Refolding studies using pressure: The folding landscape of lysozyme in the pressure–temperature plane

Behavior of enzymes under high pressure in food processing: mechanisms, applications, and developments

High pressure processing (HPP) offers the benefits of safety, uniformity, energy-efficient, and low waste, which is widely applied for microbial inactivation and shelf-life extension for foods. Over the past forty years, HPP has been extensively researched in the food industry, enabling the inactivation or activation of different enzymes in future food by altering their molecular structure and active site conformation. Such activation or inactivation of enzymes effectively hinders the spoilage of food and the production of beneficial substances, which is crucial for improving food quality. This paper reviews the mechanism in which high pressure affects the stability and activity of enzymes, concludes the roles of key enzymes in the future food processed using high pressure technologies. Moreover, we discuss the application of modified enzymes based on high pressure, providing insights into the future direction of enzyme evolution under complex food processing conditions (e.g. high temperature, high pressure, high shear, and multiple elements). Finally, we conclude with prospects of high pressure technology and research directions in the future. Although HPP has shown positive effects in improving the future food quality, there is still a pressing need to develop new and effective combined processing methods, upgrade processing modes, and promote sustainable lifestyles.

Technologies for Vitrification Based Cryopreservation

Cryopreservation is a unique and practical method to facilitate extended access to biological materials. Because of this, cryopreservation of cells, tissues, and organs is essential to modern medical science, including cancer cell therapy, tissue engineering, transplantation, reproductive technologies, and bio-banking. Among diverse cryopreservation methods, significant focus has been placed on vitrification due to low cost and reduced protocol time. However, several factors, including the intracellular ice formation that is suppressed in the conventional cryopreservation method, restrict the achievement of this method. To enhance the viability and functionality of biological samples after storage, a large number of cryoprotocols and cryodevices have been developed and studied. Recently, new technologies have been investigated by considering the physical and thermodynamic aspects of cryopreservation in heat and mass transfer. In this review, we first present an overview of the physiochemical aspects of freezing in cryopreservation. Secondly, we present and catalog classical and novel approaches that seek to capitalize on these physicochemical effects. We conclude with the perspective that interdisciplinary studies provide pieces of the cryopreservation puzzle to achieve sustainability in the biospecimen supply chain.

Biomolecules under Pressure: Phase Diagrams, Volume Changes, and High Pressure Spectroscopic Techniques

Pressure is an equally important thermodynamical parameter as temperature. However, its importance is often overlooked in the biophysical and biochemical investigations of biomolecules and biological systems. This review focuses on the application of high pressure (>100 MPa = 1 kbar) in biology. Studies of high pressure can give insight into the volumetric aspects of various biological systems; this information cannot be obtained otherwise. High-pressure treatment is a potentially useful alternative method to heat-treatment in food science. Elevated pressure (up to 120 MPa) is present in the deep sea, which is a considerable part of the biosphere. From a basic scientific point of view, the application of the gamut of modern spectroscopic techniques provides information about the conformational changes of biomolecules, fluctuations, and flexibility. This paper reviews first the thermodynamic aspects of pressure science, the important parameters affecting the volume of a molecule. The technical aspects of high pressure production are briefly mentioned, and the most common high-pressure-compatible spectroscopic techniques are also discussed. The last part of this paper deals with the main biomolecules, lipids, proteins, and nucleic acids: how they are affected by pressure and what information can be gained about them using pressure. I I also briefly mention a few supramolecular structures such as viruses and bacteria. Finally, a subjective view of the most promising directions of high pressure bioscience is outlined.

Formation of α-synuclein aggregates in aqueous ethylammonium nitrate solutions

The effect of adding ethylammonium nitrate (EAN), which is an ionic liquid (IL), on the aggregate formation of α-synuclein (α-Syn) in aqueous solution has been investigated. FTIR and Raman spectroscopy were used to investigate changes in the secondary structure of α-Syn and in the states of water molecules and EAN. The results presented here show that the addition of EAN to α-Syn causes the formation of an intermolecular β-sheet structure in the following manner: native disordered state → polyproline II (PPII)-helix → intermolecular β-sheet (α-Syn amyloid-like aggregates: α-SynA). Although cations and anions of EAN play roles in masking the charged side chains and PPII-helix-forming ability involved in the formation of α-SynA, water molecules are not directly related to its formation. We conclude that EAN-induced α-Syn amyloid-like aggregates form at hydrophobic associations in the middle of the molecules after masking the charged side chains at the N- and C-terminals of α-Syn.

