Alkaline isomerization of ferricytochrome c: identification of the lysine ligand.

Changes in the visible absorbance spectra of complexes of horse heart cytochrome c hemopeptide 1-65, peptide 66-104, and their guanidinated counterparts are compared with those characteristic of native and fully guanidinated ferricytochrome c over the pH range 7 to 11. Upon raising the pH, the methionine ligand in the guanidinated hemopeptide 1-65.peptide 66-104 complex is replaced by a strong field ligand. By contrast, the methionine ligand in the hemopeptide 1-65.guanidinated peptide 66-104 is replaced by a weak field ligand. These results demonstrate that lysine 13 does not ligate with the heme iron upon isomerization of ferricytochrome c and that the ligand in the horse heart protein is one of the eight lysine residues in the 66-104 segment of the polypeptide, most likely lysine 79.