Towards the Mechanism of N2 Fixation by Nitrogenase

Despite the invention of the Haber-Bosch process for industrial generation of NH3 from N2 more than 100 years ago, biological nitrogen fixation — the reduction of N2 to two NH3 molecules — still supports more than half the human population. This process, which involves one of the most challenging chemical transformation in biology, the reduction of the N≡N triple bond, is catalyzed by nitrogenases, primarily the Mo-dependent enzyme. Prior to our studies with Dean and Seefeldt, essentially no mechanistic details of catalytic N2 binding and reduction were known, and there was not even a consensus about the stoichiometry of NH3 formation. Our team has revealed key features of the nitrogenase catalytic mechanism by which the catalytic nitrogenase MoFe protein carries out nitrogen fixation, in particular how it is activated to cleave the N2 triple bond. This work sets the stage for studies that will deepen and enrich our understanding of the nitrogenase catalytic cycle, and may inspire the synthesis of a new generation of catalysts that can fix N2 ‘electrochemically’, namely with sustainably-derived electrons (e.g., from photovoltaic (PV) cells and water oxidation) and protons (from water), and operating under benign conditions. This would address the drawbacks to the Haber-Bosch process, which accounts for up to ~ 3% of the world’s use of fossil fuels and reduces N2 with H2 derived from CH4, a reaction that releases large quantities of CO2. As nitrogenase is the paradigm for sustainable nitrogen fixation, discovery of the molecular mechanisn by which it performs this reaction will provide a foundation for the study of chemical means to fix nitrogen. Through the use of advanced paramagnetic resonance spectroscopies, coupled with computational chemistry, we will test and extend our mechanistic conclusions and proposals: (i) with experiments whose goal is to characterize for every catalytic intermediate state both the bound substrate-derived moieties, through 1H/14/15N ENDOR, and the catalytic cofactor itself, through 57Fe/95Mo/13C ENDOR and mass spectrometry; (ii) through kinetic and photophysical experiments that yield detailed insights into the transformations between intermediates. To broaden these mechanistic studies we are investigating not only the Mo-nitrogenase, but also the Fe-only enzyme, which we have shown to share the Mo-nitrogenase catalytic mechanism.