A comparison of Fe 4 S 4 clusters in high-potential iron protein and in ferredoxin

The structures of both oxidized (HP(ox)) and reduced (HP(red)) high-potential iron protein and of oxidized ferredoxin (Fd(ox)) have been partially refined at 2.0-A resolution by methods similar to those applied to the protein rubredoxin [Watenpaugh, K. D., Sieker, L. C., Herriott, J. R. & Jensen, L. H. (1971) Cold Spring Harbor Symp. Quant. Biol. 36, 359-367]. Average bond lengths and angles in the HP(red) and Fd(ox) Fe(4)S(4) (*) clusters are the same to within the root-mean square (rms) deviation of each mean value. A preliminary comparison of the two HiPIP oxidation states indicates that the HP(ox) cluster is geometrically similar to the HP(red) cluster, but that it is smaller by 0.1-0.2 A in certain dimensions. The HiPIP and ferredoxin cluster geometry is also nearly identical to that reported recently for a synthetic analog [Herskovitz, T., Averill, B. A., Holm, R. A., Ibers, J. A., Phillips, W. D. & Weither, J. F. (1972) Proc. Nat. Acad. Sci. USA 69, 2437-2441]. An apparent paradox presented by the large difference between the HiPIP and ferredoxin electrode potentials can be resolved by the assumption that the Fe(4)S(4) (*) cluster has not two but three oxidation states. The fully oxidized (HP(ox)) and fully reduced (Fd(red)) clusters are paramagnetic, and the intermediate state is spin-paired [Tsibris, J. C. M. and Woody, R. W. (1970) Coord. Chem. Rev. 5, 417-458]. This hypothesis is supported by structural and spectroscopic evidence that the "paired-spin" state exists in both HP(red) and Fd(ox).