High-resolution proton nuclear magnetic resonance spectroscopy of cytochrome.

In cytochrome c the axial positions of the heme iron are occupied by two amino acid residues, one of which is known from X-ray studies to be histidyl. Nuclear magnetic resonance spectroscopy provides strong evidence that the sixth ligand is a methionyl residue in both the ferric and ferrous oxidation states. It is further shown that in cyanoferricytochrome c cyanide ion replaces methionyl in the first coordination sphere of the heme iron. Additional data are obtained on the protein conformation and on the electronic structure of the heme group in ferricytochrome c. As in other heme proteins, the interactions with the polypeptide chain greatly affect the unpaired electron distribution in the heme group of cytochrome c. In particular, from a comparison of ferricytochrome c and cyanoferricytochrome c, the importance of the coordination of the sixth ligand is apparent.