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

The catalytic mechanism of N2 fixation by nitrogenase remains unre-solved in how the strong N=N bond is activated and why the reduc-tive 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 dou-ble bond accompanied by a low barrier for H2 release. We empha-size that finite temperature effects smooth out mechanistic differ-ences 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.