Stephen F. Gheller

Iron-molybdenum cofactor of nitrogenase: Electrochemical determination of the electron stoichiometry of the oxidized/semi-reduced couple

The number of electrons involved in the more positive of the two redox couples of the iron-molybdenum cofactor of Azotobacter vinelandii nitrogenase has been investigated by controlled potential coulometry in both the oxidizing and reducing directions. A n value of 1 was determined for interconversion of the oxidized and semi-reduced states of the cofactor. This electron count was confirmed by double integration of the S = 3/2 electron paramagnetic resonance signal exhibited by the semi-reduced state.

Effects of oxidation state, solvent acidity and thiophenol on the electrochemical properties of iron-molybdenum cofactor from nitrogenase

Abstract Electrochemical examination of the iron-molybdenum cofactor (FeMoco) extracted from the MoFe protein of Azotobacter vinelandii nitrogenase reveals that this important biological cluster exists in a variety of chemical forms whose numbers, proportions and properties depend on oxidation state, N -methylformamide solvent acidity and presence of thiophenolate ion. The distribution of oxidized (ox) species, determined from voltammetric responses, and of semi-reduced (s-r) species, determined from EPR spectra, show that FeMoco exists as a 2:1 proportion of populations in its ox state in alkaline NMF and in its ox and s-r states in acidic NMF. A single form of FeMoco(s-r) is found in alkaline NMF. Addition of thiophenol converts all populations of FeMoco into single PhS − -coordinated ox and s-r species under all conditions; these forms exhibit quasi-reversible [FeMoco(ox)-SPh] + e − ⇌ [Femoco(s-r)-SPh] electrochemistry. The variety of cofactor species observed is attributed to various states of ligation, protonation or structural arrangement of the metal cluster. The greater distribution of forms observed in the absence versus presence of thiophenol, in acidic versus alkaline solvent and in the oxidized versus semi- reduced oxidation state suggests that FeMoco exhibits greater structural or compositional heterogeneity under the former of each of these conditions.

Mossbauer spectroscopy applied to the oxidized and semi-reduced states of the iron-molybdenum cofactor of nitrogenase

Mössbauer parameters at 125K for both the oxidized and semi-reduced states of FeMoco isolated from the MoFe protein of Azotobacter vinelandii nitrogenase of delta/Fe = 0.32 and 0.37 mm/s and delta Eq = 0.84 and 0.71 mm/s, respectively, are reported. FeMoco(ox) fits the Debye model perfectly from 4.2-125K and has a S = 0 ground state. FeMoco(ox) apparently contains 10-20% FeMoco(s-r) and vice versa, possibly as a result of the spontaneous oxidation phenomenon. Quantitation of the spectra indicates a Fe:Mo ratio of 5 +/- 1:1 and the similar quadrupole splittings and isomer shifts suggest a similar environment for all iron atoms.