title: Electrochemical and spectroscopic characterization of the conversion of the 7Fe into the 8Fe form of ferredoxin III from Desulfovibrio africanus. Identification of a [4Fe-4S] cluster with one non-cysteine ligand.
abstract: Desulfovibrio africanus ferredoxin III is a protein (Mr 6585) containing one [3Fe-4S]1+,0 and one [4Fe-4S]2+,1+ core cluster when aerobically isolated. The amino acid sequence contains only seven cysteine residues, the minimum required to ligand these two clusters. Cyclic voltammery by means of direct electrochemistry at a pyrolytic-graphite-'edge' electrode promoted by neomycin shows that, when reduced, the [3Fe-4S]0 centre reacts rapidly with Fe(II) ion to form a [4Fe-4S]2+ cluster. The latter, which can be reduced at a redox potential similar to that of the other [4Fe-4S] cluster, must include non-thiolate ligation. We propose that the carboxylate side chain of aspartic acid-14 is the most likely candidate, since this amino acid occupies the position of a cysteine residue in the sequence typical of an 8Fe ferredoxin. The magnetic properties at liquid-He temperature of this novel cluster, studied by low-temperature magnetic-c.d. and e.p.r. spectroscopy, are diamagnetic in the oxidized state and S = 3/2 in the one-electron-reduced state. This cluster provides a plausible model for the ligation states of the [4Fe-4S]1+ core in the S = 3/2 cluster of the iron protein of nitrogenase and in Bacillus subtilis glutamine:phosphoribosyl pyrophosphate amidotransferase.
authors.affiliation: School of Chemical Sciences, University of East Anglia, Norwich, U.K.
authors.forename: S J
authors.initials: SJ
authors.lastname: George
authors.forename: F A
authors.initials: FA
authors.lastname: Armstrong
authors.forename: E C
authors.initials: EC
authors.lastname: Hatchikian
authors.forename: A J
authors.initials: AJ
authors.lastname: Thomson
mesh_terms: D002942:Circular Dichroism; D003545:Cysteine; D003901:Desulfovibrio / Q000032:analysis*; D004578:Electron Spin Resonance Spectroscopy; D005288:Ferredoxins; D005296:Ferrous Compounds; D008280:Magnetics; D010084:Oxidation-Reduction; D013057:Spectrum Analysis; D013696:Temperature
chemical_list: D005288:Ferredoxins; D005296:Ferrous Compounds; D003545:Cysteine