title: Electron paramagnetic resonance studies of succinate:ubiquinone oxidoreductase from Paracoccus denitrificans. Evidence for a magnetic interaction between the 3Fe-4S cluster and cytochrome b.
abstract: Electron paramagnetic resonance (EPR) studies of succinate:ubiquinone oxidoreductase (SQR) from Paracoccus denitrificans have been undertaken in the purified and membrane-bound states. Spectroscopic "signatures" accounting for the three iron-sulfur clusters (2Fe-2S, 3Fe-4S, and 4Fe-4S), cytochrome b, flavin, and protein-bound ubisemiquinone radicals have been obtained in air-oxidized, succinate-reduced, and dithionite-reduced preparations at 4-10 K. Spectra obtained at 170 K in the presence of excess succinate showed a signal typical of that of a flavin radical, but superimposed with another signal. The superimposed signal originated from two bound ubisemiquinones, as shown by spectral simulations. Power saturation measurements performed on the air-oxidized enzyme provided evidence for a weak magnetic dipolar interaction operating between the oxidized 3Fe-4S cluster and the oxidized cytochrome b. Power saturation experiments performed on the succinate- and dithionite-reduced forms of the enzyme demonstrated that the 4Fe-4S cluster is coupled weakly to both the 2Fe-2S and the 3Fe-4S clusters. Quantitative interpretation of these power saturation experiments has been achieved through redox calculations. They revealed that a spin-spin interaction between the reduced 3Fe-4S cluster and the cytochrome b (oxidized) may also exist. These findings form the first direct EPR evidence for a close proximity (</=2 nm) of the high potential 3Fe-4S cluster, situated in the succinate dehydrogenase part of the enzyme, and the low potential, low spin b-heme in the membrane anchor of the enzyme.
authors.affiliation: Arthur Amos Noyes Laboratory of Chemical Physics, California Institute of Technology, Pasadena, California 91125, USA.
authors.forename: A R
authors.initials: AR
authors.lastname: Waldeck
authors.forename: M H
authors.initials: MH
authors.lastname: Stowell
authors.forename: H K
authors.initials: HK
authors.lastname: Lee
authors.forename: S C
authors.initials: SC
authors.lastname: Hung
authors.forename: M
authors.initials: M
authors.lastname: Matsson
authors.forename: L
authors.initials: L
authors.lastname: Hederstedt
authors.forename: B A
authors.initials: BA
authors.lastname: Ackrell
authors.forename: S I
authors.initials: SI
authors.lastname: Chan
mesh_terms: D003573:Cytochrome b Group / Q000737:chemistry*; D004227:Dithionite / Q000494:pharmacology; D004578:Electron Spin Resonance Spectroscopy; D042963:Electron Transport Complex II; D007501:Iron; D009097:Multienzyme Complexes / Q000737:chemistry*; D010088:Oxidoreductases / Q000737:chemistry*; D010231:Paracoccus denitrificans / Q000201:enzymology*; D013385:Succinate Dehydrogenase / Q000737:chemistry*; D013455:Sulfur
chemical_list: D003573:Cytochrome b Group; D009097:Multienzyme Complexes; D004227:Dithionite; D013455:Sulfur; D007501:Iron; D010088:Oxidoreductases; D042963:Electron Transport Complex II; D013385:Succinate Dehydrogenase