Isolation, characterization, and biological activity of the Methanococcus thermolithotrophicus ferredoxin

A ferredoxin has been isolated from the thermophilic methanogen Methanococcus thermolithotrophicus. The native protein was a monomer exhibiting a molecular weight of 7,262, calculated from the amino acid composition. Its absorption spectrum had two maxima at 390 and 283 nm, with an absorbance ratio A390/A283 of 0.79. The absorption at 390 nm (E = 29 mM-1 cm-1) and the content of iron of the protein are in agreement with the presence of two 4Fe-4S clusters in M. thermolithotrophicus ferredoxin. Its amino acid composition showed the presence of eight cysteine residues, which is the required number of cysteines for the binding of two 4Fe-4S clusters. The protein was characterized by the lack of histidine, arginine, and leucine and a high content of valine. It was unusually stable to high temperatures but not to oxygen. The ESR spectrum of the protein in the oxidized state showed a minor signal at g = 2.01, corresponding to an oxidized 3Fe-4S cluster. The protein, which was difficult to reduce with dithionite or reduced mediators, exhibited in its reduced state a spectrum typical of two interacting reduced 4Fe-4S clusters. M. thermolithotrophicus ferredoxin functioned as an electron acceptor for the CO dehydrogenase complex with an extract free of ferredoxin. No reaction was detected with F420 or hydrogenase.