Pressure-induced protein unfolding in the ternary system AOT-octane-water is different from that in bulk water

Perchlorate salts confer psychrophilic characteristics in α-chymotrypsin

Studies of salt effects on enzyme activity have typically been conducted at standard temperatures and pressures, thus missing effects which only become apparent under non-standard conditions. Here we show that perchlorate salts, which are found pervasively on Mars, increase the activity of α-chymotrypsin at low temperatures. The low temperature activation is facilitated by a reduced enthalpy of activation owing to the destabilising effects of perchlorate salts. By destabilising α-chymotrypsin, the perchlorate salts also cause an increasingly negative entropy of activation, which drives the reduction of enzyme activity at higher temperatures. We have also shown that α-chymotrypsin activity appears to exhibit an altered pressure response at low temperatures while also maintaining stability at high pressures and sub-zero temperatures. As the effects of perchlorate salts on the thermodynamics of α-chymotrypsin's activity closely resemble those of psychrophilic adaptations, it suggests that the presence of chaotropic molecules may be beneficial to life operating in low temperature environments.

High pressures increase α-chymotrypsin enzyme activity under perchlorate stress

Deep subsurface environments can harbour high concentrations of dissolved ions, yet we know little about how this shapes the conditions for life. We know even less about how the combined effects of high pressure influence the way in which ions constrain the possibilities for life. One such ion is perchlorate, which is found in extreme environments on Earth and pervasively on Mars. We investigated the interactions of high pressure and high perchlorate concentrations on enzymatic activity. We demonstrate that high pressures increase α-chymotrypsin enzyme activity even in the presence of high perchlorate concentrations. Perchlorate salts were shown to shift the folded α-chymotrypsin phase space to lower temperatures and pressures. The results presented here may suggest that high pressures increase the habitability of environments under perchlorate stress. Therefore, deep subsurface environments that combine these stressors, potentially including the subsurface of Mars, may be more habitable than previously thought.

Gault et al. show that high pressures increase α-chymotrypsin enzyme activity in the presence of high perchlorate concentrations. These perchlorate salts shift the folded enzyme phase space to lower temperatures and pressure and may move the optimum enzyme activity towards lower temperatures in addition to higher pressures, which has implications for Martian habitability.

A review of the structure and dynamics of nanoconfined water and ionic liquids via molecular dynamics simulation

During the past decade, the research on fluids in nanoconfined geometries has received considerable attention as a consequence of their wide applications in different fields. Several nanoconfined systems such as water and ionic liquids, together with an equally impressive array of nanoconfining media such as carbon nanotube, graphene and graphene oxide have received increasingly growing interest in the past years. Water is the first system that has been reviewed in this article, due to its important role in transport phenomena in environmental sciences. Water is often considered as a highly nanoconfined system, due to its reduction to a few layers of water molecules between the extended surface of large macromolecules. The second system discussed here is ionic liquids, which have been widely studied in the modern green chemistry movement. Considering the great importance of ionic liquids in industry, and also their oil/water counterpart, nanoconfined ionic liquid system has become an important area of research with many fascinating applications. Furthermore, the method of molecular dynamics simulation is one of the major tools in the theoretical study of water and ionic liquids in nanoconfinement, which increasingly has been joined with experimental procedures. In this way, the choice of water and ionic liquids in nanoconfinement is justified by applying molecular dynamics simulation approaches in this review article.

Infrared spectroscopy of proteins in reverse micelles

Reverse micelles are a versatile model system for the study of crowded microenvironments containing limited water, such as those found in various tissue spaces or endosomes. They also preclude protein aggregation. Reverse micelles are amenable to study by linear and nonlinear infrared spectroscopies, which have demonstrated that the encapsulation of polypeptides and enzymatically active proteins into reverse micelles leads to conformational changes not seen in bulk solution. The potential value of this model system for understanding the folding and kinetic behavior of polypeptides and proteins in biologically important circumstances warrants increased study of reverse micelle systems by infrared spectroscopy. This article is part of a Special Issue entitled: FTIR in membrane proteins and peptide studies.

Water-in-oil micro-emulsion enhances the secondary structure of a protein by confinement

A scheme is presented in which an organic solvent environment in combination with surfactants is used to confine a natively unfolded protein inside an inverse microemulsion droplet. This type of confinement allows a study that provides unique insight into the dynamic structure of an unfolded, flexible protein which is still solvated and thus under near-physiological conditions. In a model system, the protein osteopontin (OPN) is used. It is a highly phosphorylated glycoprotein that is expressed in a wide range of cells and tissues for which limited structural analysis exists due to the high degree of flexibility and large number of post-translational modifications. OPN is implicated in tissue functions, such as inflammation and mineralisation. It also has a key function in tumour metastasis and progression. Circular dichroism measurements show that confinement enhances the secondary structural features of the protein. Small-angle X-ray scattering and dynamic light scattering show that OPN changes from being a flexible protein in aqueous solution to adopting a less flexible and more compact structure inside the microemulsion droplets. This novel approach for confining proteins while they are still hydrated may aid in studying the structure of a wide range of natively unfolded proteins.

