Replacement of Methionine as the Axial Ligand of Achromobacter cycloclastes Cytochrome C₅₅₄ at High pH Values Revealed by Absorption, EPR and MCD Spectroscopy

Cytochrome c₅₅₄ from the denitrifying bacterium Achromobacter cycloclastes is a monoheme class II c-type cytochrome with a His-Met axial coordination at neutral pH. The amino acid composition and the N-terminal sequence of the cytochrome have been determined. Subsequent determination of the pH-dependence of the redox potential and examination of the EPR and MCD spectra of ferricytochrome c₅₅₄ revealed a new form at high pH values made apparent with both spectroscopies. These observations are consistent with the presence of lysine as the axial ligand for which methionine substitutes at high pH values.