Characterization of a new type of ferredoxin from Desulfovibrio africanus

A new ferredoxin designated ferredoxin III has been isolated from Desulfovibrio africanus grown on media high in iron. Native ferredoxin III is a dimer constituted by two identical subunits of approx. 7500. It is distinguished from the two other ferredoxins (I and II) isolated from this microorganism by its amino acids composition, N-terminal sequence, spectral properties and iron-sulfur content. The amino acid composition of D. africanus ferredoxin III is typical of ferredoxins with an excess of acidic over basic residues and the absence of histidine and arginine residues. The absorption spectrum of ferredoxin III exhibits two maxima, at 408 nm (epsilon = 58.5 . 10(3) M-1 . cm-1) and 285 nm (epsilon = 82 . 10(3) M-1 . cm-1), with a shoulder at 305 nm (epsilon = 75 . 10(3) M-1 . cm-1). Its A408/A285 absorbance ratio is 0.78. Ferredoxin III contains approx. 12--13 atoms each of iron and labile sulfur. This is in agreement with the high value of the extinction coefficient at 408 nm, which is slightly higher than 3-fold that of one [4Fe-4S] cluster. However, the number of cysteine residues of the protein (six residues), which is about the half that of iron atoms, is indicative of the presence of a new type of iron-sulfur cluster in ferredoxin III. The protein is unstable in a low ionic strength environment; the addition of neutral salts stabilizes the protein conformation. The data on the biological activity of ferredoxin III as compared to the two other ferredoxins from D. africanus show that the three iron-sulfur proteins function with equal effectiveness as electron carrier in the phosphoroclastic reaction and the H2-sulfite reductase system.