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Reversing nitrogen fixation


The nitrogen cycle is one of the most important biogeochemical cycles on Earth because nitrogen is an essential nutrient for all life forms. To supplement natural nitrogen fixation, farmers add large amounts of nitrogen-containing fertilizer to their soils such that nitrogen never becomes a limiting nutrient for plant growth. However, of the nitrogen added to fields — most of which is in the form of NH3 and NO3 — only 30–50% is taken up by plants, while the remainder is metabolized by soil microorganisms in processes with detrimental environmental impacts. The first of these processes, that is, nitrification, refers to the biological oxidation of NH3 to NO2 and NO3, which have low retention in soil and pollute waterways, leading to downstream eutrophication and ultimately ‘dead zones’ (low oxygen zones) in coastal waters, for example, the Gulf of Mexico. In a second process, namely, denitrification, NO3 and NO2 undergo stepwise reduction to N2O and N2. Substantial amounts of the N2O produced in this process escape into the atmosphere, contributing to climate change and ozone destruction. Recent results suggest that nitrification also affords N2O. This Review describes the enzymes involved in NH3 oxidation and N2O production and degradation in the nitrogen cycle. We pay particular attention to the active site structures, the associated coordination chemistry that enables the chemical transformations and the reaction mechanisms.

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Fig. 1: The main species and interconversions in the nitrogen cycle.
Fig. 2: Dissimilatory denitrification involves stepwise reduction of NO3 to N2.
Fig. 3: The active site of hydroxylamine oxidoreductase from Nitrosomonas europaea.
Fig. 4: Cytochrome P450 nitric oxide reductase reductively couples NO at a {haem-thiolate} active site.
Fig. 5: Comparison of the bimetallic active site of bacterial NO reductase and an engineered myoglobin.
Fig. 6: Mechanistic proposals for NO reductive coupling catalysed by bacterial NO reductase.
Fig. 7: Bonding in a small molecule Viii–N2O complex.
Fig. 8: Nitrous oxide reductase and its synthetic active site models.


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Work on P450nor models and related NO complexes by the Lehnert laboratory is supported by a grant from the National Science Foundation (CHE-1464696 to N.L.), which is gratefully acknowledged.

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Lehnert, N., Dong, H.T., Harland, J.B. et al. Reversing nitrogen fixation. Nat Rev Chem 2, 278–289 (2018).

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