text: A common motif features a four iron ions and four sulfide ions placed at the vertices of a cubane-type cluster.  The Fe centers are typically further coordinated by cysteinyl ligands. The [FeS] electron-transfer proteins ([FeS] ferredoxins) may be further subdivided into low-potential (bacterial-type) and high-potential (HiPIP) ferredoxins. Low- and high-potential ferredoxins are related by the following redox scheme:
  In HiPIP, the cluster shuttles between [2Fe, 2Fe] (FeS) and [3Fe, Fe] (FeS). The potentials for this redox couple range from 0.4 to 0.1 V. In the bacterial ferredoxins, the pair of oxidation states are [Fe, 3Fe] (FeS) and [2Fe, 2Fe] (FeS).  The potentials for this redox couple range from −0.3 to −0.7 V. The two families of 4Fe–4S clusters share the FeS oxidation state.  The difference in the redox couples is attributed to the degree of hydrogen bonding, which strongly modifies the basicity of the cysteinyl thiolate ligands. A further redox couple, which is still more reducing than the bacterial ferredoxins is implicated in the nitrogenase.
  Some 4Fe–4S clusters bind substrates and are thus classified as enzyme cofactors. In aconitase, the Fe–S cluster binds aconitate at the one Fe centre that lacks a thiolate ligand.  The cluster does not undergo redox, but serves as a Lewis acid catalyst to convert citrate to isocitrate.  In radical SAM enzymes, the cluster binds and reduces S-adenosylmethionine to generate a radical, which is involved in many biosyntheses.
  The second cubane shown here with mixed valence pairs (2 Fe3+ and 2 Fe2+), has a greater stability from covalent communication and strong covalent delocalization of the “extra” electron from the reduced Fe2+ that results in full ferromagnetic coupling.