H2 Formation Holds the Key to Opening the Fe Coordination Sites of Nitrogenase FeMo-cofactor for Dinitrogen Activation

Abstract

The present quantum-mechanical and molecular-mechanics study reveals the crucial roles of H<sub>2</sub> formation, of H<sub>2</sub>S shift and of N<sub>2</sub> bond expansion in the nitrogenase process of the reduction of N<sub>2</sub> to <a href="https://en.wikipedia.org/wiki/Ammonia">NH<sub>3</sub></a>. Proton and electron transfers to the Fe(C@Fe<sub>6</sub>S<sub>9</sub>)Mo unit of the FeMo-co complex weaken the Fe-S and Fe-H bonds and expose the <b>Fe</b> coordination sites, coupled with energy release due to H<sub>2</sub> generation. Thereby the two sites <b>Fe2</b> and <b>Fe6</b> become prepared for stronger N<sub>2</sub> adsorption, expanding and attenuating the ǀN≡Nǀ bond. After subsequent detachment of H<sub>2</sub>S from its Fe binding site into a holding site of the rearranged protein residue, the <b>Fe6</b> site becomes completely unfolded, and the N<sub>2</sub> triple bond becomes completely activated to an ‑<u>N</u>=<u>N</u>- double bond for easy subsequent hydrogenation to NH<sub>3</sub>. We explain in particular, why the obligatory H<sub>2</sub> formation is an essential step in N<sub>2</sub> adsorption and activation

How Thermal Fluctuations Influence the Function of the FeMo-cofactor in Nitrogenase Enzymes

“…Additional clues in the nitrogenase story include inhibitor studies of MoFe-protein using carbon monoxide 39,40 , hydroxo 41 and selenium 42 , and provide powerful tools to prepare the stably trapped transient states and indicate the activation of N 2 occurs on a di-iron edge of FeMo-co. A recent study reported that the dissociated HS − species can be held within a protein cavity created by the reorientation of glutamine in V-nitrogenase 43 . Together with the facile exchange of all the three bridged sulfur sets, and the lability of the S2B towards ligand exchange found from experiments of site-selective incorporation of selenium in FeMo-co 42,44 , this sets up the possibility of partial or full dissociation of bridged S2B from FeMo-co as recently discussed by us and other theoretical work [45][46][47][48][49][50] .…”