Can the E1 state in nitrogenase tell if there is an activation process prior to catalysis?

Abstract

Model calculations have been performed for the singly reduced ground state of Mo-nitrogenase, usually termed E₁. Contradictory conclusions have been reached in two recent experimental studies. In a study based on EPR, it was concluded that there is a bridging hydride in E₁, while in an X-ray study it was concluded that there is no hydride in E₁. Therefore, the EPR study implies that there is an oxidation of the cofactor going from E₀ to E₁, the X-ray study implies a reduction. DFT methods have here been used, which have previously been benchmarked on a set of redox enzymes that led to the conclusion that the accuracy is about 3 kcal mol⁻¹ in all cases, even for redox transitions. The methodology should therefore be adequate for resolving the question of the hydride presence in E₁. As a comparison, calculations are performed on both Mo- and V-nitrogenase with the same conclusion. The conclusion from the calculations has far reaching consequences for the mechanism of nitrogenase.