Iron-sulfur stoichiometry and structure of iron-sulfur clusters in three-iron proteins: Evidence for [3Fe-4S] clusters

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Abstract
Beef heart aconitase contains 3Fe clusters in its inactive and 4Fe clusters in its active form. The fully active form can be restored from the inactive one by insertion of Fe2+, whereas S2- is not required. Chemical analyses for iron and labile sulfide yield Fe/S2- ratios of 0.66-0.74 for the inactive and 0.90-1.03 for the active form. Sulfane sulfur (S0) was not detected. We propose on the basis of these data that the inactive form may arise from the active one by loss of one iron only per cluster with the sulfur remaining as S2- in a [3Fe-4S] structure. Measurements by extended x-ray absorption fine structure (EXAFS) spectroscopy on the 3Fe form of aconitase yield a Fe··S distance of 2.24 Å and a Fe··Fe distance of 2.71 Å. This Fe··Fe distance is in agreement with that obtained by EXAFS on ferredoxin II of Desulfovibrio gigas, another 3Fe protein, but disagrees with Fe··Fe distances observed for the 3Fe cluster of Azotobacter vinelandii ferredoxin I by x-ray diffraction—namely, 4.1 Å. We suggest that this difference may be due to the presence of a [3Fe-3S] structure in the Azotobacter ferredoxin I crystals vs. a [3Fe-4S] structure in liquid or frozen solutions of aconitase. The [3Fe-3S] cluster has been shown to have a relatively flat twist-boat structure, whereas a [3Fe-4S] cluster could be expected to essentially maintain the compact structure of the [4Fe-4S] cluster. This would explain the differences in Fe··Fe distances. Two possible structural models for a [3Fe-4S] cluster are discussed.