Reference:
Lauble, H., Kennedy, M. C., Emptage, M. H., Beinert, H., Stout, C. D.:
The reaction of fluorocitrate with aconitase and the crystal structure of the enzyme-inhibitor complex.
Proc Natl Acad Sci U S A 93 pp. 13699 (1996)
Aconitase has four iron atoms in a cluster in the center. It also clusters with four sulfur atoms, creating a cluster of the two atoms.
The bond distances between each sulfur and each of its three closest iron atoms range from 2.233 A to 2.418 A.
The first ion was bonded to a cysteine with a length of 2.356 A. The second iron bonded to a cysteine with a length of 2.345 A.
The third iron also bonded to a cysteine with a bong length of 2.276 A. The last iron didn't have any amino acid closer than 3.944 A.
The iron to iron distance ranges from 2.657 A to 2.913 A.
It seems at though the iron coordinates to three sulfur atoms, and then a sulfur in a cysteine.
I wonder if the fourth iron should have a cysteine present too, but it just didn't show up for some reason.
Maybe there is water there? The coordination number would then be 4.