Conformation of Cytochromes: II. COMPARATIVE STUDY OF CIRCULAR DICHROISM SPECTRA, OPTICAL ROTATORY DISPERSION, AND ABSORPTION SPECTRA OF HORSE HEART CYTOCHROME c

Abstract

The circular dichroism spectra of horse heart ferri- and ferrocytochrome c in the absence and presence of 9 M urea, pH 7.0, in the wave length region 188 to 600 mµ have been examined and the results were compared with results of optical rotatory dispersion and absorption spectra. The circular dichroism spectra of both ferri- and ferrocytochrome c in the region below 240 mµ exhibit characteristics of both an α-helix, namely, the negative minima at about 222 and 210 mµ and a positive maximum at 190 mµ, and of β structurally organized polypeptide chain, a shoulder at 218 mµ and a peak at 195 mµ. The change of oxidation state of heme iron seems to be devoid of any significant effect on the dichroic pattern in this wave length region. The dichroic spectra in the near ultraviolet and the visible spectral regions are exceedingly rich in detail and exhibit strong dependence on the valence state of the protein. The reduction of the protein results in a significant increase in the rotatory strengths of the porphyrin transitions (300 to 370 mµ), the inversion of the dichroic pattern in the Soret absorption region, possible change of sign of the dichroic bands corresponding to the aromatic absorption bands, and replacement of the broad dichroic peaks above 500 mµ by multiple small positive peaks. An internally compensated conformational change during the oxidation-reduction cycle, including the possibility of switching of the centrally coordinated amino acid side chain or the alteration of the geometry of heme iron or both, has been discussed.