Development of Novel EE/Alginate Polyelectrolyte Complex Nanoparticles for Lysozyme Delivery: Physicochemical Properties and In Vitro Safety

The number of biologic drugs has increased in the pharmaceutical industry due to their high therapeutic efficacy and selectivity. As such, safe and biocompatible delivery systems to improve their stability and efficacy are needed. Here, we developed novel cationic polymethacrylate-alginate (EE-alginate) pNPs for the biologic drug model lysozyme (Lys). The impact of variables such as total charge and charge ratios over nanoparticle physicochemical properties as well as their influence over in vitro safety (viability/proliferation and cell morphology) on HeLa cells was investigated. Our results showed that electrostatic interactions between the EE-alginate and lysozyme led to the formation of EE/alginate Lys pNPs with reproducible size, high stability due to their controllable zeta potential, a high association efficiency, and an in vitro sustained Lys release. Selected formulations remained stable for up to one month and Fourier transform-Infrared (FT-IR) showed that the functional groups of different polymers remain identifiable in combined systems, suggesting that Lys secondary structure is retained after pNP synthesis. EE-alginate Lys pNPs at low concentrations are biocompatible, while at high concentrations, they show cytotoxic for HeLa cells, and this effect was found to be dose-dependent. This study highlights the potential of the EE-alginate, a novel polyelectrolyte complex nanoparticle, as an effective and viable nanocarrier for future drug delivery applications.

Marine sulfated polysaccharides as versatile polyelectrolytes for the development of drug delivery nanoplatforms: Complexation of ulvan with lysozyme

Ulvan, a marine sulfated polysaccharide isolated from green algae, has been recently recognized as a natural biopolymer of biomedical interest. A series of lysozyme/ulvan complexes prepared under various charge ratios at physiological pH were studied. The resulting complexes were examined with light scattering techniques in order to characterize the size, the distribution and the ζ-potential of the nanocarriers, which were found to depend on the charge ratio employed. Increased complexation efficiency of lysozyme was observed for certain charge ratios, while ATR-FTIR data suggested that the protein structure after complexation was retained. Bacterial growth studies showed that lysozyme once complexed with ulvan not only retains its antibacterial activity against the Gram positive strain Staphylococcus aureus, but actually exhibits increased levels of activity. In this model study, the results highlight the potential of ulvan as a promising nanocarrier for positively charged bioactive molecules.

Formation of complexes in aqueous solutions of amphiphilic triblock polyelectrolytes of different topologies and an oppositely charged protein

The complexation of lysozyme with aggregates from two triblock amphiphilic polyelectrolytes of the same blocks but different topologies and block molar masses, namely PS-b-SCPI-b-PEO and SCPI-b-PS-b-PEO, is investigated by scattering and spectroscopy methods. Light scattering reveals that the interaction with lysozyme causes shrinkage of the self-assembled nanoparticles in the case of the hydrophobic-polyelectrolyte-hydrophilic sequence. In the polyelectrolyte-hydrophobic-hydrophilic sequence, the opposite trend is observed. Small angle neutron scattering confirms the existence of micellar and fractal aggregates and the complexation with lysozyme. The pH-dependence of the interactions and the stability of the hybrid protein/polymer nanoparticles upon salt addition are tested. The native conformation of the protein is found to be preserved during complexation. This study reveals that both micellar and fractal aggregates made of amphiphilic triblock polyelectrolytes are capable of loading with oppositely charged proteins in a controllable manner, tuned primarily by the structure of the triblock terpolymer.

Low crowding agent concentration destabilizes against pressure unfolding

The concentration of macromolecules inside a cell is very high, which can affect the behavior of the enzymes, and consequently influence vital biological processes. This is called macromolecular crowding. Since the most important effect of macromolecular crowding is the excluded volume, we performed pressure experiments, where the volume (as conjugate parameter to the pressure) is the crucial factor. We measured the temperature and pressure stability of bovine serum albumin and lysozyme with various concentrations of crowding agents, dextran, Ficoll™ and lysozyme itself. Our most interesting finding is that low concentration of all the studied crowding agents decreases the pressure stability of the proteins. We explain this by the reduced hydration volume change in the crowded environment. Furthermore, we discuss the volumetric parameters and emphasize the difference between the partial volume of the protein and the volume it influences, and their relation to the excluded volume which is responsible for the macromolecular crowding.

