Iron-sulfur cluster

An iron-sulfur cluster is a structural motif found in certain metalloproteins, such as the ferrodoxins, as well as NADH dehydrogenase and Coenzyme Q - cytochrome c reductase of the electron transfer system. There are at least three distinct kinds of iron-sulfur clusters:

• The 2 iron, 2 sulfur cluster, consisting of two iron atoms, with two inorganic sulfur atoms found between the irons and acting as bridging ligands. The irons themselves are usually coordinated to the sulfurs of 4 cysteine side chains, or to 2 cysteine sulfurs, and 2 nitrogen atoms found in the side chain of histidine.
• The 4 iron, 4 sulfur cluster, consisting of 4 irons, with 4 inorganic sulfur atoms found between the irons and acting as bridging ligands. The irons, once again, are coordinated to the protein via the sulfurs of 4 cysteine side chains.
• The 3 iron, 4 sulfur cluster, which is a 4 iron, 4 sulfur cluster with one missing iron atom.

Iron-sulfur clusters can be involved in oxidation-reduction reactions, or can be involved in catalysis.

Examples of iron-sulfur clusters can be found here