title: Isolated iron-molybdenum cofactor of nitrogenase exists in multiple forms in its oxidized and semi-reduced states.
abstract: Electrochemical and EPR spectroscopic experiments demonstrate that the isolated iron-molybdenum cofactor from the molybdenum-iron protein of nitrogenase from Azotobacter vinelandii exists in multiple forms in both its oxidized and semi-reduced states. The particular forms found in either oxidation state appear to be a function of the acid/base status of the solvent, N-methylformamide. In "alkaline" N-methylformamide, a single, detectable form of iron-molybdenum cofactor is observed for both oxidized and semi-reduced states. The semi-reduced form, termed R(s-r), is the one previously recognized with an S = 3/2 EPR spectrum with apparent g values of 4.6, 3.4, 2.0. Its oxidized counterpart, termed B(ox), is characterized electrochemically by a differential pulse voltammetric reduction peak at -0.37 V versus the normal hydrogen electrode. In "acidic" solvent, two distinct, previously unrecognized redox pairs of iron-molybdenum cofactor forms exist. The two semi-reduced forms, N(s-r) and W(s-r), are characterized by EPR spectra with g = 4.5, 3.6, 2.0 and g = 4.9, 3.1, 1.9, respectively. Their oxidized counterparts, A(ox) and C(ox), have differential pulse voltammetric reduction peaks at -0.32 and -0.43 V versus the normal hydrogen electrode, respectively. Manipulations of either the isolation protocol or the sample conditions affects both the type and distribution of forms present. Each form likely corresponds to a biologically significant state of the cofactor cluster within the protein.
authors.affiliation: Western Regional Research Center, United States Department of Agriculture-Agriculture Research Service, Albany, California 94710.
authors.forename: W E
authors.initials: WE
authors.lastname: Newton
authors.forename: S F
authors.initials: SF
authors.lastname: Gheller
authors.forename: B J
authors.initials: BJ
authors.lastname: Feldman
authors.forename: W R
authors.initials: WR
authors.lastname: Dunham
authors.forename: F A
authors.initials: FA
authors.lastname: Schultz
mesh_terms: D001395:Azotobacter / Q000201:enzymology; D004578:Electron Spin Resonance Spectroscopy; D005288:Ferredoxins / Q000302:isolation & purification*; D005559:Formamides; D007202:Indicators and Reagents; D008983:Molybdoferredoxin / Q000302:isolation & purification* / Q000378:metabolism; D009591:Nitrogenase / Q000378:metabolism*; D010084:Oxidation-Reduction
chemical_list: D005288:Ferredoxins; D005559:Formamides; D007202:Indicators and Reagents; D008983:Molybdoferredoxin; D009591:Nitrogenase; C002950:methylformamide