Novel Redox Chemistry of [3Fe−4S] Clusters:  Electrochemical Characterization of the All-Fe(II) Form of the [3Fe−4S] Cluster Generated Reversibly in Various Proteins and Its Spectroscopic Investigation in Sulfolobus acidocaldarius Ferredoxin

The novel “hyper-reduced” form of protein-bound [3Fe−4S] clusters, which is two electron equivalents below the normal reduced form [3Fe−4S]0 and thus formally composed entirely of Fe(II) subsites, has been characterized by electrochemistry and by EPR, MCD, and UV/visible spectroscopy. The two-electron reduction of [3Fe−4S]0 has been studied for a range of proteins, in particular the 7Fe ferredoxins from Sulfolobus acidocaldarius, Desulfovibrio africanus, and Azotobacter vinelandii. In each case, the reaction is chemically reversible, the product is surprisingly inert, and the pH-dependent reduction potential is in the region of −700 mV vs SHE at pH 7, regardless of the identity of the protein. Protein film voltammetry of three different ferredoxins investigated in detail over a wide pH range shows that the novel species denoted as [3Fe−4S]2- is formed by a cooperative two-electron reduction of [3Fe−4S]0 and there is a net uptake of three protons relative to the all-Fe(III) state [3Fe−4S]1+. The protons are probably bound at or close to the cluster (accounting for the strong pH dependence and insensitivity to protein host), but H2 is not evolved despite the negative potential at which [3Fe−4S]2- is formed. The hyper-reduced species which is produced reversibly in solution by four-electron electrochemical reduction of the 7Fe ferredoxin from Sulfolobus acidocaldarius contributes little absorbance in the visible spectral region, and shows an MCD spectrum with transitions below 400 nm that resemble features observed for Fe(II) rubredoxin. The EPR spectrum of the four-electron reduced protein differs significantly from that of the normal two-electron reduced form ([3Fe−4S]0, [4Fe−4S]1+); the signal at g = 12 assigned to [3Fe−4S]0 disappears and changes occur to the spectrum in the g = 1.94 region which can be attributed to alterations in spin coupling with the [4Fe−4S]1+ cluster. As an all-Fe(II) and (probably) protonated species performing two- electron redox reactions, [3Fe−4S]2- represents a fundamental entity of iron−sulfur cluster chemistry that has so far remained elusive. Structural and functional implications of this reactivity are considered.