Proton and nitrogen-15 NMR studies of ferricytochrome c cyanide complexes: remarkable conservation of the heme environment among organisms of diverse origin.

The native ferric and cyanide-bound ferric forms of nine vertebrate and two yeast cytochromes c have been investigated by high-resolution proton nuclear magnetic resonance spectroscopy. Spectral comparisons have been made among the cytochromes with emphasis on the signal positions for heme and amino acid ligand protons. Consistent with earlier more limited studies of native ferric cytochromes c, the paramagnetically shifted proton NMR signals show little variation among species with up to 50% substitution of amino acids. Proton NMR spectra for the cyanide complexes also show little variation among species. The nitrogen-15 signal for the coordinated cyanide ion is known to be highly variable among other hemoproteins, but the signal covers a range of only 855 to 865 ppm (nitrate ion reference) for vertebrate cytochromes c and 884 to 886 ppm for yeast cytochromes c. The cyanide ligand probe thus reports an amazing conservation of the heme and proximal ligand environment among the cytochromes. Comparative proton and nitrogen-15 chemical shift values are consistent with a slightly stronger proximal histidine imidazole hydrogen bond to an amino acid carbonyl function than is the case for hemoglobin and myoglobin.