Coupling of protein and hydration-water dynamics in biological membranes

Abstract

The dynamical coupling between proteins and their hydration water is important for the understanding of macromolecular function in a cellular context. In the case of membrane proteins, the environment is heterogeneous, composed of lipids and hydration water, and the dynamical coupling might be more complex than in the case of the extensively studied soluble proteins. Here, we examine the dynamical coupling between a biological membrane, the purple membrane (PM), and its hydration water by a combination of elastic incoherent neutron scattering, specific deuteration, and molecular dynamics simulations. Examining completely deuterated PM, hydrated in H2O, allowed the direct experimental exploration of water dynamics. The study of natural abundance PM in D2O focused on membrane dynamics. The temperature-dependence of atomic mean-square displacements shows inflections at 120 K and 260 K for the membrane and at 200 K and 260 K for the hydration water. Because transition temperatures are different for PM and hydration water, we conclude that ps-ns hydration water dynamics are not directly coupled to membrane motions on the same time scale at temperatures <260 K. Molecular-dynamics simulations of hydrated PM in the temperature range from 100 to 296 K revealed an onset of hydration-water translational diffusion at Ϸ200 K, but no transition in the PM at the same temperature. Our results suggest that, in contrast to soluble proteins, the dynamics of the membrane protein is not controlled by that of hydration water at temperatures <260 K. Lipid dynamics may have a stronger impact on membrane protein dynamics than hydration water. molecular dynamics simulations ͉ neutron spectroscopy ͉ dynamical transition ͉ purple membrane ͉ bacteriorhodopsin P roteins are animated by a multitude of motions occurring on various length and time scales. In a current model, a protein is not characterized by a single three dimensional structure but, rather, by a large number of conformations, so-called conformational substates, that interconvert via molecular motions (1). Natively unfolded proteins reflect this conformational heterogeneity to an extreme degree. The developing concept of a free-energy landscape has put some order into the complex world of protein motions (1). The energy landscape is a highdimensional space, in which each conformational substate is defined by the coordinates of all atoms in the protein. The energy landscape is organized in a hierarchy of levels; the top level containing only a few substates that interconvert by relatively slow large-scale movements (2). Each substate at this level contains, itself, a larger number of substates, separated by smaller barriers that are sampled by more rapid motions. At the third level, the number of substates becomes very large and the barriers very small, and motions at this level have been proposed to take place on the ps-ns time scale (2). Which of the motions on different levels are crucial for biological function and whether and how they are correlated with each other are chall...

From shell to cell: neutron scattering studies of biological water dynamics and coupling to activity

“…Energy resolved incoherent neutron scattering, especially in the elastic (EINS) and quasielastic (QENS) scattering modes, have been informing us on water dynamics under different conditions for decades. Because the method relies on incoherent scattering, samples need not be crystalline or even monodisperse and measurements can be performed on highly complex systems such as living cells 2 or stacks of natural membranes 3 . The incoherent neutron scattering cross section of hydrogen is more than an order of magnitude larger than that of other atomic nuclei usually found in biological material and their isotopes, including deuterium, and the development of in vivo specific deuterium labelling methods permitted the focus on water dynamics in complex biological samples, and its coupling with biological function and activity.…”

“…In contrast, however, the onset of hydration water translational diffusion at 200 K, in a membrane, was not observed to trigger a dynamical transition in the membrane protein 25,26 , suggesting a more complex interaction between protein, water, and the membrane lipid environment. Recent neutron scattering studies, using specific deuterium labelling, confirmed that a dynamical transition coincides with the onset of hydration water translational diffusion, in the case of a soluble proteins 27 , but not in the case of a membrane protein 3,28 .…”

Slow cooling of protein crystals

“…solvent molecules must flow past or around protein molecules. Below 200 K, solvent flow and translational diffusion are negligible (Weik et al, 2004;Wood et al, 2007Wood et al, , 2008 and so contraction should become nearly isotropic.…”

Reduced coupling of water molecules near the surface of reverse micelles

“…[10][11][12][13][14][15] Interestingly, such studies indicate coupling between the dynamics of water and biological objects which may imply a strong interaction between the solute and the hydration shell. 16,17 It is well known that structure, dynamics and even macroscopic properties of water are determined by the strong intermolecular interactions between the hydrogen-bonded water molecules. 18 However, the question which still remains unanswered is whether water molecules at interfaces, such as in the hydration layer of the membrane, display a behavior similar to the bulk water.…”