High potential iron–sulfur proteins (HiPIPs) form a unique family of FeS ferredoxins that function in anaerobic electron transport chains. Some HiPIPs have a redox potential higher than any other known iron–sulfur protein (e.g., HiPIP from Rhodopila globiformis has a redox potential of ca. -450 mV). Several HiPIPs have so far been characterized structurally, their folds belonging to the α+β class. As in other bacterial ferredoxins, the [FeS] unit forms a cubane-type cluster and is ligated to the protein via four Cys residues.