# Unravelling Catalytic Divergence in Mo- and Fe-Only Nitrogenases: The Role of the Heterometal-Site and Protein Environment from QM/MM Insights

## TL;DR

This paper investigates how molybdenum and iron-only nitrogenases differ in their catalytic behavior using quantum mechanical and molecular mechanical models.

## Contribution

The study reveals structural and electronic differences in the metal sites of Mo- and Fe-only nitrogenases that explain their distinct reactivity.

## Key findings

- FeFeco exhibits two low-lying E1 states due to sulfide protonation.
- FeMoco strictly favors μ2-sulfide protonation in its E1 state.
- FeFeco's unique μ3-sulfide-protonated state may explain its divergent reactivity.

## Abstract

Two key characteristics differentiate molybdenum-dependent
(MoFe)
and iron-only (FeFe) nitrogenases: their efficacy for N2 fixation and their product distributions for CO2 reduction,
yielding HCO2
– and CH4, respectively.
Despite their divergent properties, prior research argues that the
distinct cofactors share a common mechanism and equivalent structures
with the addition of “n” electrons
and protons at their E
n
catalytic states.
The proposed equivalence between the cofactors was foundational in
the assignment of an Fe-hydride at their E1 states based
on interpretation of FeFeco’s photolabile and thermolabile
E1 isomers (Inorg. Chem. 2022, 61, 5459–5464). However, the E0 state crystal
structures of FeMoco and FeFeco have sharp, previously unaddressed
distinctions in their metal–metal distances and proximal residue
identities at their respective octahedral M-sites (M = Mo or Fe).
Herein, we study QM/MM models of the E0 and E1 states of FeMoco and FeFeco to distinguish their geometric and electronic
structures. Our analysis shows diminished metal–metal bonding
and increased hydrogen bonding at FeFeco’s M-site, supporting
the presence of two energetically low-lying E1 states differentiated
by protonation of a μ3- or μ2-sulfide.
The calculated thermodynamic and kinetic properties of FeFeco’s
sulfide-protonated states agree with experimental data, without invoking
Fe-hydride formation. Unlike FeFeco, FeMoco’s E1 state strictly favors μ2-sulfide protonation. FeFeco’s
distinct μ3-sulfide-protonated E1 isomer
has a five-coordinate, reduced M-site that could explain its divergent
reactivity relative to FeMoco.