Why do nitrogenases waste electrons by evolving dihydrogen?

In nature, nitrogen is commonly fixed as the most reduced form, ammonia (NH₃) and as the most oxidized form, i.e. nitrate ion (NO ₃^-). Nitrogenases catalyze the reduction of N₂ into NH₃ by using protons and electrons with evolution of H ₂. However, the reason why the enzymes waste electrons by evolving H₂ has yet to be clarified. We have previously reported (J. Am. Chem. Soc. 2002; 124: 597) pH-dependent heterolytic cleavage of H₂ and subsequent reduction of NO₃^- with evolution of H ₂ by iridium complexes in water. We propose herein a catalytic mechanism of nitrogenases, which can account for evolution of H₂ in the reduction of N₂ to NH₃ in relation to a mechanism of the reduction of NO₃^-.