The Cu_A center in subunit II of cytochrome c oxidase, the terminal enzyme of aerobic respiration, transfers electrons from cytochrome c to the proton-pumping machinery in subunit I. The unique electronic absorption and EPR spectra of Cu_A exclude it from classification with the well-studied biological copper centers. High-resolution X-ray structures of Cu_A-containing proteins reveal two copper atoms approximately 2.5 Å apart, bridged by two cysteine sulfurs. Each Cu has a terminal histidine ligand and a weak ligand, methionine for one and a main chain carbonyl for the other. These structures are consistent with earlier EPR measurements and theoretical calculations, which predicted a highly delocalized mixed-valence [Cu(II),Cu(I)] Cu_A site. Here we report ^1H NMR measurements at 600 MHz on a soluble Cu_A domain from Thermus thermophilus cytochrome ba_3
