Figure 2.

A simplified scheme for the reaction between cytochrome oxidase and O₂. The binuclear site, which contains heme a₃, Cu_B, and the cross-linked, H240 - Y244 (H-Y) structure, is shown. Reduction and protonation of the oxidized form of the center produces the reduced site. This binds O₂ to form initially the oxy species, which reacts further to produce P and F intermediates, before regenerating the oxidized form of the enzyme. The reduction of P and F are limited by proton transfer reactions, as indicated. The steps between P and the reduced form of the site have been implicated in proton pumping processes, which are indicated by red arrows. The stoichiometry of these steps is a matter of current investigation, although up to four protons can be pumped during the complete cycle.