Abstract
Copper is essential for proper functioning of cytochrome c oxidases, and therefore for cellular respiration in eukaryotes and many bacteria. Here we show that a new periplasmic protein (PCu(A)C) selectively inserts Cu(I) ions into subunit II of Thermus thermophilus ba(3) oxidase to generate a native Cu(A) site. The purported metallochaperone Sco1 is unable to deliver copper ions; instead, it works as a thiol-disulfide reductase to maintain the correct oxidation state of the Cu(A) cysteine ligands.