title: Isotopic hybrids of nitrogenase. Mössbauer study of MoFe protein with selective 57Fe enrichment of the P-cluster.
abstract: Previous Mössbauer and EPR studies of the MoFe protein (approximately 30 Fe and 2 Mo) of nitrogenase have revealed the presence of two unique clusters, namely, the P-clusters (presumably of the Fe4S4 type) and the molybdenum- and iron-containing cofactors (or M-clusters). Mössbauer components D (approximately 10-12 Fe) and Fe2+ (approximately 4 Fe) represent subsites of the P-clusters while component S (approximately 2 Fe) appeared to belong to a separate, unidentified cluster. In order to refine the analyses of Mössbauer spectra, we have constructed an isotopic hybrid of the Klebsiella pneumoniae protein which contains 57Fe-enriched P-clusters and 56Fe-enriched M-clusters. The highly resolved 57Fe Mössbauer spectra of this hybrid show that component S behaves spectroscopically like the P-cluster sites D and Fe2+ in oxidized and reduced MoFe protein. This suggests that S is a subset of the P-clusters rather than a different cluster type. The present study shows, for the first time, that the Debye-Waller factors of different P-cluster subsites have a different temperature dependence. Thus, the Fe2+/D absorption ratio is 4.0:10.0 at 4.2 K and 4.0:11.6 at 173 K. We propose that the reduced MoFe protein contains two pairs of P-clusters: one pair containing one Fe2+ and three D-sites and the other one Fe2+, two D, and one S-site. We have argued previously that the oxidized P-clusters occur in pairs as well.
authors.affiliation: Department of Chemistry, Massachusetts Institute of Technology, Cambridge 02139.
authors.forename: P A
authors.initials: PA
authors.lastname: McLean
authors.forename: V
authors.initials: V
authors.lastname: Papaefthymiou
authors.forename: W H
authors.initials: WH
authors.lastname: Orme-Johnson
authors.forename: E
authors.initials: E
authors.lastname: Münck
mesh_terms: D001395:Azotobacter / Q000201:enzymology; D004227:Dithionite / Q000494:pharmacology; D004578:Electron Spin Resonance Spectroscopy; D005288:Ferredoxins / Q000378:metabolism*; D007503:Iron Isotopes; D007711:Klebsiella pneumoniae / Q000201:enzymology; D008983:Molybdoferredoxin / Q000378:metabolism*; D009591:Nitrogenase / Q000378:metabolism*; D010084:Oxidation-Reduction; D010640:Phenothiazines / Q000494:pharmacology; D013057:Spectrum Analysis
chemical_list: D005288:Ferredoxins; D007503:Iron Isotopes; D008983:Molybdoferredoxin; D010640:Phenothiazines; D004227:Dithionite; D009591:Nitrogenase; C009469:thionine