Amino acid osmolytes disrupting intradomain interactions reduce the amyloidogenicity of α-synuclein: studies with charged <scp>l</scp> -amino acids and their derivatives

Aggregation of α-synuclein (α-syn) is a hallmark of Parkinson's and dementia with Lewy bodies pathogenesis. The high plasticity and lack of stable tertiary structure make α-syn more susceptible to its surrounding environment. Under stress conditions, small organic molecules known as osmolytes accumulate inside the cells. They affect the conformational states and fibrillation pathways of proteins. Here, the effects of eight different amino acid osmolytes (charged: l-glutamate, l-aspartate, l-lysine, l-arginine; amide side chains: l-glutamine, l-asparagine; and <i>N</i>-acetylated: <i>N</i>-acetyl-l-glutamic acid, <i>N</i>-acetyl-l-lysine) on the fibrillation of human α-synuclein were examined. Arginine and <i>N</i>-acetyl-l-lysine inhibited the fibrillation at concentrations above 0.2 and 0.4 M, respectively. Lysine, asparagine, and glutamate accelerated the fibrillation by reducing lag time. <i>N</i>-acetyl-l-glutamic acid induced lag-independent fibrillation, whereas glutamine and asparagine showed concentration-dependent effects on the fibrillation with reduced lag time at higher concentrations. Molecular dynamics simulations revealed that interdomain contacts facilitated the fibrillation. The amino acids interacting predominantly through van der Waals interactions reduced the lag time of α-syn. However, the amino acids having strong electrostatic interactions with the protein disrupted intradomain contacts, favored extended conformation, and inhibited the fibril formation.