Figure 1.

X-ray structure of the active sites and a schematic representation of the catalytic cycle of bovine heart CcO.A, X-ray structure of the active sites. Metal sites are indicated by brown, violet, and beige spheres for iron, copper, and magnesium ions, respectively. Porphyrins of heme a and heme a₃ are represented by the magenta stick models as labeled. Within the stick models of the amino acid residues, dark blue, red, and yellow portions are nitrogen, oxygen, and carbon, respectively. A beige arrow indicates the location of the electron transfer passage, while two black arrows indicate those for protons for producing water molecules. The hydrogen-bond network and the water channel of the H-pathway are indicated by the red and blue portions of the leftmost curved arrow, respectively. The Mg/H₂O cluster (the blue area) is attached to the hydrogen-bond network of the H-pathway via a short hydrogen-bond network (the gray area). Small blue spheres in the Mg/H₂O cluster mark the positions of water molecules, while small beige spheres mark the other water molecule positions. The formyl group and one of the propionate groups of heme a are hydrogen-bonded with Arg³⁸ and a fixed water molecule in the hydrogen-bond network of the H-pathway. The inset shows the overall locations of the redox-active metal sites and pathways for transportation of electrons and protons within the CcO structure, indicated by Cα-backbone traces. This figure was prepared from the X-ray diffraction data of PDB 5B1A. B, a schematic representation of the structural changes in the O₂ reduction site of CcO; Fe_a3 and Cu_B are the iron and copper ions; Tyr-OH and Tyr-O• denote Tyr²⁴⁴ located in its protonated and neutral radical states, respectively. The six intermediate forms in the catalytic cycle are designated as A-to R-forms. The reaction steps coupled with proton pumping are indicated by straight arrows marked with “H⁺”. This figure is a slightly modified version of the previous paper (23).