The aggregation of α-synuclein (AS) is selectively enhanced by copper in vitro, and the interaction is proposed to play a potential role in vivo. In this work, we report the structural, residue-specific characterization of Cu(I) binding to AS and demonstrate that the protein is able to bind Cu(I) with relatively high affinity in a coordination environment that involves the participation of Met1 and Met5 residues. This knowledge is a key to understanding the structural-aggregation basis of the copper-catalyzed oxidation of AS.

Amyloid aggregation of α-synuclein (AS) has been linked to the pathological effects associated with Parkinson's disease (PD). Cu(II) binds specifically at the N-terminus of AS and triggers its aggregation. Site-specific Cu(I)-catalyzed oxidation of AS has been proposed as a plausible mechanism for metal-enhanced AS amyloid formation. In this study, Cu(I) 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 (1)MDVFM(5) constitute key structural determinants for the high-affinity binding of Cu(I) to the N-terminal region of AS. The replacement of one Met residue by Ile causes a dramatic decrease in the binding affinity for Cu(I), 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-Cu(I) 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 Cu(II) and Cu(I) 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.

Growing evidence supports a link between brain copper homeostasis, the formation of alpha-synuclein (AS)-copper complexes, and the development of Parkinson disease (PD). Recently it was demonstrated that the physiological form of AS is N-terminally acetylated (AcAS). Here we used NMR spectroscopy to structurally characterize the interaction between Cu(I) and AcAS. We found that the formation of an AcAS-Cu(I) complex at the N-terminal region stabilizes local conformations with α-helical secondary structure and restricted motility. Our work provides new evidence into the metallo-biology of PD and opens new lines of research as the formation of AcAS-Cu(I) complex might impact on AcAS membrane binding and aggregation.

α-Synuclein (AS) aggregation is associated to neurodegeneration in Parkinson's disease (PD). At the same time, alterations in metal ion homeostasis may play a pivotal role in the progression of AS amyloid assembly and the onset of PD. Elucidation of the structural basis directing AS-metal interactions and their effect on AS aggregation constitutes a key step towards understanding the role of metal ions in AS amyloid formation and neurodegeneration. Despite of the reported evidences that link Zn(2+) with the pathophysiology of PD and the fact that this metal ion was shown to promote AS fibrillation in vitro, neither the structural characterization of the binding sites nor the identification of the amino acids involved in the interaction of Zn(2+) with the protein AS has been carried out. By using NMR spectroscopy, we have addressed here unknown structural details related to the binding of Zn(2+) to the protein AS through the design of site-directed and domain truncated mutants of AS. The binding of zinc to the Aβ peptide was also studied and discussed comparatively. Although the results of this study contribute to the understanding of the structural and molecular basis behind the acceleration of AS fibrillation mediated by Zn(2+), the low affinity that characterizes the interaction of Zn(2+) with AS contrasts strongly with the high-affinity features reported for the binding of this metal ion to other target proteins linked to human amylodosis such as Aβ peptide and the Islet Amyloid Polypeptide (IAPP), challenging the biological relevance of zinc interactions in the pathogenesis of PD.