Di-cluster, seven-iron ferredoxins from hyperthermophilic Sulfolobales

Seven-iron ferredoxins from the thermoacidophilic archaea Acidianus ambivalens, A. infernus, Metalosphaera prunae and Sulfolobus metallicus were extensively characterised, allowing study of their expression under aerobic and anaerobic growth conditions as well as the putative role in thermal stability of a recently described zinc centre. The archaeon S. metallicus was found to express, under the same growth conditions, two ferredoxins in almost identical amounts, a novelty among Archaea. Most interestingly, these two ferredoxins differ at the N-terminal amino acid sequence in that one has a zinc binding motif (FdA) and the other does not (FdB); in agreement with these:findings, FdA contains a zinc ion and FdB does not. These two ferredoxins have identical thermal stabilities, indicating that the zinc atom is not determinant in the protein thermostability. Further, the presence of the additional zinc centre does not interfere with the redox properties of the iron-sulfur clusters since their reduction potentials are almost identical. From the other three archaea, independently of the growth mode in respect to oxygen, only a single zinc-containing ferredoxin was found. EPR studies on the purified proteins, both in the oxidised and dithionite reduced states, allowed the identification of one [3Fe-4S](1+/0) centre and one [4Fe-4S](2+/1+) centre in all proteins studied. The complete sequence of A. ambivalens ferredoxin is reported. Together with the data gathered in this study, the properties of the seven-iron ferredoxins from Sulfolobales so far known are re-discussed.