Copper is typically coordinated by histidine, cysteine, or methionine residues in enzyme or chaperones and these residues are particularly sensitive to oxidation. However, it remains unclear whether copper-coordinating residues exhibit different sensitivity to oxidation compared to their non-coordinating counterparts, nor whether their oxidation susceptibility varies between the apo and copper-bound states of the protein. To evaluate this, we combined redox proteomics to structural analysis using the Cereibacter sphaeroides copper chaperone PcuC, which participates in the assembly of bacterial respiratory cytochrome oxidase. PcuC contains a canonical copper binding site made of two Met and two His (H51xnM63x22H86xM88), binding a Cu(I), and an unfolded C-terminal extension, rich in Met and His, proposed to coordinate an extra Cu atom. Our results showed that, in the apo-PcuC, Met135 in the C-terminal extension is highly sensitive to oxidation contrary to copper-coordinating Met, but that in the holo-PcuC, the presence of Cu strongly increased their sensitivity.