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Published November 20, 2019 | Accepted Version
Journal Article Open

Rethinking the Nitrogenase Mechanism: Activating the Active Site

Abstract

Biological nitrogen fixation, the conversion of dinitrogen (N₂) to ammonia (NH₃) catalyzed by nitrogenases (N₂ases), provides Earth with ∼50% of its bioavailable nitrogen. Without N₂ fixation, the chemically inert N₂ that comprises 80% of the Earth's atmosphere would not be accessible to life. Although the reduction of N₂ to NH₃ is thermodynamically favored at ambient conditions, both the biological and industrial N₂ fixation processes have a significant energy requirement. Understanding the catalytic mechanism of N₂ase may lead to the development of new catalysts that can operate under mild conditions in water closer to equilibrium. Most catalysts for N₂-to-NH₃ fixation, whether homogeneous, heterogeneous, or biological, contain transition metal centers that bind N₂ to lower the kinetic barrier for reduction. Exemplifying this theme, the active site cofactor of N₂ase contains eight transition metals. Although the N₂ase active site must become highly reactive during the catalytic cycle, the cofactor in the as-isolated state of N₂ase is a stable species that does not bind N₂. Consequently, cofactor activation is required prior to N₂ binding and functionalization. Herein, we discuss potential routes for cofactor activation based on recent studies of the Mo and V N₂ases. Like other transition metal systems capable of N₂ reduction, such as the Mittasch catalyst used in the Haber-Bosch process, structural rearrangements to the catalyst precursor are required to generate metal centers sufficiently reactive enough to bind N₂. Characterization of the activated state(s) of the cofactor that binds N₂ and the structural rearrangements constituting cofactor activation/deactivation processes will be key to establishing the mechanism of biological N₂ fixation. These mechanistic insights will be important for guiding the development of new catalysts or improved reaction conditions for N_2 reduction with more favorable energetic requirements than current processes.

Additional Information

© 2019 Elsevier Inc. Available online 10 October 2019. We thank Professor James B. Howard, Professor Jonas C. Peters, Dr. Thomas Spatzal, Professor Markus Ribbe, Professor Yilin Hu, Ailiena Maggiolo, Dr. Rebeccah Warmack, Javier Fajardo, Jr., and Professor Shabnam Hematian for insightful discussions and NIH grant GM045162 for funding. Author Contributions: T.M.B. wrote the manuscript, and T.M.B. and D.C.R. revised the manuscript.

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August 22, 2023
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