Mechanism of N₂ Reduction Catalyzed by Fe-Nitrogenase Involves Reductive Elimination of H₂

Of the three forms of nitrogenase (Mo-nitrogenase, V-nitrogenase, and Fe-nitrogenase), Fe-nitrogenase has the poorest ratio of N₂ reduction relative to H₂ evolution. Recent work on the Mo-nitrogenase has revealed that reductive elimination of two bridging Fe-H-Fe hydrides on the active site FeMo-cofactor to yield H₂ is a key feature in the N₂ reduction mechanism. The N₂ reduction mechanism for the Fe-nitrogenase active site FeFe-cofactor was unknown. Here, we have purified both component proteins of the Fe-nitrogenase system, the electron-delivery Fe protein (AnfH) plus the catalytic FeFe protein (AnfDGK), and established its mechanism of N₂ reduction. Inductively coupled plasma optical emission spectroscopy and mass spectrometry show that the FeFe protein component does not contain significant amounts of Mo or V, thus ruling out a requirement of these metals for N₂ reduction. The fully functioning Fe-nitrogenase system was found to have specific activities for N₂ reduction (1 atm) of 181 ± 5 nmol NH₃ min^-1 mg^-1 FeFe protein, for proton reduction (in the absence of N₂) of 1085 ± 41 nmol H₂ min^-1 mg^-1 FeFe protein, and for acetylene reduction (0.3 atm) of 306 ± 3 nmol C₂H₄ min^-1 mg^-1 FeFe protein. Under turnover conditions, N₂ reduction is inhibited by H₂ and the enzyme catalyzes the formation of HD when presented with N₂ and D₂. These observations are explained by the accumulation of four reducing equivalents as two metal-bound hydrides and two protons at the FeFe-cofactor, with activation for N₂ reduction occurring by reductive elimination of H₂.