Slow electrogenic events in the cytochrome bc1-complex of Rhodobacter sphaeroides. The electron transfer between cytochrome b hemes can be non-electrogenic.

The flash-induced formation of transmembrane electric potential differences (measured by carotenoid bandshift) and redox changes of cytochrome bh (b561) were monitored spectrophotometrically in Rb. sphaeroides chromatophores in a pH range from 7.5 to 10.0. It is shown that in the presence of antimycin A and at pH less than 8.3 the myxothiazol-sensitive, antimycin-insensitive component of the carotenoid bandshift is kinetically coupled to cytochrome bh reduction. The kinetics of both processes can be described by a single exponent with a rise time of about 10 ms. Alkalization of the medium (8.3 less than or equal to pH less than or equal to 9.2) causes the appearance of an additional constituent in this phase of the carotenoid response with the rise time varying in the range of 100-300 ms. With a further pH increase (pH greater than 9.2), the electrogenic constituent, kinetically linked to cytochrome bh reduction, diminishes. The obtained data are discussed within the framework of the scheme, assuming that the electron transfer between bl and bh hemes in the bc1 complex is, under certain conditions, accompanied by proton transfer in the same direction.