Figure 1. Postulated mechanisms for the reaction of oxygen with the reduced Cu_B-heme a₃ binuclear center in CcO and for the reaction of H₂O₂ with the oxidized enzyme.

In this scheme the reaction with O₂ with the reduced binuclear center starts from the lower left corner and progresses to the P_M intermediate shown in the center. P_M is associated with a radical on Y244, which donates an electron to the binuclear center following the O-O bond cleavage reaction. The reaction sequence progresses to the P_R and F intermediates and ultimately to the oxidized state on the top left. The reaction with H₂O₂ is initiated by its binding to the heme a₃ of the oxidized enzyme. The oxidized bCcO preparation used for the present work contains a peroxide bridged between Cu and Fe.^{25, 56} Thus, the O species in the scheme are likely to be formed by the H₂O₂ reduction of the bridged peroxide. The O-O bond cleavage of the resulting hydroperoxy intermediate leads to a Compound I species with a porphyrin π-cation, which can accept an electron from Y244 to form P_H, leaving a radical on Y244 (Pathway A). The Y244 radical can be re-reduced by Y129 to form F^•. Alternatively, the porphyrin π-cation can accept an electron directly from Y129 to form F^• without passing through Y244 (Pathway B). The P_H intermediate formed via Pathway A is the same as that (P_M) generated in the O₂ reaction and depending on the pH can progress to form P_R and F.