SPECTROSCOPIC AND MUTAGENESIS STUDIES ON THE CU-A CENTER FROM THE CYTOCHROME-C-OXIDASE COMPLEX OF PARACOCCUS-DENITRIFICANS

Cytochrome-e oxidase contains an unusual copper centre (Cu-A) located in subunit II. This centre mediates one-electron transfer from cytochr ome c to low-spin heme a. Recent spectroscopic and biochemical studies have shown that this centre is a valence delocalised dinuclear [Cu(+1 .5)-Cu(+1.5)] centre. We have measured the absorption, EPR and variabl e-temperature magnetic circular dichroism spectra of the Cu-A-binding domain isolated from Paracoccus denitrificans cytochrome aa(3). The EP R spectrum showed the following signals: g(parallel to) = 2.18; g(perp endicular to) = 2.03. g(parallel to) exhibited a seven-line hyperfine splitting pattern, with an intensity ratio showing that the single unp aired electron interacted equally with two copper nuclei. The magnetic circular dichroism spectrum was identical to those from Cu-A in bovin e heart cytochrome-c oxidase and centre A of nitrous-oxide reductase, showing the close structural similarity between the three centres. To identify the ligands of Cu-A, all the conserved putative ligands in th e P. denitrificans Cu-A domain were substituted. Only five residues, C ys244, Cys248, His209, His252, and Met255, were required for correct a ssembly of the Cu-A centre. Replacement of Met255 caused protein misfo lding. Hence, methionine may have a structural role for the folding of the protein rather than being a Cu-A ligand. Given that both copper i ons must have identical coordination geometries, the number of possibl e structures is limited. Two models are proposed: one involves the thi olate side-chains of Cys244 and Cys248 bridging a pair of copper ions with one histidine coordinating each copper ion, and the other has ter minal ligation of each copper ion by one cysteine and one histidine re sidue. In both models, the metal-metal distance can be sufficiently sh ort to permit direct d-orbital overlap of the copper ions. The magneti c circular dichroism transitions at 475 nm and 525 nm are assigned to thiolate-to-copper charge-transfer processes polarised perpendicular t o one another, although the magnetic circular dichroism intensities sh ow that the excited states were heavily mixed with copper d-orbitals. These intensities can be interpreted in the thiolate bridged model in terms of transitions within a Cu-2(SR)(2) rhomb. In the model involvin g terminal cysteine ligation, exciton coupling of two thiolate-to-copp er charge-transfer transitions of similar energy, polarised along the Cu-S bonds, would contribute two transitions perpendicular to one anot her. This requires that the cysteine ligands have a cis orientation re lative to one another. The spectral properties of the H252N variant (h istidine at position 252 replaced by asparagine) and the high-pH form of I? denitrificans Cu-A were similar, showing that one copper ion had lost one histidine ligand in the latter form. The dimer was converted into a valence trapped [Cu(+1)-Cu(+2)] state, which may retain the me tal-metal interaction.