Figure 1. Energy profile for the Cu ⁺ transfer reaction for different types of metallochaperone/partner interactions. In (a), the equilibrium constant for Cu ⁺ exchange between the chaperone and the partner is close to 1, while in (b) the copper affinity of the partner is higher than that of the chaperone. In both panels, the dashed line describes the situation where the intermediate adduct does not accumulate to a significant extent in the test tube, and therefore might not be detectable, while the thick line describes the situation where the adduct is more stable than the separate proteins.

Figure 2. Mechanism for Cu ⁺ transfer between metallochaperone and copper‐receiving protein partner. The Cu ⁺ chaperone and Cu ⁺‐receiving protein partner are shown in magenta and violet, respectively. Sulfur‐coordinating ligands and the metal ion are shown. The coordination of Cu ⁺ involves a rapid equilibrium of various species.

Figure 3. Comparison of a selected peak region for the – HSQC NMR spectra of (A) ‐labeled C15A‐Atx1 alone and in the presence of equimolar unlabeled wild‐type (WT)‐Ccc2a, (B) ‐labeled C18A‐Atx1 alone and in the presence of equimolar unlabeled WT‐Ccc2a, (C) ‐labeled WT‐Atx1 alone and in the presence of equimolar unlabeled C13A‐Ccc2a, and (D) ‐labeled WT‐Atx1 alone and in the presence of equimolar unlabeled C16A‐Ccc2a. The inset shows a close‐up view of the peak of C15. Only (B) and (D) correspond to instances of complex formation. All spectra have been recorded on samples containing also one equivalent (with respect to Atx1) of Cu ⁺. The spectra of Cu ⁺‐Atx1 alone are in green, those in the presence of Ccc2a are in red.