Iron-Sulfur Center
This is a 4-Fe, 4-S center from a fumarate reductase enzyme 1qla.
The 4 iron atoms are colored magenta to distinguish them from the 4 yellow sulfide ions and the 4 cysteine residues. This iron is tightly bound to the enzyme. Each iron is coordinated by 2 sulfide ions and by 2 Cysteine sulfurs.
(Note: a 5th Cys is shown, but it does not help coordinate this FeS center.)