title: Formation and characterization of an all-ferrous Rieske cluster and stabilization of the [2Fe-2S]0 core by protonation.
abstract: The all-ferrous Rieske cluster, [2Fe-2S](0), has been produced in solution and characterized by protein-film voltammetry and UV-visible, EPR, and Mössbauer spectroscopies. The [2Fe-2S](0) cluster, in the overexpressed soluble domain of the Rieske protein from the bovine cytochrome bc(1) complex, is formed at -0.73 V at pH 7. Therefore, at pH 7, the [2Fe-2S](1+/0) couple is 1.0 V below the [2Fe-2S](2+/1+) couple. The two cluster-bound ferrous irons are both high spin (S = 2), and they are coupled antiferromagnetically (-J > or = 30 cm(-1), H =-2JS1.S2) to give a diamagnetic (S = 0) ground state. The ability of the Rieske cluster to exist in three oxidation states (2+, 1+, and 0) without an accompanying coupled reaction, such as a conformational change or protonation, is highly unusual. However, uncoupled reduction to the [2Fe-2S](0) state occurs at pH > 9.8 only, and at high pH the intact cluster persists in solution for <1 min. At pH < 9.8, the all-ferrous cluster is stabilized significantly by protonation. A combination of experimental data and calculations based on density functional theory suggests strongly that the proton binds to one of the cluster mu(2)-sulfides, consistent with observations that reduced [3Fe-4S] clusters are protonated also. The implications for our understanding of coupled reactions at iron-sulfur clusters and of the factors that determine the relative stabilities of their different oxidation states are discussed.
authors.affiliation: Medical Research Council Dunn Human Nutrition Unit, Wellcome Trust/Medical Research Council Building, Hills Road, Cambridge CB2 2XY, United Kingdom.
authors.forename: Ellen J
authors.initials: EJ
authors.lastname: Leggate
authors.forename: Eckhard
authors.initials: E
authors.lastname: Bill
authors.forename: Timm
authors.initials: T
authors.lastname: Essigke
authors.forename: G Matthias
authors.initials: GM
authors.lastname: Ullmann
authors.forename: Judy
authors.initials: J
authors.lastname: Hirst
mesh_terms: D000818:Animals; D002417:Cattle; D003001:Cloning, Molecular; D003545:Cysteine / Q000378:metabolism; D014450:Electron Transport Complex III / Q000737:chemistry* / Q000378:metabolism*; D007506:Iron-Sulfur Proteins / Q000737:chemistry* / Q000378:metabolism*; D007700:Kinetics; D008024:Ligands; D012996:Solutions; D015204:Spectroscopy, Mossbauer; D013440:Sulfides / Q000378:metabolism
chemical_list: D007506:Iron-Sulfur Proteins; D008024:Ligands; C037989:Rieske iron-sulfur protein; D012996:Solutions; D013440:Sulfides; D014450:Electron Transport Complex III; D003545:Cysteine