Critical computational analysis illuminates the reductive-elimination mechanism that activates nitrogenase for N 2 reduction

Significance This report critically evaluates the mechanism by which nitrogenase cleaves the N≡N triple bond. It assesses the thermodynamic driving force provided by the accompanying, apparently “wasteful,” reductive elimination of an H 2 , and explains how the enzyme mechanistically couples exothermic H 2 formation to endothermic triple-bond cleavage in a nearly thermoneutral equilibrium process, thereby preventing the “futile” generation of two H 2 without N 2 reduction. This evaluation rests on a critical assessment of the density functional theory flavors needed to properly treat nitrogenase, and a demonstration that to prevent spurious disruption of FeMo-co upon 4[ e − /H + ] accumulation, one must employ a nitrogenase structural model that includes all residues interacting directly with FeMo-co, either via specific H-bond interactions, nonspecific electrostatic interactions, or steric confinement.