Microscopic Origin of Hysteresis in Water Sorption on Protein Matrices

Abstract

Despite the importance of water sorption isotherms for a fundamental understanding of protein-water interactions, the microscopic origin of hysteresis between the adsorption and desorption branches is not well understood. Using our recently developed simulation technique, we compute the water sorption isotherms of two proteins, lysozyme and Trp-cage, a miniprotein. We explicitly compare protein-water interactions in adsorption and desorption processes, by analyzing local hydration in terms of hydrogen bonding, water density, and solvent-accessible surface area. We find that significant differences in hydration behavior between adsorption and desorption manifest themselves at the individual amino acid level, in particular around polar or charged residues. We confirm this observation by demonstrating that Trp-cage's hysteresis can be significantly reduced by mutating charged residues to alanine, a neutral and nonpolar amino acid.

Effects of disulfide bridges and backbone connectivity on water sorption by protein matrices

“…This adsorption behavior of the amino acid mixture at lower hydrations has some resemblance with lysozyme in the sense that in both cases not all amino acids are equally exposed to water. This is consistent with recent observation that hysteresis correlates with localized differences in hydration 21 .…”

“…As mentioned earlier, we have recently identified a correlation between the water sorption hysteresis and localized, residue-level differences in hydration that occur between the adsorption and desorption branches, even at the same water content 21 . Such local differences occur mostly at charged or polar residues, which are often exposed to water while the non-polar residues are buried in the protein’s interior core.…”

“…That study, along with the present observation of reduction of hysteresis in the presence of disulfide linkages, suggests a close connection between rigidity of proteins and shrinkage of hysteresis between the adsorption and desorption isotherms. Recently, we identified residue-level differences in the local hydration of charged/polar residues between the adsorption and desorption processes as the microscopic signature of hysteresis in water sorption isotherms 21 . Following the same procedures 21 , we characterized the difference in local, residue-level hydration between adsorption and desorption processes at a hydration level of 0.2 g/g, where the magnitude of hysteresis is the largest (maximum difference in relative humidity between adsorption and desorption branches) in the absence of disulfide bridges (see Supplementary Fig.…”

“…Bennett’s acceptance ratio method 32 was used to compute μ w , which performs numerous insertions and deletions of test water molecules. Consistent with our previous work 21 , for each simulation frame we performed 20 × 10 6 insertions and a number of deletions that corresponds to the number of water molecules in the system. Chemical potential of saturated water vapor ( ) was calculated using the ideal gas law and the vapor pressure of the SPC/E water model at 300 K (0.01 bar) 33 .…”

“…These intrinsic factors could, for example, influence the accessibility of hydrophilic residues to the water molecules, thereby altering protein-water interactions 12 – 18 . Recently, it has been shown computationally that water sorption isotherms are significantly affected when a protein’s flexibility 19 , 20 and its fraction of charged amino acids 21 are altered. However, despite numerous water sorption studies, and the well-known fact that most globular proteins and protein products at ambient conditions exhibit a generic sigmoidal water sorption isotherm (type II in IUPAC convention 22 ) with a pronounced hysteresis between adsorption and desorption branches, the effects of the above-mentioned intrinsic factors on water sorption behavior have not been studied systematically, and hence remain poorly understood.…”