Electrogenic events in chromatophores fromRhodobacter sphaeroides lacking high-potential cytochrome b of thebc1-complex

Abstract

The ubiquinol:cytochrome c oxidoreductase (bc tcomplex), contains a cytochrome (cyt) b subunit, which contributes to both quinol oxidizing (Qz) and quinone reductase (Qc) sites. As shown by electrochromic measurements [1,2], the antimycin-insensitive electrogenic reaction linked to electron transfer between cyt bj and b h contributes about 50% of the total electrogenesis of the bcrcomplex. The nature of the remaining antimycin-sensitive electrogenic step is controversial. The antimycin sensitivity of both the forward and the reverse (in the presence of myxothiazol) reaction shows that the phase is linked to reduction of Q at the Qc site by cyt b h [1-5]. Direct electrometry [3] suggests that the electron transfer between b h and Qc is not electrogenic. Probably, the antimycin-sensitive electrogenesis is at least partially due to the protonation that accompanies the reduction of QcAnother possible electrogenic step could be the release of protons, trapped at the Qz site on oxidation of QH 2, as b, becomes oxidized [6]. The goal of the present work is to clarify these points by measurement of the flash-induced electrical potential difference (A~0) generated by chromatophores from Rhodobacter sphaeroides mutants which have lost the heme of cyt b,. Replacement of the axial ligands of cyt b, (His-lll by either Asp (Hl l lD) or Asn (Hll lN), or His-212 by Asp (H212D)) resulted in selective loss of cyt b, but retention of cyt b~ [7]. Flash kinetic studies of the H l l l N mutant showed that the