How thermal fluctuations influence the function of the FeMo cofactor in nitrogenase enzymes

The catalytic mechanism of N₂ fixation by nitrogenase remains unresolved in how the strong N≡N bond is activated and why the reductive elimination of H₂ is required. Here, we use density functional theory and physiologically relevant thermal simulations to elucidate the mechanism of the complete nitrogenase catalytic cycle. Over the accumulation of four reducing equivalents, we find that protons and electrons transfer to the FeMo cofactor to weaken and break its bridge Fe–S bond, leading to temporary H₂S formation that exposes the Fe sites to weakly bind N₂. Remarkably, we find that subsequent H₂ formation is responsible for chemical activation to an N=N double bond accompanied by a low barrier for H₂ release. We emphasize that finite temperature effects smooth out mechanistic differences between DFT functionals observed at 0 K, thus leading to a consistent understanding as to why H formation is an obligatory step in N₂ adsorption and activation.