Location of the heme moiety of cytochrome c by solvent perturbation.

The exposure of the heme moiety of cytochrome c to various solvents was investigated by the technique of solvent perturbation. The heme hexadecapeptide in a chymotryptic digest of the protein was used as a model compound for the fully exposed chromophore. While the Soret absorption of both ferriand ferrocytochrome c was enhanced in 20% ethylene glycol, the extent of enhancement, AC/E, was substantially less than that measured for the model compound or for the unfolded protein in 8 M urea. The percentage maximal perturbation of ferrocytochrome c was found to be independent of the radius of the perturbant over the range from 1.4 to 3.6 A. It was concluded that only a portion of the heme moiety of native cytochrome c is in contact with the solvent. Replacement of one of the two protein-heme coordinate-covalent bonds by cyanide increased the exposure of the heme to the solvent.