Very Rapid, Cooperative Two-Electron/Two-Proton Redox Reactions of [3Fe−4S] Clusters: Detection and Analysis by Protein-Film Voltammetry

Abstract

The “hyper-reduced” [3Fe−4S]2- cluster (consisting formally of three Fe(II) atoms and four sulfides (S2-)) possesses a remarkable capability for very rapid and reversible two-electron/two-proton oxidation (Eo‘ > −650 mV below pH 7) that is strongly suggestive of disulfide-based (as opposed to Fe-based) redox chemistry. This otherwise elusive reactivity is most readily revealed by performing fast-scan protein-film voltammetry on ferredoxins that contain a [3Fe−4S] cluster using an electrolyte composed of D2O. Fast, cooperative two-electron/two-proton transfer is observed after first generating the fully reduced [3Fe−4S]2- state and then cycling rapidly to more oxidizing potentials. The unusual voltammetric characteristics can be modeled by using a coupled electron-transfer scheme involving multiple states of both the [3Fe−4S]0 and [3Fe−4S]2- forms. Rapid two-electron/two-proton oxidation produces an unstable species, most likely a disulfide, which may either be rapidly re-reduced or undergo an internal redox reaction to produce the normal [3Fe−4S]0 form, which formally comprises two Fe(III) and one Fe(II). Relaxation to the normal “0” form is a factor of 4 faster when the experiment is conducted in H2O, thus making the fast couple more difficult to observethe retardation observed in D2O may be attributed to the need to rearrange the hydrogen-bonding interactions in the cluster binding domain.