Electronic and Structural Heterogeneity in the Diiron Center of Sulerythrin: Insights From Hybrid QM/MM Calculations

Sulerythrins (SulE) are ferritin‐like proteins from obligate aerobes such as Sulfolobus tokodaii , forming a domain‐swapped dimer with a four‐helix‐bundle scaffold and a heterobimetallic Fe–Zn center. The diFe‐SulE variant resembles diiron carboxylate proteins and contains two bimetallic active sites coordinated by histidines, glutamates, and bridging oxo ligands. High‐resolution crystallography revealed slight differences in Fe–Fe distances and mixed‐valence states, but the precise chemical nature of the oxo species remains unclear. To clarify the electronic and structural properties of diFe‐SulE, we performed hybrid quantum mechanical/molecular mechanics (QM/MM) calculations on models varying in protonation, dioxo ligands, and iron redox states of the active site. Our results reveal at least three electronic states for diFe‐SulE: (i) a diferrous center with an end‐on di‐μ‐hydroperoxo ligand; (ii) a diferric center with hydroxo ligands interacting with protonated Glu95; and (iii) a diferrous center bridged by a di‐μ‐peroxo ligand, also interacting with protonated Glu95. These states are consistent with the structural heterogeneity observed experimentally. Overall, the hybrid QM/MM approach refines the crystallographic models and offers subatomic‐level insight into the electronic structure and reactivity of the SulE diiron center, deepening our understanding of nonheme diiron enzymes.