Fig. 1: The NifEN complex in FeMo-cofactor maturation.
From: Trafficking of a nitrogenase FeMo-cofactor assembly intermediate
a, Mo-nitrogenase cofactors. The active site FeMo-co is assembled along a complex pathway ex situ and only inserted into apo NifDK as the final step of assembly. The key intermediate of cofactor assembly is NifB-co (L-cluster), whose final modification steps take place at the NifEN complex. b, Overview of maturases and chaperones involved in FeMo-co biogenesis. The NifSU machinery produces [4Fe:4S] cubanes. The maturase NifB fuses two of them and inserts a carbide from SAM and a sulfide to yield the [8Fe:9S:C] NifB-co. NifX shuttles NifB-co to the NifEN complex, where Mo and homocitrate are inserted before the chaperone NafY transports and inserts the complete FeMo-co into apo NifDK. c, Cryo-EM maps for A. vinelandii NifEN at 2.14-Å resolution in front and top view. NifE, dark blue; NifN, light blue. The resolved N termini of two bound NifX proteins are shown in green. d, Three-domain architecture of NifE and overlay of the Cα backbone of the NifEN cryo-EM structure with the crystal structure (PDB 3PDI) for the third Rossmann fold domain of NifE (αIII) that is most relevant to NifB-co binding.
