The mechanism by which the human copper (Cu) chaperone Atox1 delivers Cu to metal-binding domains of Wilson disease (WD) protein for insertion into cuproenzymes is unclear. Using near-UV circular dichroism as a new tool to probe chaperone-target interactions, in combination with gel filtration and molecular dynamics simulations, we here demonstrate that Atox1 forms a stable Cu-dependent adduct with the fourth metal-binding domain of WD (WD4). Using point-mutated Atox1 variants, we show that the adduct forms in the absence of conserved residues M10 or T11 but K60 is essential for heterocomplex formation and Cu transfer. Dissection of heterocomplex energetic components reveals a crucial role for K60-mediated electrostatic interaction.