Site‐specific oxidation state assignments of the irons in the [4Fe:4S]2+/1+/0 states of the nitrogenase Fe‐protein

The nitrogenase iron protein (Fe‐protein) serves as the electron donor for biological nitrogen fixation. Understanding how the Fe‐protein controls electron transfer to the active site is critical in addressing the mechanism of substrate reduction. The Fe‐protein contains an unusual [4Fe:4S] iron‐sulphur cluster that is stable in three oxidation states: 2+, 1+ and 0. Here, we combine structural and spectroscopic techniques, including spatially resolved anomalous dispersion refinement (SpReAD), to report oxidation assignments for individual irons in the cluster for each overall state. Additionally, we report the 1.13‐Å resolution structure for the Fe‐protein with bound ADP, the highest resolution Fe‐protein structure presently determined. In the dithionite‐reduced [4Fe:4S]1+ state, the SpReAD analysis supports the oxidation state assignment of a delocalized Fe2.5+ pair and a reduced Fe2+ pair. Our work identifies the Fe2.5+ pair as coordinated by the solvent exposed Cys97, while the Fe2+ pair faces the protein interior and is coordinated by Cys132. It is proposed that binding of ATP to the Fe‐protein promotes an internal redox rearrangement such that the solvent‐exposed Fe becomes reduced, thereby facilitating electron transfer to the nitrogenase molybdenum iron‐protein. The SpReAD analysis supports a uniform oxidation state assignment of Fe2+ for all irons in the titanium citrate‐reduced [4Fe:4S]0 state, while all irons in the IDS oxidized [4Fe:4S]2+ state are assigned to the valence delocalized Fe2.5+ state.

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