Site-Specific Copper-Catalyzed Oxidation of α‑Synuclein: Tightening the Link between Metal Binding and Protein Oxidative Damage in Parkinson’s Disease

Amyloid aggregation of α-synuclein (AS) has been linked to the pathological effects associated with Parkinson’s disease (PD). CuII binds specifically at the N-terminus of AS and triggers its aggregation. Site-specific CuI-catalyzed oxidation of AS has been proposed as a plausible mechanism for metal-enhanced AS amyloid formation. In this study, CuI binding to AS was probed by NMR spectroscopy, in combination with synthetic peptide models, site-directed mutagenesis, and C-terminal-truncated protein variants. Our results demonstrate that both Met residues in the motif 1MDVFM5 constitute key structural determinants for the high-affinity binding of CuI to the N-terminal region of AS. The replacement of one Met residue by Ile causes a dramatic decrease in the binding affinity for CuI, whereas the removal of both Met residues results in a complete lack of binding. Moreover, these Met residues can be oxidized rapidly after air exposure of the AS-CuI complex, whereas Met-116 and Met-127 in the C-terminal region remain unaffected. Met-1 displays higher susceptibility to oxidative damage compared to Met-5 because it is directly involved in both CuII and CuI coordination, resulting in closer exposure to the reactive oxygen species that may be generated by the redox cycling of copper. Our findings support a mechanism where the interaction of AS with copper ions leads to site-specific metal-catalyzed oxidation in the protein under physiologically relevant conditions. In light of recent biological findings, these results support a role for AS–copper interactions in neurodegeneration in PD.