ABSTRACT
The interconversion of Fe3S4 and Fe4S4 clusters by uptake or loss of one iron ion, respectively, is a well-known phenomenon that represents the simplest conversion reaction of protein-bound iron-sulfur clusters (see [135-137] and references therein). This property has been exploited to prepare and characterize heterometal MFe3S4 clusters [138-142]. In general, a Fe3S4 cluster in a protein can be converted to a Fe4S4 cluster by addition of iron(II) under reducing conditions; the additional iron ion complements the cubane geometry by occupying the empty comer of the Fe3S4 clus ter. This iron ion retains a pseudotetrahedral coordination geometry by recruiting a fourth ligand either from the protein (often, but not necessarily, a cysteine) or from solution (e.g., a hydroxide ion). The reverse reaction requires oxidative removal of an iron ion from an [Fe4S4]2+ cluster to form an [Fe3S4]° cluster, as observed often during aerobic purification of iron-sulfur proteins (see [143] for an example). Also ferricyanide is long known to be an efficient oxidant for the generation of Fe3S4 centers from Fe4S4 clusters [144,145]. Two physiologically relevant examples of Fe3S4/Fe4S4 cluster conversions are the ferredoxin from Desulfovibrio gigas and the enzyme aconitase that is representative for a class of dehydratases [5].
