STRUCTURE OF THE BINUCLEAR HEME IRON-COPPER SITE IN THE QUINOL-OXIDIZING CYTOCHROME AA(3), FROM BACILLUS-SUBTILIS

Cytochrome aa(3)-600 is a terminal quinol oxidase of Bacillus subtilis, belonging to the large family of structurally and functionally related respiratory enzymes to which the mitochondrial cytochrome c oxidase also belongs. However, the Cu-A center typical of the cytochrome c oxidases is lacking from cytochrome aa(3)-600. The presence of only one copper, viz. Cu-B of the binuclear heme iron-copper site, makes cytochrome aa(3)-600 especially suitable for XAS analysis of this structure. Cu and Fe XAS data for fully oxidized cytochrome aa(3)-600 indicate a structure for the binuclear site similar to that previously reported for mitochondrial cytochrome c oxidase (see Powers et al. (1981) Biophys. J. 34, 465-468). Heme Fe-a3 has a proximal histidine nitrogen ligand 2.10 +/- 0.02 Angstrom from the iron, and a distal S or Cl ligand at 2.36 +/- 0.03 Angstrom. The latter is also a ligand of Cu-B (2.21 +/- 0.02 Angstrom), and apparently forms a bridge between the two metals which are 3.70 +/- 0.06 Angstrom apart. Cu-B has two more close-lying ligands at 1.95 +/- 0.02 Angstrom which are likely histidine nitrogens. The similarity between EXAFS of Cu-B and type 1 'blue' copper is contrasted to EPR and optical spectroscopic properties of Cu-B and the nature of the bridging ligand is discussed.