Temperature dependence of lysozyme hydration and the role of elastic energy

Hai-Jing Wang, Alfred Kleinhammes, Pei Tang, Yan Xu, and Yue Wu

Phys. Rev. E 83, 031924 – Published 31 March 2011

Abstract

Water plays a critical role in protein dynamics and functions. However, the most basic property of hydration—the water sorption isotherm—remains inadequately understood. Surface adsorption is the commonly adopted picture of hydration. Since it does not account for changes in the conformational entropy of proteins, it is difficult to explain why protein dynamics and activity change upon hydration. The solution picture of hydration provides an alternative approach to describe the thermodynamics of hydration. Here, the flexibility of proteins could influence the hydration level through the change of elastic energy upon hydration. Using nuclear magnetic resonance to measure the isotherms of lysozyme in situ between 18 and 2 °C, the present work provides evidence that the part of water uptake associated with the onset of protein function is significantly reduced below 8 °C. Quantitative analysis shows that such reduction is directly related to the reduction of protein flexibility and enhanced cost in elastic energy upon hydration at lower temperature. The elastic property derived from the water isotherm agrees with direct mechanical measurements, providing independent support for the solution model. This result also implies that water adsorption at charged and polar groups occurring at low vapor pressure, which is known for softening the protein, is crucial for the later stage of water uptake, leading to the activation of protein dynamics. The present work sheds light on the mutual influence of protein flexibility and hydration, providing the basis for understanding the role of hydration on protein dynamics.

Received 31 August 2010

DOI:https://doi.org/10.1103/PhysRevE.83.031924

Authors & Affiliations

Hai-Jing Wang¹, Alfred Kleinhammes¹, Pei Tang^2,3,4, Yan Xu^2,3,5, and Yue Wu^1,*

¹Department of Physics and Astronomy, University of North Carolina, Chapel Hill, North Carolina 27599-3255, USA
²Department of Anesthesiology, University of Pittsburgh School of Medicine, Pittsburgh, Pennsylvania 15260, USA
³Department of Pharmacology & Chemical Biology, University of Pittsburgh School of Medicine, Pittsburgh, Pennsylvania 15260, USA
⁴Department of Computational Biology, University of Pittsburgh School of Medicine, Pittsburgh, Pennsylvania 15260, USA
⁵Department of Structural Biology, University of Pittsburgh School of Medicine, Pittsburgh, Pennsylvania 15260, USA

^*yuewu@physics.unc.edu

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Vol. 83, Iss. 3 — March 2011

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Images

Figure 1
(a) A van der Waals representation of a lysozyme molecule colored with a hydrophobic scale. White: hydrophobic; red (dark gray): hydrophilic; orange (gray): positive or negative charge. (b) A lattice model of protein hydration. The white chain represents a lysozyme molecule occupying $x$ lattice cells. The blue (gray) spheres are hydration water, each of which occupy one lattice cell. Their locations are obtained from hydration water of <0.34 nm away from the protein in molecular dynamics simulations shown in Sec. 3f.Reuse & Permissions
Figure 2
(a) NMR spectra of lysozyme powder before (open circle) and after pumping (filled circle). (b) The sharp NMR peak corresponding to the hydration water of protein as temperature decreasing from 4 to –34 °C. The initial hydration level is $h$ ∼ 0.21 at 4 °C. The inset shows the hydration levels estimated from the integrated intensities of such sharp peaks.Reuse & Permissions
Figure 3
Water sorption isotherms in lysozyme powder from 18 to 2 °C. The isotherms at high relative pressure are fitted as the implicit form of Eq. (2) with parameters shown in Table . Inset: $N_{(el)}^{0}$ versus temperature.Reuse & Permissions
Figure 4
Plots of $[\ln (P / P_{0}) - \ln v_{1} - v_{2}] v_{2}^{2}$ versus $[(v_{2}^{- 1 / 3} - 1) (5 / 3 v_{2}^{- 1 / 3} - 1) (5 / 3 v_{2}^{- 1 / 3} - 1) v_{2}^{2} / v_{2}^{2}]$ for the isotherms data following Eq. (2). Plots show linear portion when $[\ln (P / P_{0}) - \ln v_{1} - v_{2}] v_{2}^{2} > 0.6$ . The fitting parameters are shown in Table for $η$ $=$ 1. Inset: Comparison of the temperature dependence of the elastic modulus (blue square) derived from isotherms in this study, with the Young's modulus of hydrated lysozyme (red circle) measured in [31].Reuse & Permissions
Figure 5
A ribbon representation of lysozyme structure from an averaged structure from the last 5 ns of 11-ns NPT simulations at 18 °C and $P$ $/ P_{0}$ $=$ 1. The residues with elevated fluctuations and correlated motions by hydration water are highlighted in red (dark gray) (residues 44–53) and in yellow (light gray) (residues 59–81). The residues with low fluctuations in the $β$ domain are shown in orange (gray) (residues 54–58). Residues that are essential for the completion of the enzymatic reaction (Glu35 and Asp52) are shown in an atomic representation. The rest of the residues are shown in a narrow ribbon representation.Reuse & Permissions
Figure 6
All-mode correlation plots of hen egg-white lysozyme from normal mode analysis using the Gaussian network model. The NPT simulations are under comparable conditions of the experiments at relative vapor pressure of (a) $P$ $/ P_{0}$ $=$ 0.6 ( $N_{hydration}$ $=$ 124) and (b) $P$ $/ P_{0}$ $=$ 1 ( $N_{hydration}$ $=$ 360) and a temperature of 18 °C. Averaged structures from the last 5 ns of 11-ns MD simulations were used for the GNM analyses. Cross correlation of motion is color-coded using the color (gray) scale to the right. The most profound increases in diagonal intensities are from residues 44–53 [upper left dashed squares and the red (dark gray) region in Fig. 5] and residues 59–81 [lower right dashed squares and the yellow (light gray) region in Fig. 5]. The corresponding increases in off-diagonal intensities between these two residue groups are labeled by rectangles.Reuse & Permissions

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