Electrospray-ionization mass spectrometry of molecular variants of a [2Fe-2S] ferredoxin

The [2Fe-2S] ferredoxin from Clostridium pasteurianum is a homodimeric protein of which each subunit contains one [2Fe-2S] cluster. In previous investigations, the five cysteine residues in positions 11, 14, 24, 56 and 60 had been mutated into serine or alanine. The wild type ferredoxin and several of its molecular variants have now been analyzed by electrospray-ionization mass spectrometry. In the negative-ion detection mode, depending on the infusion solvent used, molecular peaks attributable to the apoprotein, to the monomeric holoprotein, and to the dimeric holoprotein were detected in all cases. The data confirmed the presence of the expected mutations, showed that all of these proteins contain one [2Fe-2S] cluster per subunit, and indicated that the dimeric structure of these ferredoxins could be retained in the conditions of the electrospray ionization. This investigation establishes the power of electrospray-ionization mass spectrometry for the analysis of oligomeric proteins containing labile metal clusters.