Binding of cyanide to cytochrome c' from Chromatium vinosum

Spectroscopic evidence is presented which demonstrates the binding of cyanide to the ferric cytochrome c' from Chromatium vinosum. The cytochrome was shown to bind one equivalent of cyanide with an equilibrium constant of 2.1 X 10(4) at pH 7.0 and 25 degrees C. This finding represents the first observation of the binding of an anionic ligand to the heme iron in a ferric cytochrome c'. These results suggest that the binding site of the ferric Chromatium cytochrome c' may be significantly more accessible than previously indicated.