Circular dichroic spectroscopy of membrane haemoproteins. The molecular determinants of the dichroic properties of the b cytochromes in various ubiquinol:cytochrome c reductases

The circular dichroism (CD) of dihaem cytochrome b from mitochondrial and bacterial ubiquinol:cytochrome-c reductase (bc1 complex) has been characterized. The dichroic properties of the yeast purified cyt b are very similar to those of the native cyt b within the mitochondrial bc1 complex. The CD spectra in the Soret region of the native cytochrome b present in all species studied show an intense bisignate Cotton effect having a zero-crossing wavelength close to the absorbance maximum. In preparations partially or completely depleted of the low-potential b haem (b1) the CD spectra exhibit a single positive Cotton effect resembling the corresponding absorption spectrum. This is particularly evident in the purified cytochrome b-562 from Rhodobacter sphaeroides R26, which contains only the high-potential b haem (bh). These spectral features together with the reconstitution of the cytochrome b1 haem have been used to resolve the CD contribution of each haem to the CD spectra of cytochrome b. The mechanisms which might be responsible for the optical activity have been examined. It appears that the CD spectra of cytochrome b derive from both the mutual interaction of its two haems (giving rise to exciton coupling) and to the interaction of each haem with nearby aromatic residues, other than the pairs of histidines which coordinate the iron. The dipole coupling between haem and aromatic residues appears to be more important than exciton coupling in the CD spectra of oxidized b cytochromes and correlations have been made between the CD features and the proposed structure of cytochrome b.