Structure analysis of bovine heart cytochrome c oxidase at 2.8 angstrom resolution

The crystal structure of bovine heart cytochrome c oxidase has been determi ned at 2.8 Angstrom resolution by the multiple isomorphous replacement (MIR ) method with three heavy-atom derivatives. An asymmetric unit of the cryst al has a molecular weight of 422 kDa. Eight heavy atoms as main sites of a CH3HgCl derivative were clearly located by solving the difference Patterson function. The electron density obtained by the MIR method was refined by d ensity modification, consisting of solvent flattening, histogram matching a nd noncrystallographic symmetry averaging. The enzyme exhibits a dimeric st ructure in the crystal. Out of 3606 amino-acid residues in 26 subunits in t he dimer, 3560 residues were located in the electron-density map. The struc ture was refined by X-PLOR. The final R factor and the free R factor were 0 .199 and 0.252 at 2.8 Angstrom resolution, respectively. One monomer in the dimeric structure with a stronger packing interaction has a lower averaged temperature factor than the other, by 16 Angstrom(2). The region +/-12 Ang strom from the centre of the transmembrane part is almost 100% alpha-helix, despite the glycine residue content being as high as 7.1% in the transmemb rane region. The residues around haem a of animals have evolved away from t hose of bacteria in contrast with the residues of the haem a(3), The hierar chy of the structural organization of the enzyme complex has been proposed on the basis of intersubunit interactions.