An Isc-type extremely thermostable [2Fe-2S] ferredoxin from Aquifex aeolicus. Biochemical, spectroscopic, and unfolding studies

Analysis of the genome of the hyperthermophilic bacterium Aquifex aeolicus has revealed the presence of a previously undetected gene potentially encoding a plant- and mammalian-type [2Fe−2S] ferredoxin. Expression of that gene in Escherichia coli has yielded a novel thermostable [2Fe−2S] ferredoxin (designated ferredoxin 5) whose sequence is most similar to those of ferredoxins involved in the assembly of iron−sulfur clusters (Isc-Fd). It nevertheless differs from the latter proteins by having deletions near its N- and C-termini, and no cysteine residues other than those involved in [2Fe−2S] cluster coordination. Resonance Raman, low-temperature MCD and EPR studies show close spectral similarities between ferredoxin 5 and the Isc-Fd from Azotobacter vinelandii. Mossbauer spectra of the reduced protein were analyzed with an S = 1/2 spin Hamiltonian and interpreted in the framework of the ligand field model proposed by Bertrand and Gayda. The redox potential of A. aeolicus ferredoxin 5 (−390 mV) is in keepi...