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. Structural and spectroscopic techniques, including spatially resolved anomalous dispersion (SpReAD) refinement, were combined to report oxidation assignments for individual irons in the cluster for each overall state.

Abstract

The nitrogenase iron protein (Fe‐protein) contains an unusual [4Fe:4S] iron‐sulphur cluster that is stable in three oxidation states: 2+, 1+, and 0. Here, we use spatially resolved anomalous dispersion (SpReAD) refinement to determine oxidation assignments for the individual irons for each state. Additionally, we report the 1.13‐Å resolution structure for the ADP bound Fe‐protein, the highest resolution Fe‐protein structure presently determined. In the dithionite‐reduced [4Fe:4S]¹⁺ state, our analysis identifies a solvent exposed, delocalized Fe^2.5+ pair and a buried Fe²⁺ pair. We propose that ATP binding by the Fe‐protein promotes an internal redox rearrangement such that the solvent‐exposed Fe pair becomes reduced, thereby facilitating electron transfer to the nitrogenase molybdenum iron‐protein. In the [4Fe:4S]⁰ and [4Fe:4S]²⁺ states, the SpReAD analysis supports oxidation states assignments for all irons in these clusters of Fe²⁺ and valence delocalized Fe^2.5+, respectively.

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