Proton-coupled electron transfer reactions at Rieske [2Fe-2S] clusters: three oxidation states and four protonation states

Rieske clusters are unusual amongst [2Fe-2S] clusters because they are ligated by two cysteine and two histidine residues, and because their reduction potentials ([2Fe-2S]) are strongly pH dependent. The pH-dependence arises from deprotonation of the two histidine ligands, which coordinate the redox-active iron centre. Rieske clusters are important components of two respiratory enzymes, the cytochrome bc1 and b6f complexes, and it is possible that they are used for proton transfer, as well as electron transfer, during catalytic turnover. Recently, we have used protein-film voltammetry to study the proton-coupled electron transfer reactions of Rieske clusters [1 3]. In addition to the [2Fe-2S] (all-ferric) and [2Fe2S] states, Rieske clusters can exist in the [2Fe-2S] (all-ferrous) state also [4]. Reduction to the all-ferrous state requires a very low potential, and is accompanied by protonation of the cluster. Therefore, Rieske clusters can adopt three oxidation states (+2, +1 and 0) and four protonation states (both histidine ligands can be protonated or deprotonated, and the cluster can also be protonated) [4]. This talk describes the different oxidation and protonation states of the Rieske cluster, and studies of them by voltammetric (protein-film voltammetry) and spectroscopic techniques.