Figure 10.

A schematic representation of the catalytic cycle of bovine CcO based on the X-ray structures of the catalytic intermediate forms of bovine CcO. The O₂ reduction site includes Fe_a3, Cu_B and Tyr²⁴⁴OH group. When the R-form is attained (A), the Fe_a3²⁺ receives O₂ to yield the A-form providing a weak interaction between Cu_B¹⁺ and the bound O₂, which is displayed by the absence of any line between Cu_B¹⁺ and the O₂ (B). The present OxyMb-type structure supports the weak interaction between O₂ and Cu_B¹⁺ in the A-form, which has been proposed by resonance Raman and X-ray structural analyses (1, 12, 27). The A-form is relaxed into the P-form in which the bound O₂ has been completely reduced, giving Fe_a3⁴⁺ = O²⁻, Cu_B²⁺-OH⁻, and Tyr²⁴⁴ radical with a hydrogen-bonded water (the interstitial water) (C). During the P→F transition, the Tyr²⁴⁴ radical is transformed to Tyr²⁴⁴OH by a proton-coupled electron transfer. The red arrow in (C) marks the substrate proton transfer pathway from D-pathway to the Tyr²⁴⁴ radical through the OH⁻ group at the Cu_B²⁺, the O²⁻ at the Fe_a3⁴⁺, and the interstitial water. (The bent structure of HisN-Fe_a3⁴⁺ = O²⁻ of the P-form (24) is illustrated schematically in C.) The second proton-coupled electron transfer to Fe_a3⁴⁺ = O²⁻ of the F-form from the D-pathway via the OH⁻ group, as illustrated by a red arrow in (D), forms the O-form (E). The low-barrier hydrogen bonds between Fe_a3⁴⁺ = O²⁻ and Cu_B²⁺-OH⁻ in both the P- and F-forms are shown by thick dotted lines in C and D. The unusually long distance between Cu_B²⁺ and OH⁻ is illustrated by deleting any lines between them (E). In the O to E transition (E to F), the substrate protons are transferred from K-pathway to the OH⁻ group near the Cu_B²⁺ through the interstitial water and the OH⁻ group at the Fe_a3³⁺ (E), giving Cu_B¹⁺-H₂O (F) as illustrated by red arrows (E). In the O- and E-forms, normal hydrogen bonds are detectable between the two oxygen atoms as illustrated by thin dotted lines (E and F). The forth proton-coupled electron transfer through K-pathway and the interstitial water illustrated by a red arrow (F) regenerates the R-form (A), releasing two water molecules. The interstitial water molecules, hydrogen-bonded to Tyr²⁴⁴, in C–F could be transferred reversibly from a storage site near the O₂-reduction site, not from the bulk water phase.