Fig. 4.

Model of the stepwise mechanism of P-cluster formation. Before maturation, the P-cluster species (i.e., precursor) in ΔnifH MoFe protein is composed of a pair of [Fe₄S₄]-like clusters in each αβ dimer (A). Upon incubation with Fe protein, MgATP and dithionite, the two P-cluster precursors in ΔnifH MoFe protein are matured in a stepwise fashion, i.e., the precursor in one αβ dimer is converted to a fully matured P-cluster (B) before the other in the second αβ dimer (C). Concurrent with the sequential formation of the P-clusters, the FeMoco sites in MoFe protein are converted from a closed conformation to an open one, permitting the subsequent insertion of FeMoco into these sites.