Osmolyte-Induced Macromolecular Aggregation Is Length-Scale Dependent

The protein-stabilizing effects of TMAO in aqueous and non-aqueous conditions

We present a new water-dependent molecular mechanism for the widely-used protein stabilizing osmolyte, trimethylamine N-oxide (TMAO), whose mode of action has remained controversial. Classical interpretations, such as osmolyte exclusion from the vicinity of protein, cannot adequately explain the behavior of this osmolyte and were challenged by recent data showing the direct interactions of TMAO with proteins, mainly via hydrophobic binding. Solvent effect theories also fail to propose a straightforward mechanism. To explore the role of water and the hydrophobic association, we disabled osmolyte-protein hydrophobic interactions by replacing water with hexane and using lipase enzyme as an anhydrous-stable protein. Biocatalysis experiments showed that under this non-aqueous condition, TMAO does not act as a stabilizer, but strongly deactivates the enzyme. Molecular dynamics (MD) simulations reveal that TMAO accumulates near the enzyme and makes many hydrogen bonds with it, like denaturing osmolytes. Some TMAO molecules even reach the active site and interact strongly with the catalystic traid. In aqueous solvent, the enzyme functions well: the extent of TMAO interactions is reduced and can be divided into both polar and non-polar terms. Structural analysis shows that in water, some TMAO molecules bind to the enzyme surface like a surfactant. We show that these interactions limit water-protein hydrogen bonds and unfavorable water-hydrophobic surface contacts. Moreover, a more hydrophobic environment is formed in the solvation layer, which reduces water dynamics and subsequently, rigidifies the backbone in aqueous solution. We show that osmolyte amphiphilicity and protein surface heterogeneity can address the weaknesses of exclusion and solvent effect theories about the TMAO mechanism.

Zwitterionic Osmolytes Revive Surface Charges under Salt Stress via Dual Mechanisms

To counter the stress of a salt imbalance, the cell often produces low molecular weight osmolytes to resuscitate homeostasis. However, how zwitterionic osmolytes would tune the electrostatic interactions among charged biomacromolecular surfaces under salt stress has eluded mainstream investigations. Here, via combination of molecular simulation and experiment, we demonstrate that a set of zwitterionic osmolytes is able to restore the electrostatic interaction between two negatively charged surfaces that had been masked in the presence of salt. Interestingly, the mechanisms of resurrecting charge interaction under excess salt are revealed to be mutually divergent and osmolyte specific. In particular, glycine is found to competitively desorb the salt ions from the surface via its direct interaction with the surface. On the contrary, TMAO and betaine counteract salt stress by retaining adsorbed cations but partially neutralizing their charge density via ion-mediated interaction. These access to alternative modes of osmolytic actions would provide the cell the required flexibility in combating salt stress.

Hydrophobic association and solvation of neopentane in urea, TMAO and urea-TMAO solutions

A detailed knowledge of hydrophobic association and solvation is crucial for understanding the con-formational stability of proteins and polymers in osmolyte solutions. Using molecular dynamics simulations, it is found that the hydrophobic association of neopentane molecules is greater in a mixed urea-TMAO-water solution in comparison to that in 8 M urea solution, 4 M TMAO solution and neat water. The neopentane association in urea solution is greater than that in TMAO solution or neat water. We find the association is even less in TMAO solution than pure water. From free energy calculations, it is revealed that the neopentane sized cavity creation in mixed urea-TMAO-water is most unfavorable and that causes the highest hydrophobic association. The cavity formation in urea solution is either more unfavorable or comparable to that in TMAO solution. Importantly, it is found that the population of neopentane-neopentane contact pair and the free energy contribution for the cavity formation step in TMAO solution are very sensitive towards the choice of TMAO force-fields. A careful construction of TMAO force-fields is important for studying the hydrophobic association. Interestingly it is observed that the total solute-solvent dispersion interaction energy contribution is always the most favorable in mixed urea-TMAO-water. The magnitude of this interaction energy is greater in urea solution relative to TMAO solution for two different force-fields of TMAO, whereas the lowest value is obtained in pure water. It is revealed that the extent of the overall hydrophobic association in osmolyte solutions is mainly governed by the cavity creation step and it nullifies the contribution coming from the solute-solvent interaction contribution.

Cryo vs Thermo: Duality of Ethylene Glycol on the Stability of Proteins

Osmolytes are known to stabilize proteins under stress conditions. Thermal denaturation studies on globular proteins (β-lactoglobulin, cytochrome c, myoglobin, α-chymotrypsin) in the presence of ethylene glycol (EG), a polyol class of osmolyte, demonstrate a unique property of EG. EG stabilizes proteins against cold denaturation and destabilizes them during heat-induced denaturation. Further, chemical denaturation experiments performed at room temperature and at a sub-zero temperature (-10 °C) show that EG stabilizes the proteins at subzero temperature but destabilizes them at room temperature. The experiments carried out in the presence of glycerol, however, showed that glycerol stabilizes proteins against all of the denaturing conditions. This differential effect has not been reported for any other polyol class of osmolyte and might be specific to EG. Moreover, molecular dynamics simulations of all of the four proteins were carried out at three different temperatures, 240, 300, and 340 K, in the absence and presence of EG (20 and 40%). The results suggest that EG preferably accumulates around the hydrophobic residues and reduces the hydrophobic hydration of the proteins at a low temperature leading to stabilization of the proteins. At 340 K, the preferential hydration of the proteins is significantly reduced and the preferential binding of EG destabilizes the proteins like common denaturants.

Pressure, Peptides, and a Piezolyte: Structural Analysis of the Effects of Pressure and Trimethylamine-N-oxide on the Peptide Solvation Shell

The osmolyte trimethylamine-N-oxide (TMAO) is able to increase the thermodynamic stability of folded proteins, counteracting pressure denaturation. Herein, we report experimental solubility data on penta-alanine (pAla) in aqueous TMAO solutions (at pH = 7 and pH = 13) together with molecular simulation data for pAla, penta-serine (pSer), and an elastin-like peptide (ELP) sequence (VPGVG) under varying pH and pressure conditions. The effect of the peptide end groups on TMAO-peptide interactions is investigated by comparing the solvation of zwitterionic and negatively charged pentamers with the solvation of pentamers with charge-neutral C- and N-termini and linear, virtually infinite, peptide chains stretched across the periodic boundaries of the simulation cell. The experiments and simulations consistently show that TMAO is net-depleted from the pAla-water interface, but local accumulation of TMAO is observed just outside the first hydration shell of the peptide. While the same observations are also made in the simulations of the zwitterionic pentamers (Ala, Ser, and ELP) and virtually infinite peptide chains (Ala and ELP), weak preferential binding of TMAO is instead observed for pAla with neutral end groups at a 1 M TMAO concentration and for an ELP pentamer with capped neutral end groups at a 0.55 M TMAO concentration studied in previous work (Y.-T. Liao et al. Proc. Natl. Acad. Sci. USA, 2017, 114, 2479-2484). The above observations made at 1 bar ambient pressure remain qualitatively unchanged at 500 bar and 2 kbar. Local accumulation of TMAO correlates with a reduction in the total number of peptide-solvent hydrogen bonds, independent of the peptide's primary sequence and the applied pressure. By weakening water hydrogen bonds with the protein backbone, TMAO indirectly contributes to stabilizing internal hydrogen bonds in proteins, thus providing a protein stabilization mechanism beyond net depletion.