Mutated forms of a [2Fe-2S] ferredoxin with serine ligands to the iron-sulfur cluster

The [2Fe-2S] ferredoxin from Clostridium pasteurianum contains five cysteine residues in positions 11, 14, 24, 56 and 60. Residues 24, 56 and 60 have been separately mutated into serine. The modified ferredoxins have been purified and were all found to contain a [2Fe-2S]-type cluster. The electronic absorption and EPR spectra of the C24S protein were only slightly different from those of the native one. In contrast, the C56S and C60S ferredoxins displayed spectroscopic features witnessing the presence of an oxygen ligand to the iron-sulfur cluster: the UV-visible absorption bands were shifted to higher energy by ca. 20 nm, and the high field components of the EPR spectra were shifted from gx = 1.92 and gy = 1.95 to gx = 1.88 and gy = 1.92, respectively.