A Radical Route for Nitrogenase Carbide Insertion

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Science  28 Sep 2012:
Vol. 337, Issue 6102, pp. 1617-1618
DOI: 10.1126/science.1229088

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Nitrogenase is the only known biological catalyst for the conversion of dinitrogen (N2) to ammonia (NH3), a reaction that provides the “fixed” nitrogen required by most organisms. Biological breakdown of N2 by bacterial nitrogenase enzymes occurs at a complex metallocofactor consisting of seven iron (Fe) ions, nine sulfides (S2−), a molybdenum (Mo) ion (most commonly), and a homocitrate ligand (1, 2). Twenty years ago, crystal structures of nitrogenase provided the first glimpses of this FeMo cofactor, also called the M cluster, revealing that it resembles two [4Fe-4S] cubes (a more ordinary type of iron-sulfur metallocluster) linked by an additional sulfur atom and with one Fe ion replaced by the Mo ion (3). Ten years ago, near–atomic resolution crystallographic data revealed an atom in the middle of the cluster, bonded to six of the Fe ions (4). Its identity remained under debate until late last year, when two separate groups identified it as a carbide ion (5, 6). Remarkably, less than a year later, Wiig et al. (7) on page 1672 of this issue identify both the enzyme that inserts the carbon, NifB, and the molecule that supplies it, the ubiquitous biological carbon donor S-adenosyl-l-methionine (SAM).