Molecular Interplay of Small Molecules and Calcium Ions with α-Synuclein Revealed by NMR and Molecular Dynamics Simulations

Human alpha-synuclein is an intrinsically disordered protein concentrated at presynaptic terminals and strongly linked to Parkinson's disease and other synucleinopathies. Its dynamic C-terminal region mediates interactions with small molecules and metal ions. Here, we used high-resolution nuclear magnetic resonance spectroscopy (NMR) and molecular dynamics (MD) simulations to characterize interactions between the C-terminal alpha-synuclein construct, the small molecule fasudil, and calcium ions. NMR data show that fasudil and Ca2+ bind preferentially to overlapping regions enriched in alternating tyrosine and acidic residues while preserving the protein's disordered nature. Side-chain-resolved spectra indicate distinct driving forces for fasudil and calcium binding. MD simulations reveal that Ca2+ modifies the local electrostatic environment, decreasing fasudil interaction frequency through electrostatic screening and steric effects. Despite this, fasudil retains dynamic, reversible contacts with key tyrosine residues. Overall, exposed alpha-synuclein conformations allow simultaneous, ligand-specific interactions, highlighting side-chain hotspots governing binding in Ca2+-rich conditions.