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

To compare the catalytic effect of the active center of nitrogenase (iron-m olybdenum cofactor (FeMoco)) under nonenzymatic conditions with the behavio r of FeMoco incorporated in a protein, the kinetics of C2H2 reduction with Zn and Eu amalgams was examined in the presence of the cofactor extracted f rom the MoFe protein of nitrogenase (the specific activity of the extracted FeMoco after its integration into the cofactor-deficient MoFe protein of K p 5058 was 200 +/- 20 mol of C2H4 (mol of Mo)(-1) min(-1). It was found tha t under exposure to reducing agents of different strength-Zn amalgam (I) (- 0.84 V with respect to a normal hydrogen electrode (NHE)) and Eu amalgam (I I) (-1.4 V with respect to NHE)-different reduction states of FeMoco were p roduced. They differed in the number and properties of substrate- and inhib itor-coordinating active sites. For I, the rate of ethylene formation was d escribed by a hyperbolic function of substrate concentration (K-M = 0.045 a tm). Carbon monoxide reversibly inhibited the reduction of acetylene (K-i = 0.05). For II, a sigmoid relationship between the rare of accumulation of C2H4 or C2H6 and substrate concentration was found. This relationship was e xplained by the occurrence of three interrelated sites of acetylene coordin ation and reduction with the apparent constant K-M = 0.08 atm in the FeMoco cluster reduced by europium amalgam. In this case, the specific activity w as 40-60 mol of C2H4 (mol of Mo)(-1) min(-1). For the system with Eu (Hg), the CO inhibition constants were 0.004 and 0.009 atm for the formation of e thylene and ethane, respectively. The behavior of FeMoco as a catalyst for acetylene reduction and the inhibition of this reaction by carbon monoxide in various reducing protein and nonprotein media were compared. This compar ison demonstrated that typical features of the catalytic behavior of FeMoco depend primarily on its composition acid structure and only secondarily on the type of the reducing agent and on the reaction medium.