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

Extinction coefficients for cytochrome b and c1 in the isolated cytochrome bc1 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 c1, for the difference between the reduced and the oxidized state. Cytochrome b is present in two forms in the complex. One form has an Em7 of 50 mV, an alpha-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 Em7 of -90 mV, a double alpha-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 bc1 complexes from Rhodopseudomonas sphaeroides and from mitochondria are identical.