An atomic-level mechanism for molybdenum nitrogenase. Part 2. Proton reduction, inhibition of dinitrogen reduction by dihydrogen, and the HD formation reaction

Quantum calculations have been used to examine the energetics of the reactions of diazene and isodiazene with H-2 and the properties of the Fe and Mo sites of the nitrogenase iron-molybdenum cofactor with respect to the binding of H and H-2. The results have been used to extend the model for N-2 reduction by nitrogenase given in the preceding paper to describe the formation of HD from D-2. The proposed mechanism for HD formation invokes a combination of two well-established chemical reactions, namely, competitive protonation of metal N-2 Species at either the metal or at N-2, followed by scrambling of D-2 at a metal hydride. The model is evaluated against the available biochemical data for the nitrogenase HD formation reaction and extended to account for H-2 inhibition of N-2 reduction and the reduction of H+ in the absence of other substrates.