Characterization of cytochrome b in the isolated ubiquinol‐cytochrome c2 oxidoreductase from Rhodopseudomonas sphaeroides GA

Abstract

Extinction coefficients for cytochrome b and c ₁ in the isolated cytochrome bc ₁ complex from Rhodopseudomonas sphaeroides GA have been determined. They are 25 mM⁻¹.cm⁻¹ at 561 nm for cytochrome b and 17.4 mM⁻¹.cm⁻¹ at 553 nM for cytochrome c ₁ for the difference between the reduced and the oxidized state. Cytochrome b is present in two forms in the complex. One form has an E _m7 of 50 mV, an α-peak of 557 nm at liquid N₂ temperature and of 561 nm at RT, which is red-shifted by antimycin A. The other form has an E _m7 of −90 mV, a double α-peak of 555 and 561 nm at liquid N₂ temperature corresponding to 559 and 566 nm at RT. The absorption at 566 nm is red-shifted by myxothiazol. The two shifts are independent of each other. Both midpoint potentials of cytochromes b are pH-dependent. The redox center compositions of the cytochrome bc ₁ complexes from Rhodopseudomonas sphaeroides and from mitochondria are identical.