The intrinsic stability of the second intermediate following the dioxygen-bound form in the O2 reduction by cytochrome c oxidase.

Aeration of a two-electron reduced cytochrome c oxidase provides a species with two Raman bands at 804 and 356 cm(-1), identifying it as the second intermediate following the O2-bound species in the enzymatic O2 reduction process. It degrades directly to the fully oxidized form with a half-life time of 70 min at pH 8.0. The stability suggests an effective insulation for the active site in an extremely high oxidation state (Fe4+ with one oxidative equivalent nearby) against spontaneous electron leaks, which would dissipate proton motive force. The formation and degradation of the second intermediate are pH-dependent.