Reductive Titrations of Iron-Sulfur Proteins Containing Two to Four Iron Atoms

Anaerobic reductive titrations with solid dithionite were carried out on seven iron-sulfur proteins. Of these, six were of the plant ferredoxin type with 2 iron and 2 labile sulfur atoms per molecule in the molecular weight range of from 12,000 to 25,000. In addition, the high potential iron protein of Chromatium with 4 iron and labile sulfur atoms per molecule was titrated. The electron paramagnetic resonance (EPR) and in most cases also the optical spectra were measured as titration progressed. In the ferredoxin group of proteins the end point was determined by maximal development of the characteristic EPR signal (g⊥ ≈ 1.94; g|| ≈ 2.0) of the reduced forms and maximal bleaching of color. With the protein from Chromatium, disappearance of the EPR signal at g = 2 of the oxidized form indicated the end point. By these criteria it was found that in every instance 1 electron is taken up per molecule of protein, i.e. 1 electron/2 iron atoms by the ferredoxin-type compounds and 1 electron/4 iron atoms by the Chromatium protein. On double integration of the EPR signals recorded at temperatures between 30 and 80° K, the reducing equivalents taken up by these proteins could be accounted for quantitatively. These findings are in agreement with the results of other approaches which indicate that these proteins have a 2-iron center which is able to accommodate 1 additional electron on reduction.