Cytochrome c′, a c-type cytochrome with unique spectroscopic and magnetic properties, has been characterized in a variety of denitrifying and photosynthetic bacteria. Cytochrome c′ has a role in defence and/or removal of NO but the mechanism of action is not clear. To examine the function of cytochrome c′ from Neisseria meningitidis, the protein was purified after heterologous overexpression in Escherichia coli. The electronic spectra of the oxidized c′ demonstrated a pH-dependent transition (over the pH range of 6–10) typical of known c′-type cytochromes. Interestingly, the form in which NO is supplied determines the redox state of the resultant haem-nitrosyl complex. Fe(III)–NO complexes were formed when Fe(II) or Fe(III) cytochrome c′ was sparged with NO gas, whereas an Fe(II)–NO complex was generated when NO was supplied using DEA NONOate (diazeniumdiolate).

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