A key step in the global nitrogen cycle is the reduction of atmospheric dinitrogen to ammonia by nitrogenase, a complex metalloenzyme. The catalytic component of this enzyme, the molybdenum-iron protein NifDK, comprises two unusual metallo-clusters, the [8Fe-7S] P cluster and the [Mo-7Fe-9S-X-homocitrate] M cluster. Likewise, the protein NifEN displays sequence similarity to NifDK, but it plays a role in nitrogenase assembly: It converts an iron-only precursor form to the mature molybdenum cluster and delivers this to NifDK.
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