Ferredoxins from Bacillus polymyxa. Low potential iron-sulfur proteins which appear to contain single four iron, four sulfur centers accepting a single electron on reduction.

Two iron-sulfur proteins from Bacillus polymyxa (ferredoxins I and II) have been shown to transfer 1 electron per 4 iron atoms at a potential of -380 and -420 mv, respectively, at pH 7. The proteins contain 4 iron and 4 labile sulfur atoms and 4 cysteine residues in molecules of about 9,000 daltons; electron paramagnetic resonance and optical properties suggest that each contains an iron-sulfur cluster similar to one of those present in bacterial-type ferredoxins which contain 8 iron atoms distributed between two clusters. These proteins therefore represent an example of a subclass of the bacterial-type ferredoxins, in which 4 iron atoms appear to occur in a cluster which transfers 1 electron at a potential near that of the hydrogen couple. These potential, electron paramagnetic resonance, and absorbance properties distinguish this subclass of proteins from the “high potential iron-sulfur proteins” of Chromatium sp. and Rhodopseudomonas sp.