Nitrogenase is a complex metalloenzyme that is best known for its function in biological nitrogen fixation.~(1,2) Harbored in a group of microbes called diazotrophs, nitrogenase catalyzes the reduction of nitrogen (N_2) to ammonia (NH_3) in a reaction that is usually depicted as N_2 + 8H~+ + 16MgATP + 8e~- → 2NH_3 + H_2 + 16MgADP + 16P_i. This reaction not only represents a key step in the global nitrogen cycle, but also embodies the formidable chemistry of breaking the exceptionally stable NN triple bond. Recently, nitrogenase was shown to reduce carbon monoxide (CO) to hydrocarbons under the same reaction conditions of biological nitrogen fixation,~(3-6) defining it as a versatile metalloenzyme that is capable of activating N_2 and CO and converting them into products of agronomic and economic values. Interestingly, the reactions of N_2- and CO-reduction by nitrogenase parallel two important processes in industry: the Haber-Bosch process, which is used for ammonia production from N_2 and hydrogen (H_2);~7 and the Fischer-Tropsch process, which is used for carbon fuel production from CO and H_2.~8 However, contrary to the industrial processes, the nitrogenase-catalyzed reactions occur under ambient conditions, making this enzyme a fascinating subject from the perspective of chemical energy.
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