Structures and Locations of the Metal Sites

The long alkyl side chain of heme a is in an almost extended conformation whereas that of heme % is in a U-shaped conformation (Tsukihara 

FIGURE 5. A schematic representation of the metal site locations in hovine heart cytochrome c oxidase. The molecular surface is determined from the electron density map at 5 resolution and is shown by the cage. 

FIGURE 6. X-ray structure of Fea3—Cub site of the fully oxidized form at 2.3 A resolution. The (Fo-Fc) difference Fourier map of the oxidized form calculated by omitting His240 and Tyr244, and any ligand between Fcys and Cub from the Fc calculation. Contours are drawn at 7 a level (lo = 0.0456e-/A3). 

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X-ray structures of 2.8-A resolution of bovine heart cytochrome c oxidase with the metals in the fully oxidized state were reported in 1995 (Tsukihara et al., 1995). The X-ray structure of cytochrome c oxidase from Paracoccus denitrificans in the fuUy oxidized azide-bound state at 2.8-A resolution was also published in the same week (Iwata et al., 1995). The structure and location of the metal sites of the two enzymes are astonishingly similar at that resolution. Later, the resolution of the bovine enzyme structure was improved to 2.3A (Yoshikawa et al., 1998). However, resolution of the Paracoccus enzyme has been improved to 2.7-A resolution (Ostermeier et al., 1997). Recently another bacterial ba3-type oxidase at 2.3-A resolution (Soulimane et al., 2000) and Escherichia coli quinol oxidase at 3.5-A resolution were reported (Abramson et al., 2000). X-ray structures of the protein and its redox-active metal sites are discussed in terms of the bovine enzyme below. 

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