The pressure dependence of hydrophobic interactions is consistent with the observed pressure denaturation of proteins

Abstract

Proteins can be denatured by pressures of a few hundred MPa. This finding apparently contradicts the most widely used model of protein stability, where the formation of a hydrophobic core drives protein folding. The pressure denaturation puzzle is resolved by focusing on the pressure-dependent transfer of water into the protein interior, in contrast to the transfer of nonpolar residues into water, the approach commonly taken in models of protein unfolding. Pressure denaturation of proteins can then be explained by the pressure destabilization of hydrophobic aggregates by using an information theory model of hydrophobic interactions. Pressure-denatured proteins, unlike heat-denatured proteins, retain a compact structure with water molecules penetrating their core. Activation volumes for hydrophobic contributions to protein folding and unfolding kinetics are positive. Clathrate hydrates are predicted to form by virtually the same mechanism that drives pressure denaturation of proteins.A decade ago, Walter Kauzmann (1) challenged the commonly held view that a hydrophobic core stabilizes globular proteins, by poignantly remarking that the ''liquid-hydrocarbon model (2) fails almost completely when one attempts to extend it to the effects of pressure on protein folding.'' Although a variety of forces stabilize folded proteins (3-6), the formation of a hydrophobic core is thought to play a dominant role. This view is supported by the temperature dependence of hydrophobic contributions to protein unfolding showing remarkable similarities to the transfer of hydrocarbons from a nonpolar phase into water, notably a convergence of the entropy of transfer (2,7,8). However, Kauzmann (1) pointed out that the pressure dependence of protein unfolding is at odds with the hydrophobic-core model: The volume change ⌬V upon unfolding is positive at low pressures but negative at pressures of about 100-200 MPa. The transfer of hydrocarbons into water shows exactly the opposite behavior, with ⌬V being negative at low pressures and positive at high pressures.Evidently, pressure unfolding of a protein (9-16) does not correspond to the transfer of a nonpolar molecule from a nonpolar environment into aqueous solution. Unlike heatdenatured proteins, the ensemble of pressure-denatured proteins retains elements of structural organization (13, 17). Consequently, an understanding of the thermodynamics of pressure denaturation might focus on the free energy of water transfer into the hydrophobic core of the protein (18) rather than transfer of nonpolar solutes into water. Our conceptual framework for pressure denaturation is as follows: the protein interior is largely composed of efficiently packed residues, more likely hydrophobic than those at the surface (19). Increasing hydrostatic pressure then forces water molecules into the protein interior, gradually filling cavities, and eventually breaking the protein structure apart.We therefore study the effects of pressure on the association of nonpolar residues in water. We use the informat...

Low-temperature and high-pressure induced swelling of a hydrophobic polymer-chain in aqueous solution

“…At 280 K and 3000 bar and 260 K and 4500 bar, however, the situation has already changed, and the tendency to explore more extended configurations leads to an increase in R G which is quickly progressing upon cooling. The low temperature destabilization of the collapsed state is in line with the decreased stability of hydrophobic contacts observed for elevated pressures [18,19,45]. Figure 3 implies that the transition towards a swelled state at 3000 bar and 4500 bar occurs in a rather narrow temperature interval, suggesting a rather large enthalpy difference between collapsed and swelled state.…”

“…We would like to point out that the location and slope of the swelling transition shows remarkable similarity to the given cold-denaturation lines. The hydrophobic polymer seemingly behaves as suggested by the water penetration scenario according Hummer et al [45]. In the present case the energy stabilization of the swelled configuration is dominated the energy gain of the hydration water.…”

“…This seemed to be inconsistent with the assumption that protein unfolding is equivalent to the transfer of hydrophobic groups from the protein interior to the aqueous solvent, since the volume change upon transfer of hydrophobic groups to water are positive. Hummer et al [45] suggested a scenario in which pressure unfolding of proteins is modeled as the transfer of water into the protein hydrophobic core with increasing pressure. The transfer of water molecules into the protein interior is essential for the pressure unfolding process, leading to the dissociation of close hydrophobic contacts and subsequent swelling of the hydrophobic protein interior through insertions of water molecules [45].…”

“…Hummer et al [45] suggested a scenario in which pressure unfolding of proteins is modeled as the transfer of water into the protein hydrophobic core with increasing pressure. The transfer of water molecules into the protein interior is essential for the pressure unfolding process, leading to the dissociation of close hydrophobic contacts and subsequent swelling of the hydrophobic protein interior through insertions of water molecules [45]. The characteristic features of water-mediated interactions between hydrophobic solutes in water are found to be pressure-dependent.…”

“…In this contribution, we focus particularly on the scenario proposed by Hummer et al [45] of water penetrat-ing into the protein interior at elevated pressures. We study, however, a very much simplified model system of protein: A fully hydrated polymer-chain, consisting of 20 interconnected hydrophobic particles.…”