EFFECTS OF OXIDATION-STATE, SOLVENT ACIDITY AND THIOPHENOL ON THE ELECTROCHEMICAL PROPERTIES OF IRON MOLYBDENUM COFACTOR FROM NITROGENASE

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. © 1990.