A Comparison of Fe4S4 Clusters in High-Potential Iron Protein and in Ferredoxin
Abstract
The structures of both oxidized (HPox) and reduced (HPred) high-potential iron protein and of oxidized ferredoxin (Fdox) have been partially refined at 2.0-Å 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 HPred and Fdox Fe4S4 * 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 HPox cluster is geometrically similar to the HPred cluster, but that it is smaller by 0.1-0.2 Å 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 Fe4S4 * cluster has not two but three oxidation states. The fully oxidized (HPox) and fully reduced (Fdred) 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 HPred and Fdox.
Footnotes
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↵ ‡ This work is taken from a thesis submitted in partial fulfillment of the requirements for the degree of Doctor of Philosophy at the University of California, San Diego.
