A Seven‐iron Ferredoxin from the Thermoacidophilic Archaeon Desulfurolobus ambivalens

A seven‐iron ferredoxin was isolated from aerobically grown cells of the hyperthermoacidophilic archaeon Desulfurolobus ambivalens (DSM 3772). The protein is monomeric, with an apparent molecular mass of 15 kDa and contains 7 iron atoms/molecule. The N‐terminal sequence shows a large similarity (70% identity) with that of the ferredoxin isolated from the archaeon Sulfolobus acidocaldarius. The EPR characteristics in both the native (oxidized) and dithionite‐reduced states of this protein allowed an unequivocal identification of a [3Fe‐4S]^1+/0 center, with a reduction potential of −270 ± 20 mV, at pH 7.5. The protein also contains a [4Fe‐4S]^2+/1+ center with a very low reduction potential (Eo = ‐540 mV, pH 7.0), which yields a rhombic EPR spectrum upon reduction with sodium dithionite at high pH. The reduction potentials of both centers are slightly pH dependent between pH 6 and 9. The [3Fe‐4S] ferredoxin center is able to accept electrons from pyruvate oxidase and NADH oxidase isolated from D. ambivalens. This ferredoxin is present in large amounts (at least 130 mg/kg wet cells), which allowed the unequivocal observation of oxidized [3Fe‐4S] clusters in intact D. ambivalens cells.