How Does Mother Nature Tackle the Tough Triple Bond Found in Nitrogen?

March 21, 2019

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How Does Mother Nature Tackle the Tough Triple Bond Found in Nitrogen?
Researchers explain how the nitrogenase enzyme couples the energy-releasing formation of hydrogen (H2) (reductive elimination) to the energy-requiring cleavage of the triple bond in nitrogen (N2) (oxidative addition) (bottom, center). The enzyme's release of H2 (top) is the thermodynamic driving force (see energy in kiloJoules per mole) for breaking apart the bonds in N2.

The Science

We use a great deal of ammonia—most of it for fertilizers to grow our food. Currently, most of our ammonia is made by breaking apart nitrogen in an energy-demanding reaction. But nature performs this same chemistry in a more environmentally benign way through the action of an enzyme called nitrogenase. Oddly, the enzyme also produces molecular hydrogen. Because hydrogen contains a considerable amount of energy, chemists have long viewed this process of producing molecular hydrogen as inefficient. With this recent closer look into the enzyme, researchers concluded that the hydrogen formation actually enables the process to help break nitrogen apart.

The Impact

Nitrogenase works at atmospheric pressure and room temperature. Industrial processes, on the other hand, require significant amounts of heat, pressure, and hydrogen. Fossil fuel—primarily natural gas—provides the heat, pressure, and hydrogen for industrial ammonia production, which is why these plants account for 1 to 3 percent of the world's total energy-related carbon emissions. Knowing what lends nitrogenase its bond-breaking muscle can foster new ideas for the design of synthetic catalysts to make ammonia.

Summary

For every molecule of nitrogen transformed to ammonia, nitrogenase makes at least one molecule of hydrogen (H2), which has been a puzzling mystery. Why would the enzyme make this seemingly "wasteful" byproduct? The researchers found that this phenomenon actually helps nitrogenase tackle nitrogen's strong bonds by coupling the production of hydrogen, which releases energy, with cleaving nitrogen, which requires energy. To arrive at the results, the team used a blend of theoretical and experimental methods. The researchers focused on the nitrogenase catalytic core, composed of iron, molybdenum, and sulfur (FeMo-co). During the catalytic event, when FeMo-co has acquired a critical number of electrons and protons (H+) in the form of two bridging hydrides (Fe-H-Fe) in its peripheral belt, generation of an H2 bound to FeMo-co and its displacement by N2 provides the give and take of energy to spark the first critical step in nitrogen reduction. The team seeks to extend the research by examining the fine details of electron- and proton-accumulation at the active site of nitrogenase and exactly how the N2 bond is broken to form ammonia.

Contact

Simone Raugei
Pacific Northwest National Laboratory
simone.raugei@pnnl.gov  

Funding

Simone Raugei and Lance Seefeldt were funded by the Department of Energy (DOE), Office of Science, Basic Energy Sciences, Division of Chemical Sciences, Geosciences, and Biosciences. Work by Brian Hoffman was supported by National Institutes of Health, National Science Foundation, and DOE, Office of Science, Basic Energy Sciences, Division of Chemical Sciences, Geosciences, and Biosciences. Computer resources were provided by the Environmental Molecular Sciences Laboratory, a DOE Office of Science user facility at Pacific Northwest National Laboratory and managed for DOE Office of Science's Office of Biological and Environmental Research and the National Energy Research Scientific Computing Center at Lawrence Berkeley National Laboratory and managed for the DOE Office of Science's Office of Advanced Scientific Computing Research.

Publications

S. Raugei, L.C. Seefeldt, and B.M. Hoffman, "Critical computational analysis illuminates the reductive-elimination mechanism that activates nitrogenase for N2 reduction." Proceedings of the National Academy of Sciences 115, E10521 (2018). [DOI: 10.1073/pnas.1810211115]

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