title: Selenol binds to iron in nitrogenase iron-molybdenum cofactor: an extended x-ray absorption fine structure study.
abstract: The biological N2-fixation reaction is catalyzed by the enzyme nitrogenase. The metal cluster active site of this enzyme, the iron-molybdenum cofactor (FeMoco), can be studied either while bound within the MoFe protein component of nitrogenase or after it has been extracted into N-methylformamide. The two species are similar but not identical. For example, the addition of thiophenol or selenophenol to isolated FeMoco causes its rather broad S = 3/2 electron paramagnetic resonance signal to sharpen and more closely approach the signal exhibited by protein-bound FeMoco. The nature of this thiol/selenol binding site has been investigated by using Se-K edge extended x-ray absorption fine structure (EXAFS) to study selenophenol ligated to FeMoco, and the results are reported here. EXAFS data analysis at the ligand Se-K edge was performed with a set of software, GNXAS, that provides for direct calculation of the theoretical EXAFS signals and least-squares fits to the experimental data. Data analysis results show definitively that the selenol (and by inference thiol) binds to Fe at a distance of 2.4 A. In contrast, unacceptable fits are obtained with either Mo or S as the liganded atom (instead of Fe). These results provide quantitative details about an exchangeable thiol/selenol binding site on FeMoco in its isolated, solution state and establish an Fe atom as the site of this reaction. Furthermore, the utility of ligand-based EXAFS as a probe of coordination in polynuclear metal clusters is demonstrated.
authors.affiliation: Department of Chemistry, Stanford University, CA 94305.
authors.forename: S D
authors.initials: SD
authors.lastname: Conradson
authors.forename: B K
authors.initials: BK
authors.lastname: Burgess
authors.forename: W E
authors.initials: WE
authors.lastname: Newton
authors.forename: A
authors.initials: A
authors.lastname: Di Cicco
authors.forename: A
authors.initials: A
authors.lastname: Filipponi
authors.forename: Z Y
authors.initials: ZY
authors.lastname: Wu
authors.forename: C R
authors.initials: CR
authors.lastname: Natoli
authors.forename: B
authors.initials: B
authors.lastname: Hedman
authors.forename: K O
authors.initials: KO
authors.lastname: Hodgson
mesh_terms: D016948:Azotobacter vinelandii / Q000737:chemistry; D001555:Benzene Derivatives / Q000737:chemistry* / Q000378:metabolism; D005583:Fourier Analysis; D008983:Molybdoferredoxin / Q000737:chemistry* / Q000378:metabolism; D016566:Organoselenium Compounds / Q000737:chemistry* / Q000378:metabolism; D011188:Potassium / Q000737:chemistry; D012643:Selenium / Q000737:chemistry; D013057:Spectrum Analysis / Q000379:methods*; D013223:Statistics as Topic; D014965:X-Rays
chemical_list: D001555:Benzene Derivatives; D008983:Molybdoferredoxin; D016566:Organoselenium Compounds; C018846:benzeneselenol; D012643:Selenium; D011188:Potassium