Translational diffusion of hydration water correlates with functional motions in folded and intrinsically disordered proteins

Abstract

Hydration water is the natural matrix of biological macromolecules and is essential for their activity in cells. The coupling between water and protein dynamics has been intensively studied, yet it remains controversial. Here we combine protein perdeuteration, neutron scattering and molecular dynamics simulations to explore the nature of hydration water motions at temperatures between 200 and 300 K, across the so-called protein dynamical transition, in the intrinsically disordered human protein tau and the globular maltose binding protein. Quasi-elastic broadening is fitted with a model of translating, rotating and immobile water molecules. In both experiment and simulation, the translational component markedly increases at the protein dynamical transition (around 240 K), regardless of whether the protein is intrinsically disordered or folded. Thus, we generalize the notion that the translational diffusion of water molecules on a protein surface promotes the large-amplitude motions of proteins that are required for their biological activity.

Figure 1. Dynamical transition in hydrated proteins.

MSDs of the IDP tau in a hydrated (full blue diamonds) and a dry (open black diamonds) state, measured by elastic incoherent neutron scattering on the backscattering spectrometer IN16 (0.9 μeV resolution, ILL, Grenoble). Inset: the MSD difference between the hydrated and the dry protein highlights an onset of large-amplitude protein motions at around 240 K. Protein samples were not deuterated and were hydrated in D₂O. Experimental error bars are indicated as vertical lines.

Figure 2. Neutron spectra reveal a change in hydration water dynamics at 240 K.

QENS spectra of D-tau-H₂O at different temperatures and at q=0.78 Å⁻¹. The spectrum in red corresponds to the experimental resolution function, obtained by a measurement of the same sample at 20 K, and was truncated below and above −4 and 4 μeV. Inset: zoom into the quasi-elastic spectra between −8 and −0.6 μeV for 230, 240, 250 and 260 K, highlighting the change at 240 K.

Figure 3. Neutron spectra and fits.

QENS spectra of D-tau-H₂O (a,c) and D-MBP-H₂O (b,d) at 260 K and for q=0.78 Å⁻¹ (a,b) and q=1.66 Å⁻¹ (c,d). The continuous lines represent the fitting curves with a model in which water molecules either translate, rotate or remain immobile.

Figure 4. Different populations of water molecules as a function of temperature.

Fractions of different dynamic contributions to the quasi-elastic spectra as a function of temperature: centre-of-mass translation of water molecules (a), rotation of water molecules around their centre-of-mass (b) and water molecules not moving in the dynamic window investigated (c). Dashed lines are guides to the eye. Red circles, MBP; blue diamond, tau.

Figure 5. Analysis of MD simulations of hydration water dynamics on tau and MBP surfaces.

(a) Continuous HB relaxation rates (1/τ_HBC) as a function of temperature. (b) Intermittent HB relaxation rates (1/τ_HBI) as a function of temperature. (c) MSDs of hydration water oxygen atoms at 100 ps as a function of temperature. Error bars on the relaxation rates and MSDs, estimated by computing the respective quantities separately over the two halves of the trajectory segments used for the analysis, are smaller than the plotting symbols.