A new electrogenic step in the ubiquinol:cytochrome c2 oxidoreductase complex of Rhodopseudomonas sphaeroides.

Myxothiazol, an inhibitor of the ubiquinol oxidase site of the ubiquinol:cytochrome c2 oxidoreductase complex, has been shown in the present work to inhibit a part of the electrogenic process indicated by phase III of the carotenoid change, in addition to the part of the change inhibited by antimycin. This finding shows that there is an antimycin-insensitive, but myxothiazol-sensitive portion of the slow phase, which indicates the existence of an electrogenic event within the ubiquinol:cytochrome c2 oxidoreductase complex, in addition to that linked to oxidation of cytochrome b-561 which has been previously characterized. Redox titrations show that the appearance of the new electrogenic step is correlated with the amount of cytochrome b-561 available in the oxidized form before the flash. The rate of the antimycin-insensitive and myxothiazol-sensitive portion of the carotenoid change correlates well with the rate of reduction of cytochrome b-561. No carotenoid change associated with reduction of cytochrome b-566 was seen. These findings suggest that the newly identified electrogenic process is linked to electron transfer between cytochrome b-566 and b-561. Calculations of the contribution of this new electrogenic step to the total electrogenic event within the complex show that electrons passing from cytochrome b-566 to cytochrome b-561 pass about 35-50% of the distance across the whole membrane.