to Crofts' linear model of the redox chain and to Mitchell's Q cycle. Kinetic discrepancies between H+ uptake and cytochrome b50 reduction at high ambient redox potentials are interpreted in terms of a special, cytochrome b50-independent, yet Rieske FeS-protein-dependent mode of H+ transport.