Brownian dynamics simulation of substrate motion near active site of enzyme entrapped inside reverse micelle

Brownian dynamics simulation has been applied to analyze the influence of the electrostatic field of a reverse micelle on the enzyme-substrate complex formation inside a micelle. The probability that the enzyme-substrate complex will form from serine protease (trypsin) and the specific hydrophilic cationic substrate Nalpha-benzoyl-L: -arginine ethyl ester has been studied within the framework of the encounter complex formation theory. It has been shown that surfactant charge, dipole moments created by charged surfactant molecules and counterions, and permittivity of the inner core of reverse micelles can all be used as regulatory parameters to alter the substrate orientation near the active site of the enzyme and to change the probability that the enzyme-substrate complex will form.

Kinetics of reactions catalyzed by enzymes in solutions of surfactants

The effect of surfactants, both in water-in-oil microemulsions (hydrated reverse micelles) and aqueous solutions upon enzymatic processes is reviewed, with special emphasis on the effect of the surfactant upon the kinetic parameters of the process. Differences and similarities between processes taking place in aqueous and organic solvents are highlighted, and the main models currently employed to interpret the results are briefly discussed.

Enhancing the hydrophobic effect in confined water nanodrops

The distribution of hydrophobic solutes, such as methane, enclosed in a nanosized water droplet contained in a reverse micelle of diameter 2.82 nm is investigated using Monte Carlo simulations. The effect of the hydrophobic solute's atomic diameter on the solute-solute potential of mean force is also studied. The study reveals that confinement has a strong influence on the solute's tendency to associate. The potential of mean force exhibits only a single minimum, indicating that the contact pair is the only stable configuration between solutes. The solvent-separated pair that is universally observed for small solutes in bulk water is conspicuously absent. This enhanced hydrophobic effect is attributed to the lack of sufficient water to completely hydrate and stabilize the solvent-separated configurations. The study is expected to be important in understanding the role of hydrophobic forces during protein folding and nucleation under confinement.

Protein stability and dynamics in the pressure–temperature plane

The pressure-temperature stability diagram of proteins and the underlying assumptions of the elliptical shape of the diagram are discussed. Possible extensions, such as aggregation and fibril formation, are considered. An important experimental observation is the extreme pressure stability of the mature fibrils. Molecular origins of the diagram in terms of models of the partial molar volume of a protein focus on cavities and hydration. Changes in thermal expansivity, compressibility and heat capacity in terms of fluctuations of the enthalpy and volume change of the unfolding should also focus on these parameters. It is argued that the study of water-soluble polymers might further our understanding of the stability diagram. Whereas the role of water in protein behaviour is unquestioned, the role of cavities is less clear.

Incorporation of α-chymotrypsin into the 3D channels of bicontinuous cubic lipid mesophases

The effects of protein entrapment on the structure and phase behavior of periodically curved lipid mesostructures have been examined by synchrotron small-angle X-ray diffraction and FT-IR spectroscopy. The study was directed towards a better understanding of the effect of confinement in a lipid environment on the stability and unfolding behavior of alpha-chymotrypsin, and, vice versa, the effect of the entrapped protein on the lipid's mesophase structure and temperature- and pressure-dependent phase behavior. We compare the interaction of protein molecules of two different sizes (cytochrome c, 12.4 kDa, and alpha-chymotrypsin, 25.8 kDa) with the cubic Ia3d phase of monoolein (MO), which forms spontaneously in water. The cubic structure changes significantly when cyt c is incorporated: above a protein concentration of 0.2 wt.%, the interaction between the positively charged protein and the lipid headgroups leads to an increase in interfacial curvature which promotes the formation of a new micellar cubic phase, presumably of crystallographic space group P4(3)32, which the lipid system does not form on its own. The larger alpha-chymotrypsin leads to a different scenario. On the basis of an examination of the calculated geometric parameters and water volume fractions, it is concluded that the alpha-chymotrypsin molecules cannot be located exclusively in the water channels of the cubic Ia3d or P4(3)32 phases, but rather form new, less ordered (presumably cubic Pn3m) structures. The new structure disappears above the unfolding temperature of chymotrypsin and exhibits a pressure stability, which-- in contrast to cyt c in MO-- decreases with increasing chymotrypsin concentration in the system. While the secondary structure of cyt c remains unaffected in the confining lipid environment, the structure of alpha-chymotrypsin gets destabilized slightly, and the protein tends to aggregate even at relatively low concentrations.

Mechanical compression affecting the thermal-induced conformational stability and denaturation temperature of human fibrinogen

Thermal-induced conformational stability and changes in denaturation temperature of human fibrinogen (FBG) after different mechanical compressions were investigated by a simultaneous Fourier transform infrared microspectroscopy equipped with thermal analyzer (thermal FTIR microscopic system). The confocal Raman microspectroscopy was also applied to determine the thermal reversibility of solid FBG. FBG powder was pressed on one KBr pellet (1 KBr method) or sealed within two KBr pellets (2 KBr method) by different mechanical compressions. The result indicates that there was no marked difference in the thermal behavior for the solid FBG samples prepared by 1 KBr method in the heating process even under different mechanical compression pressures, in which the thermal-induced denaturation temperatures from native to denatured state were maintained constant at 66-67 degrees C. However, the denaturation temperature for the solid FBG samples prepared by 2 KBr method was shifted from 55 to 62 degrees C with the increase of mechanical compression pressure. A good linear correlation was also found between the denaturation temperature and mechanical compression pressure for FBG samples prepared by 2 KBr method. The solid FBG sample, whether prepared by 1 KBr or 2 KBr method, was also found to show the thermal-irreversible property.