Assembly scaffold NifEN: A structural and functional homolog of the nitrogenase catalytic component

NifEN is a biosynthetic scaffold for the cofactor of Mo-nitrogenase (designated the M-cluster). Previous studies have revealed the sequence and structural homology between NifEN and NifDK, the catalytic component of nitrogenase. However, direct proof for the functional homology between the two proteins has remained elusive. Here we show that, upon maturation of a cofactor precursor (designated the L-cluster) on NifEN, the cluster species extracted from NifEN is spectroscopically equivalent and functionally interchangeable with the native M-cluster extracted from NifDK. Both extracted clusters display nearly indistinguishable EPR features, X-ray absorption spectroscopy/extended X-ray absorption fine structure (XAS/EXAFS) spectra and reconstitution activities, firmly establishing the M-cluster-bound NifEN (designated NifENM) as the only protein other than NifDK to house the unique nitrogenase cofactor. Iron chelation experiments demonstrate a relocation of the cluster from the surface to its binding site within NifENM upon maturation, which parallels the insertion of M-cluster into an analogous binding site in NifDK, whereas metal analyses suggest an asymmetric conformation of NifENM with an M-cluster in one alpha beta-half and an empty cluster-binding site in the other alpha beta-half, which led to the proposal of a stepwise assembly mechanism of the M-cluster in the two alpha beta-dimers of NifEN. Perhaps most importantly, NifENM displays comparable ATP-independent substrate-reducing profiles to those of NifDK, which establishes the M-cluster-bound alpha beta-dimer of NifENM as a structural and functional mimic of one catalytic alpha beta-half of NifDK while suggesting the potential of this protein as a useful tool for further investigations of the mechanistic details of nitrogenase.