Impact of N-terminal acetylation on Cu(I) coordination by alpha synuclein protein

Amyloid aggregation of alpha-synuclein (AS) protein is associated with Parkinson's disease. Physiologically, AS plays a crucial role in the uptake, storage, and recycling of neurotransmitter vesicles. AS has three independent binding sites for Cu(II) and Cu(I) ions. N-terminal acetylation of AS impacts the highest-affinity site of Cu, encompassing the first five residues; it prevents Cu(II) coordination, enhances Cu(I) binding affinity, raises its redox potential, and extends the α-helix to the first ten residues. In this study, X-ray absorption spectroscopy and electronic structure calculations are employed to provide a detailed molecular description of the highest affinity Cu(I) binding site in AS, both in the acetylated AS (AcAS) and non-acetylated forms of the protein. The roles of methionine residues Met1 and Met5 in Cu(I) binding are also evaluated using peptide fragment models. Our findings indicate that in both cases, the coordination sphere is tetracoordinated, with the two sulfur atoms from Met1 and Met5 serving as the primary anchors for Cu(I) coordination. At the same time, Met1 plays a crucial role in stabilizing Cu(I). While both complexes include the carboxylate oxygen of Asp2, a key difference lies in the fourth ligand: the Cu(I)-AS complex utilizes the N-terminal group, whereas the Cu(I)-AcAS complex uses a carbonyl oxygen from the N-terminal acetyl group. These results provide deeper insights into how acetylation impacts the chemical properties of the high-affinity copper binding site in AS and contribute to a better understanding of the role of Cu(I) binding in the physiological function of AS.