What can be learned from the electrostatic environments within nitrogenase enzymes?

Nitrogen fixation is a fundamental, and yet challenging, chemical transformation due to the intrinsic inertness of dinitrogen (N₂). Whereas industrial ammonia synthesis relies on the energy-intensive Haber–Bosch process, nitrogenase enzymes achieve this transformation under ambient conditions—yet at the expense of a remarkably high ATP demand. Understanding their mode of operation could inspire the development of more efficient synthetic catalysts. In this study, we scrutinize the role of electrostatics in nitrogenase’s active site, surrounding the so-called M-cluster. Strikingly, all M-clusters reveal similar trends, exhibiting distinct electrostatic environments at the metal sites that have been proposed as potential N2-coordination sites. Specifically, a strong local electric field pointing away from the Fe2 site favors the cleavage of the Fe6–S–Fe2 sulfido bridge, exposing the Fe6 center for N₂ binding. Moreover, an oriented long-range electric field along the Fe2–Fe6 axis is identified, which may assist in N₂ activation towards hydrogenation, once the nitrogen takes on a bridging configuration between both metal sites. Our findings suggest that nitrogenases likely exploit electrostatic effects in an unconventional manner; rather than directly favoring the coordination of N2 to the M-cluster, they primarily modulate the kinetics (and thermodynamics) of key mechanistic steps preceding, and following, the absorption step. Overall, this study highlights the importance of local electric fields in enzymatic catalysis, even for substrates that have only limited susceptibility to electric fields, and provides insights that could inform the design of improved nitrogen fixation catalysts.