How Thermal Fluctuations Influence the Function of the FeMo-cofactor in Nitrogenase Enzymes

Abstract

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 H2 formation is an obligatory step in N2 adsorption and activation.