Site‐Specific Oxidation State Assignments of the Iron Atoms in the [4Fe:4S]^2+/1+/0 States of the Nitrogenase Fe‐Protein

Abstract

The nitrogenase iron protein (Fe-protein) contains an unusual [4Fe:4S] iron-sulphur cluster that is stable in three oxidation states:2 + ,1 + ,a nd 0. Here,w eu se spatially resolved anomalous dispersion (SpReAD) refinement to determine oxidation assignments for the individual irons for each state.A dditionally,w er eport the 1.13-resolution structure for the ADP bound Fe-protein, the highest resolution Fe-protein structure presently determined. In the dithionitereduced [4Fe:4S] 1+ state,o ur analysis identifies as olvent exposed, delocalized Fe 2.5+ pair and ab uried Fe 2+ pair.W e propose that ATPb inding by the Fe-protein promotes an internal redoxrearrangement such that the solvent-exposed Fe pair becomes reduced, therebyf acilitating electron transfer to the nitrogenase molybdenum iron-protein. In the [4Fe:4S] 0 and [4Fe:4S] 2+ states,t he SpReAD analysis supports oxidation states assignments for all irons in these clusters of Fe 2+ and valence delocalized Fe 2.5+ ,respectively.

Biological nitrogen fixation in theory, practice, and reality: a perspective on the molybdenum nitrogenase system

“…Another arena for important experimental advances is in the development of Fe protein‐independent systems that utilize a photo‐activated supply of electrons [ 108 , 109 ] or electrocatalysis [ 110 , 111 ] to reduce the MoFe protein; this would allow for the electron supply to be better controlled to more effectively generate homogenous samples in defined E n states for analysis. The recent revolution in electron microscopy is opening up structural studies of solutions of nitrogenase in defined states [ 112 , 113 , 114 ], as well as under turnover conditions [ 114 ]. Computational analyses will benefit from advances that allow for more realistic treatment of complex metalloclusters [ 115 , 116 ], as well as to incorporate more of the surrounding protein and account for the dynamics of the protein structure on the chemistry within the active site.…”