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Science  16 Nov 2007:
Vol. 318, Issue 5853, pp. 1041
DOI: 10.1126/science.318.5853.1041a

Nitrogenase enzymes use an elaborate metal cluster to catalyze the reduction of dinitrogen to ammonia under remarkably mild conditions. Two questions about this process continue to puzzle researchers: What are the elementary steps underlying the scission of the nitrogen triple bond, and how is the cluster that guides these steps assembled? Curatti et al. shed light on the latter question by reconstituting from purified components a system for in vitro synthesis of the cluster—which contains 7 Fe, 9 S, Mo, homocitrate, and one as-yet unidentified light atom and is called the FeMo cofactor. Of 11 nitrogen fixation (Nif) proteins previously shown to be involved in FeMo cofactor biosynthesis, they find that NifB, NifEN, and NifH are key. NifB assembles ferrous iron, sulfide, and S-adenosylmethionine into the NifB cofactor (a precursor of the FeMo cofactor) under reducing conditions; NifEN pushes the synthesis one step further by converting the NifB cofactor into the VK cluster, to which molybdate and homocitrate are then added in a NifH-dependent fashion. The other Nif proteins are thought to supply the relevant forms of Fe, S, and Mo under in vivo conditions and also to protect labile intermediates. The catalytic competency of the synthesized FeMo cofactor (whose structure is still unknown) was confirmed by its ability to combine with apo-NifDK into a holoenzyme that reduced nitrogen. — GJC

Proc. Natl. Acad. Sci. U.S.A. 104, 17626 (2007).

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