Structural evidence for solvent-stabilisation by aspartic acid as a mechanism for halophilic protein stability in high salt concentrations

Abstract

Halophilic organisms have adapted to survive in high salt environments, where mesophilic organisms would perish. One of the biggest 4 challenges faced by halophilic proteins is the ability to maintain both structure and function under molar concentrations of salt. A distinct 5 adaptation of halophilic proteins, compared to mesophilic homologues, is the abundance of aspartic acid on the protein surface. 6Mutagenesis and crystallographic studies of halophilic proteins suggest an important role for solvent interactions with the surface aspartic 7 acid residues. This interaction, between the regions of acidic protein surface and solvent, is thought to maintain a hydration layer around 8 the protein under molar salt concentrations thereby allowing halophilic proteins to retain their functional state. Here we present neutron 9 diffraction data of monomeric zwitterionic form of aspartic acid solutions at physiological pH in 0.25 M and 2.5 M concentration of 10 potassium chloride, to mimic mesophilic and halophilic-like environmental conditions. We have used isotopic substitution in combination 11 with empirical potential structure refinement to extract atomic-scale information from the data. Our study provides structural insight that 12 support the hypothesis that carboxyl groups on acidic residues bind water more tightly under high salt conditions, in support of the 13 residue-ion interaction model of halophilic protein stabilisation. Furthermore our data show that in the presence of high salt the self 14 association between the zwitterionic form of aspartic acid molecules is reduced, suggesting a possible mechanism through which protein 15 aggregation is prevented.

Bead‐Level Characterization of Early‐Stage Amyloid β42 Aggregates: Nuclei and Ionic Concentration Effects

“…The sharp and soaring peak (0.22 nm interval) in the first shell indicates the most population of ions, especial of the cationic Na + , being comparable to the 0.25 nm interval of K + −O (O=carboxyl oxygen atom of aspartic acid) determined by EPSR . In detail, the ionic concentration in the range 0.51–8.40 nm keeps a higher distribution than that in the surrounding bulk solution in the first two systems with lower ionic concentration.…”

“…The distributions of WF (antifreeze water) beads (Figure S3 in the Supporting Information) and Cl − (Figure c) around the 75mer and next in the series of shells are always lower than that in the peripheral bulk solutions, reflecting the repulsion effect between the negative charges (75 e − from the 75mer) and either WF or anionic Cl − , and resultant of keeping away from each other. Although the distributions of water (W beads) are not calculated herein, this rule should also be applicable for it, as verified by experiments by Lenton et al., where the water distribution was discussed in the KCl‐ and aspartic acid‐containing solutions.…”

Temperature-Dependent Segregation in Alcohol–Water Binary Mixtures Is Driven by Water Clustering

“…EPSR has previously been used to study a wide range of biologically-relevant molecules in aqueous solution. [27][28][29][30][31][32] In this paper we refer to the different atomic constituents of methanol and ethanol according to the chemical structures shown in Figure 1. Simulated boxes of molecules were constructed at the same concentration, temperature and atomic number density as the experimentally measured samples.…”

Highly compressed water structure observed in a perchlorate aqueous solution

“…Besides their potential extraterrestrial role, perchlorate solutions are also essential in many electrochemical studies since perchlorate holds the position of being the most chaotropic anion in the Hofmeister series 18 . Many of the unique properties of water are caused by the hydrogen bonding between water molecules and other constituents in the solution and it is well known that dissolved ions and cryopreservative molecules can significantly alter the structure of water in solution 19 – 24 .…”