Catalytic behavior of the nitrogenase iron-molybdenum cofactor extracted from the enzyme in the reduction of C2H2 under nonenzymatic conditions

Effect of the potential of an external electron donor on C2H2 reduction catalyzed by the nitrogenase active center (FeMoco) isolated from the enzyme

“…8 Then a number of chemical steps occur, namely, coordination of acetylene, intramolecular elec tron transfer, and C 2 H 2 protonation to form C 2 H 4 and C 2 H 6 as products.…”

“…We did not raise the temperature further because it has previously been found that FeMoco possesses a restricted thermal stability and decomposes, to a great extent, at 40 °C. 8 Since all measurements for different temperatures were carried out at the same concentrations of all reactants and other reaction parameters being the same, the rate values were used for the calculation of activation energies in stead of the rate constants. The apparent activation en ergy of acetylene reduction with C 2 H 4 formation was calculated using the Arrhenius equation and was found to be equal to 18.3±1.5 kcal mol -1 , and that of the reduction to C 2 H 6 was approximately 1.5 fold lower (12.8±2 kcal mol -1 ) (Fig.…”

“…This implies that, as for enzymatic catalysis, N 2 and С 2 Н 2 as ligands compete for binding at the same coordination site on the FeMoco cluster re duced by amalgam, with quantitative parameters charac teristic of nitrogenase enzyme in vitro, both wild type 12, 13 and mutant. 13, 14 Our data [8][9][10][11] show that in many aspects FeMoco iso lated from the enzyme catalyzes C 2 H 2 reduction quite similarly to M center of nitrogenase. Therefore, it can be concluded that the FeMoco cluster is an obligatory and, to some extent, sufficient active center of the enzyme: it is responsible for the formation of a substrate-enzyme complex and specificity of the nitrogenase action as a catalyst.…”

“…8, 9 The reaction proceeds in aprotic sol vents (DMF, N methylformamide, THF) where FeMoco is stable in the absence of oxygen for a long time, in the presence of electron donors, for instance, zinc amalgam (Zn/Hg; Е 0 = -0.84 V against the standard hydrogen electrode (SHE)) or europium amalgam (Eu/Hg; Е 0 = -1.4 V against SHE) 8 and proton donors, e.g., thiophenol.…”

Effect of the acidity and chemical nature of the protonating agent on the rate of acetylene reduction catalyzed by the nitrogenase active site isolated from the enzyme

“…The integrity of the cluster framework after its isolation from the protein and after catalytic reactions involving FeMoco in nonprotein systems was esti mated as the ability of FeMoco to reproduce the enzy matic activity of the cofactor deficient MoFe protein of Klebsiella pneumoniae Kp5058. We demonstrated [12][13][14] that the iron-molybdenum cofactor is an active reduction catalyst for a variety of nitrogenase substrates in a dimethylformamide (DMF) medium containing a Zn or Eu amalgam as the reductant and thiophenol as the proton donor. To gain insight into the mechanism of the catalytic action of FeMoco in nonenzymatic systems, we carried out a detailed kinetic study of the FeMoco catalyzed reduction of nitrogenase substrates [11] and of the reaction of FeMoco with nitrogenase inhibitors.…”