Fig. 2: Neutron structures and protonation states at the active site of D₂O₂-soaked, reduced, and oxidized Trp161Phe MnSOD.

a D₂O₂-soaked Trp161Phe MnSOD for chain B with a singly-protonated dioxygen ligand (denoted LIG). Chain A is displayed as an inset to highlight differences in the difference density shapes that led to the identification of LIG in chain B. b D₂O₂-soaked Trp161Phe MnSOD for chain B. c Trp161Phe Mn²⁺SOD for chain A. d Active site overlay of wildtype Mn²⁺SOD and Trp161Phe Mn²⁺SOD demonstrating differences in hydrogen bond strength of the Gln143 amide anion and movement of WAT1. Inset highlights the position of the residue 161 mutation. e Trp161Phe Mn³⁺SOD for chain A. f Active site overlay of wildtype Mn³⁺SOD and Trp161Phe Mn³⁺SOD. Inset highlights the position of the residue 161 mutation. Green, orange, and magenta omit |F_o | - |F_c| difference neutron scattering length density of protons displayed at 2.5σ, 3.0σ, and 3.5σ, respectively. Light blue 2|F_o | - |F_c| density is displayed at 1.0σ. Distances are in Å. Dashed lines indicate typical hydrogen bonds and hashed lines indicate SSHBs that are hydrogen bonds <1.8 Å. All neutron structures were solved to 2.3 Å resolution, including those of wildtype MnSOD from our previous work²⁷. For the resting state structures, only one active site is shown due to high structural similarities; see Supplementary Fig. 2 for the other active site. All neutron structures were solved to 2.3 Å resolution.