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X-ray crystallographic study of the ferredoxin-like protein (iron-sulfur protein
III) from Azotobacter vinelandii has been extended to 2.5-A resolution. A 4.0-A
resolution electron density map revealed that the molecule contains two Fe-S
clusters of differing size and shape separated by 12 A (Stout, C.D. (1979)
Nature 279, 83-84). Recent Mössbauer results by Emptage et al. (Emptage, M.H.,
Kent, T.A., Huynh, B.H., Rawlings, J., Orme-Johnson, W.H., and Münck, E. (1980)
J. Biol. Chem. 255, 1793-1796) have shown that the molecule contains 7 iron
atoms as a high potential iron protein-like Fe4 center and a novel 3-Fe center.
The 2.5-A electron density map shows two distinctly different Fe-S clusters. The
larger cluster consists of a tetranuclear [4Fe-4S] core ligated to the protein
at each iron atom. The smaller cluster is distinctly planar and cannot be
modeled with [2Fe-2S] or [4Fe-4S] structures. The best model for this cluster is
a [3Fe-3S] core. The protein makes six contacts with this cluster.
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