Quantitative mapping of methionine sensitivity to oxidation in the copper-bound PcuC chaperone

Copper is typically coordinated by histidine, cysteine, or methionine in proteins, and these residues are particularly sensitive to oxidation. However, it remains unclear whether copper-coordinating residues are more prone to oxidation than non-coordinating ones, and how their susceptibility changes between the apo and copper-bound states. The copper chaperone PcuC, important for cytochrome c oxidase assembly in bacteria, contains a canonical binding site composed of two histidines and two methionines (H51xnM63x22H86xM88), as well as a disordered C-terminal extension enriched in methionine and histidine. To quantify methionine oxidation sensitivity in both apo- and Cu-bound PcuC, we used a methionine-specific oxaziridine probe combined with mass spectrometry and compared labeling patterns to those generated by 18O-labeled hydrogen peroxide. We show that methionine residues display distinct oxidation sensitivities in the apoprotein, and that the oxaziridine reacts similarly to H₂18O₂. Importantly, this probe enables quantification of methionine oxidation independently of hydroxyl radicals generated by copper-driven Fenton chemistry, which lacks residue specificity. In the copper-bound form, Cu binding strongly alters methionine reactivity, with a marked increase in oxidation of the coordinating Met63 and Met88. Structural analysis revealed that two copper ions occupy the canonical site, while the C-terminal extension does not contribute to coordination. Comparison of structural features and oxidation sensitivity showed that methionine susceptibility correlates with solvent exposure in the folded domain, but with local positive charge in the disordered region. These findings demonstrate that copper coordination modulates methionine oxidation, and that oxaziridine-based probes provide a powerful tool for mapping oxidation sensitivity in (metallo)proteins.