Stabilization of the peroxy intermediate in the oxygen splitting reaction of cytochrome cbb₃

The proton-pumping cbb₃-type cytochrome c oxidases catalyze cell respiration in many pathogenic bacteria. For reasons not yet understood, the apparent dioxygen (O₂) affinity in these enzymes is very high relative to other members of the heme-copper oxidase (HCO) superfamily. Based on density functional theory (DFT) calculations on intermediates of the oxygen scission reaction in active-site models of cbb₃- and aa ₃-type oxidases, we find that a transient peroxy intermediate (I _P, Fe[III]-OOH^-) is ~ 6 kcal/mol more stable in the former case, resulting in more efficient kinetic trapping of dioxygen and hence in a higher apparent oxygen affinity. The major molecular basis for this stabilization is a glutamate residue, polarizing the proximal histidine ligand of heme b₃ in the active site.