Coordination Features and Affinity of the Cu2+ Site in the α-Synuclein Protein of Parkinson's Disease

Parkinson's disease (PD) is the second most prevalent age-related, neurodegenerative disorder, affecting >1% of the population over the age of 60. PD pathology is marked by intracellular inclusions composed primarily of the protein alpha-synuclein (alpha-syn). These inclusions also contain copper, and the interaction of Cu2+ with alpha-syn may play an important role in PD fibrillogenesis. Here we report the stoichiometry, affinity, and coordination structure of the Cu2+ alpha-syn complex. Electron paramagnetic resonance (EPR) titrations show that monomeric alpha-syn binds 1.0 equiv of Cu2+ at the protein N-terminus. Next, an EPR competition technique demonstrates that alpha-syn binds Cu2+ with a K-d of approximate to 0.10 nM. Finally, EPR and electron spin echo modulation (ESEEM) applied to a suite of mutant and truncated alpha-syn constructs reveal a coordination sphere arising from the N-terminal amine, the Asp2 amide backbone and side chain carboxyl group, and the His50 imidazole. The high binding affinity identified here, in accord with previous measurements, suggests that copper uptake and sequestration may be a part of alpha-syn's natural function, perhaps modulating copper's redox properties. The findings further suggest that the long-range interaction between the N-terminus and His50 may have a weakening effect on the interaction of alpha-syn with lipid membranes, thereby mobilizing monomeric alpha-syn and hastening fibrillogenesis.