WT and A53T α-Synuclein Systems: Melting Diagram and Its New Interpretation

Abstract

The potential barriers governing the motions of α-synuclein (αS) variants' hydration water, especially energetics of them, is in the focus of the work. The thermodynamical approach yielded essential information about distributions and heights of the potential barriers. The proteins' structural disorder was measured by ratios of heterogeneous water-binding interfaces. They showed the αS monomers, oligomers and amyloids to possess secondary structural elements, although monomers are intrinsically disordered. Despite their disordered nature, monomers have 33% secondary structure, and therefore they are more compact than a random coil. At the lowest potential barriers with mobile hydration water, monomers are already functional, a monolayer of mobile hydration water is surrounding them. Monomers realize all possible hydrogen bonds with the solvent water. αS oligomers and amyloids have half of the mobile hydration water amount than monomers because aggregation involves less mobile hydration. The solvent-accessible surface of the oligomers is ordered or homogenous in its interactions with water to 66%. As a contrast, αS amyloids are disordered or heterogeneous to 75% of their solvent accessible surface and both wild type and A53T amyloids show identical, low-level hydration. Mobile water molecules in the first hydration shell of amyloids are the weakest bound compared to other forms.

Keywords: NMR spectroscopy; aggregation; bond energy; hydration; proteins.

Figure A1

Illustration of the method applied to measure the fraction of the mobile water component. The slowly decaying part of the FID signal is extrapolated to t = 0 (by a stretched exponential, gives narrow spectrum line). The extrapolated signal intensity is normalized to the signal intensity measured above 0 °C where the whole sample is in liquid state. Inset: typical time-spreads of FID signal components produced by ¹H nuclei in ice, in protein and in mobile water.

Figure 1

α-synuclein (αS) monomers, oligomer and amyloids, wild type (dark blue open up triangles, dark blue dot-dashed lines) and A53T mutant (cyan solid down triangles, dotted cyan lines) variants dissolved in pure water: (a) αS monomers, (b) αS oligomers, (c) αS amyloids, melting diagrams; (d) αS monomers, (e) αS oligomers, (f) αS amyloids, derivatives of the melting diagrams, i.e., potential barrier distributions related to moving hydration water. There are no reliable measured data in the range −1–0 °C (0.995–1.00 T_fn). Data are given for 50 mg ml⁻¹ protein concentration. The red line on panel (a) is at the predicted hydration value h = 4(1)·10².