Enzymes

Nitrogenase branches out

A vanadium-containing nitrogenase enzyme can reduce carbon monoxide to ethane and propane.

The Haber–Bosch process converts the small and stable nitrogen and hydrogen molecules into ammonia. This widespread and energy-intensive industrial process roughly copies what nitrogenase enzymes do in nature. These two reactions are rarely considered together with the Fischer–Tropsch reaction, which converts the small and stable carbon monoxide and hydrogen molecules into higher hydrocarbons. But this may change now Chi Chung Lee and colleagues from the University of California at Irvine have discovered1 a nitrogenase enzyme that also performs Fischer–Tropsch-style chemistry.

The team were studying molybdenum- and vanadium-containing nitrogenases and discovered that the presence of carbon monoxide adversely affected the performance of the vanadium enzyme, whereas the molybdenum one was unaffected. When they noticed that adenosine triphosphate — the 'fuel' for the reaction — was still being consumed at the same rate by the vanadium enzyme, they realized the enzyme could be acting on the carbon monoxide. Using gas chromatography and mass spectroscopy, Lee and co-workers detected ethane, ethene and propane being produced by the vanadium enzyme under a 100% carbon monoxide atmosphere, but no hydrocarbons from molybdenum enzymes in the same conditions. Radiolabelling studies confirmed that the carbon source was the carbon monoxide.

Nitrogenases use separate electrons and protons to reduce nitrogen, unlike the Haber–Bosch process that uses intact hydrogen, and hydrogen is in fact produced by the nitrogenase reaction. Increasing the level of hydrogen in the atmosphere inhibited the carbon-monoxide-reducing reaction of the vanadium enzyme, as it does for the nitrogen reaction, implying that the two reactions proceed in a similar manner.

References

  1. 1

    Lee, C. C., Hu, Y. & Ribbe, M. W. Vanadium nitrogenase reduces CO. Science 329, 642 (2010).

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Withers, N. Nitrogenase branches out. Nature Chem (2010). https://doi.org/10.1038/nchem.839

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