Nitrogenase reactivity: insight into the nitrogen-fixing process through hydrogen-inhibition and HD-forming reactions

The dihydrogen reactions of nitrogenase are H2 evolution, H2 inhibition of N2 reduction, and HD production from H2/D2O or D2/H2O. The relationships among these dihydrogen reactions are studied to gain insight into the mechanism of N2 reduction. Detailed studies have probed (1) the formation of HD by nitrogenase as a function of partial pressures of N2, D2, and CO, (2) the formation of TOH from T2 under N2-fixing conditions, and (3) the reduction of hydrazine by nitrogenase. Experiments under T2 demonstrate that negligible tritium is incorporated into water compared to the HD produced under similar conditions. Studies of total electron flow, in the presence or absence of D2, establish a requirement of 1 mol of electrons/mol of HD formed. These findings show definitively that HD formation is not due to a simple H2O/D2 exchange mechanism. Kinetic analysis shows that HD is produced by two separate processes. In the minor process, the HD formed is proportional to the H2 evolved, electron requiring, and partially inhibited by 1% CO. In the major process, HD formation is dependent on N2 pressure, electron requiring, and completely inhibited by CO. A mechanism is proposed whereby HD from the N2-dependent process is formed from a bound, reduced dinitrogen intermediate. This mechanism is supported by studies using hydrazine as a substrate for nitrogenase and leads to the conclusion that H2 inhibition of nitrogen fixation and N2-dependent HD formation are manifestations of the same molecular process.