title: Direct spectroscopic evidence for the presence of a 6Fe cluster in an iron-sulfur protein isolated from Desulfovibrio desulfuricans (ATCC 27774)
abstract: A novel iron-sulfur protein was purified from the extract of Desulfovibrio desulfuricans (ATCC 27774) to homogeneity as judged by polyacrylamide gel electrophoresis. The purified protein is a monomer of 57 kDa molecular mass. It contains comparable amounts of iron and inorganic labile sulfur atoms and exhibits an optical spectrum typical of iron-sulfur proteins with maxima at 400, 305, and 280 nm. Mössbauer data of the as-isolated protein show two spectral components, a paramagnetic and a diamagnetic, of equal intensity. Detailed analysis of the paramagnetic component reveals six distinct antiferromagnetically coupled iron sites, providing direct spectroscopic evidence for the presence of a 6Fe cluster in this newly purified protein. One of the iron sites exhibits parameters (delta EQ = 2.67 +/- 0.03 mm/s and delta = 1.09 +/- 0.02 mm/s at 140 K) typical for high spin ferrous ion; the observed large isomer shift indicates an iron environment that is distinct from the tetrahedral sulfur coordination commonly observed for the iron atoms in iron-sulfur clusters and is consistent with a penta- or hexacoordination containing N and/or O ligands. The other five iron sites are most probably high spin ferric. Three of them show parameters characteristic for tetrahedral sulfur coordination. In correlation with the EPR spectrum of the as-purified protein which shows a resonance signal at g = 15.3 and a group of signals between g = 9.8 and 5.4, this 6Fe cluster is assigned to an unusual spin state of 9/2 with zero field splitting parameters D = -1.3 cm-1 and E/D = 0.062. Other EPR signals attributable to minor impurities are also observed at the g = 4.3 and 2.0 regions. The diamagnetic Mössbauer component represents a second iron cluster, which, upon reduction with dithionite, displays an intense S = 1/2 EPR signal with g values at 2.00, 1.83, and 1.31. In addition, an EPR signal of the S = 3/2 type is also observed for the dithionite-reduced protein.
authors.affiliation: Centro de Tecnologia Química e Biológica, Universidade Nova de Lisboa, Oeiras, Portugal.
authors.forename: I
authors.initials: I
authors.lastname: Moura
authors.forename: P
authors.initials: P
authors.lastname: Tavares
authors.forename: J J
authors.initials: JJ
authors.lastname: Moura
authors.forename: N
authors.initials: N
authors.lastname: Ravi
authors.forename: B H
authors.initials: BH
authors.lastname: Huynh
authors.forename: M Y
authors.initials: MY
authors.lastname: Liu
authors.forename: J
authors.initials: J
authors.lastname: LeGall
mesh_terms: D000595:Amino Acid Sequence; D001426:Bacterial Proteins; D003901:Desulfovibrio / Q000378:metabolism*; D004578:Electron Spin Resonance Spectroscopy; D004591:Electrophoresis, Polyacrylamide Gel; D007501:Iron / Q000737:chemistry; D007506:Iron-Sulfur Proteins / Q000737:chemistry / Q000302:isolation & purification*; D008969:Molecular Sequence Data
chemical_list: D001426:Bacterial Proteins; D007506:Iron-Sulfur Proteins; C111122:fuscoredoxin protein, Desulfovibrio desulfuricans; D007501:Iron