Membrane-bound cytochrome cz couples quinol oxidoreductase to the P840 reaction center complex in isolated membranes of the green sulfur bacterium Chlorobium tepidum.

The reaction of quinol oxidoreductase and membrane-bound c-type cytochromes was studied in chlorosome-depleted membranes isolated from Chlorobium tepidum. Rapid oxidations of c-type cytochromes were detected after flash excitation. Their re-reductions occurred in parallel with the reduction of cytochrome b, especially in the presence of antimycin A, whereas reductions of both cytochromes c and b were suppressed by added stigmatellin. These results indicate the tight coupling between the photosynthetic reaction center and quinol oxidoreductase. Turnovers of two types of cytochromes c were detected. One was assigned to the monoheme-type cytochrome c (designated cytochrome cz), which is known to be tightly bound to the reaction center complex. The other was a new c-type cytochrome, cytochrome c-556, which functions the same as cytochrome c1. The steps of electron-transfer scheme, menaquinol --> Rieske FeS center --> cytochrom c-556 --> cytochrome cz --> P840, are estimated to have reaction times of 20 ms and 560, 150, and 40 microseconds, respectively. We conclude that quinol oxidoreductase and the reaction center complex in Chlorobium tepidum are linked by two distinct membrane-bound cytochromes, cz and c-556, with no involvement of water-soluble cytochromes.