Imidazole, the ligand trans to mercaptide in ferric cytochrome P-450. An EPR study of proteins and model compounds.

A crystal field analysis of EPR data for various low spin ferric cytochromes P-450 suggests that in all of them, regardless of source or method of induction, the heme ligands are a sulfur atom, presumably from cysteine, and an imidazole from histidine. The imidazole can be displaced in the ferric protein by cyanide, guanidine, or by an amine, analogous to its displacement by CO or NO in the ferrous protein. The resulting changes in the EPR parameters for the ferric protein are consistent with similar substitutions in heme thiol model compounds. The analysis of the latter can be understood on the basis of alterations of the electronic structure of the ligands to the heme iron.