Complex Formation Between Lysozyme and Stabilized Micelles with a Mixed Poly(ethylene oxide)/Poly(acrylic acid) Shell

The electrostatic complexation between lysozyme and stabilized polymeric micelles (SPMs) with a poly(acrylic acid) (PAA) or a mixed poly(ethylene oxide)/poly(acrylic acid) (PEO/PAA) shell (SPMs with a mixed shell, SPMMS) and a temperature-responsive poly(propylene oxide) (PPO) core was investigated by means of dynamic, static, and electrophoretic light scattering. The SPMs and different types of SPMMS used resulted from the self-assembly of PAA-PPO-PAA triblock copolymer chains, or PAA-PPO-PAA and PEO-PPO-PEO triblock copolymer chain mixtures (with varying chain lengths and molar ratios) in aqueous solutions at pH 10 and the subsequent cross-linking of their PPO cores via loading and photo-cross-linking of pentaerythritol tetraacrylate (PETA). The solution behavior, structure and properties of the formed complexes at pH 7 and 0.01 M ionic strength, were studied as a function of the protein concentration in the solution (the concentration of the stabilized micelles was kept constant) or equivalently the ratio of the two components. The complexation process and properties of the complexes proved to be dependent on the protein concentration, while of particular interest was the effect of the structure of the shell of the SPMs on the stability/solubility of the complexes. Finally, the fluorescence and mid infrared spectroscopic investigation of the structure of the complexed protein showed that, although a small stretching of the protein molecules occurred in some cases, no protein denaturation takes place upon complexation.

Protein Denaturation on p-T Axes--Thermodynamics and Analysis

Proteins are essential players in the vast majority of molecular level life processes. Since their structure is in most cases substantial for their correct function, study of their structural changes attracted great interest in the past decades. The three dimensional structure of proteins is influenced by several factors including temperature, pH, presence of chaotropic and cosmotropic agents, or presence of denaturants. Although pressure is an equally important thermodynamic parameter as temperature, pressure studies are considerably less frequent in the literature, probably due to the technical difficulties associated to the pressure studies. Although the first steps in the high-pressure protein study have been done 100 years ago with Bridgman's ground breaking work, the field was silent until the modern spectroscopic techniques allowed the characterization of the protein structural changes, while the protein was under pressure. Recently a number of proteins were studied under pressure, and complete pressure-temperature phase diagrams were determined for several of them. This review summarizes the thermodynamic background of the typical elliptic p-T phase diagram, its limitations and the possible reasons for deviations of the experimental diagrams from the theoretical one. Finally we show some examples of experimentally determined pressure-temperature phase diagrams.

Pressure Effects on the Intermolecular Interaction Potential of Condensed Protein Solutions

Knowledge of the intermolecular interaction potential of proteins as a function of their solution conditions is essential for understanding protein aggregation, crystallization, and the phase behavior of proteins in general. Here, we report on a combined small-angle X-ray scattering and liquid-state theoretical approach to study dense lysozyme solutions as a function of temperature and pressure, but also in the presence of salts and osmolytes of different nature. We show that the pressure-dependent interaction potential of lysozyme changes in a nonlinear fashion over a wide range of temperatures, salt and protein concentrations, indicating that changes of the bulk water structure mediate the pressure dependence of the intermolecular forces. We present also results on the effect of high hydrostatic pressure on the phase behavior of dense lysozyme solutions in the liquid-liquid phase-coexistence region. As also shown in this study, the application of pressure can be used to fine-tune the second virial coefficient of protein solutions, which can be used to control nucleation rates and hence protein crystallization, or to prevent protein aggregation. Moreover, these results are also important for understanding the hydration behavior of biological matter under extreme environmental conditions, and the high stability of dense protein solutions (as they occur intracellularly) in organisms thriving under hydrostatic pressure conditions such as in the deep sea, where pressures up to the 100 MPa-level are reached.

Ionic liquid-induced formation of the α-helical structure of β-lactoglobulin

Structural modification of bovine milk β-lactoglobulin (β-LG) in aqueous 1-butyl-3-methylimidazolium nitrate ([bmim][NO3]) and ethylammonium nitrate ([EAN][NO3]) solutions has been investigated by Fourier transform infrared and circular dichroism spectroscopy. Remarkably, high ionic liquid (IL) concentrations (>15 mol %IL) caused formation of a non-native α-helical structure of β-LG and disruption of its tertiary structure. Furthermore, while [bmim][NO3] promoted protein aggregation, [EAN][NO3] inhibited it probably owing to differences in the unique solution structure (nanoheterogeneity) of the ILs by the different cationic species. The IL-induced α-helical formation of β-LG shows a behavior similar to the alcohol denaturation, but a disordered structure-rich state was observed in the β-α transition process by adding IL, in contrast to the case of an aqueous alcohol solution of protein. We propose that the molten salt-like property of aqueous IL solutions strongly support α-helical formation of proteins.

Differences in the structural stability and cooperativity between monomeric variants of natural and de novo Cro proteins revealed by high-pressure Fourier transform infrared spectroscopy

It is widely accepted that pressure affects the structure and dynamics of proteins; however, the underlying mechanism remains unresolved. Our previous studies have investigated the effects of pressure on fundamental secondary structural elements using model peptides, because these peptides represent a basis for understanding the effects of pressure on more complex structures. This study targeted monomeric variants of naturally occurring bacteriophage λ Cro (natural Cro) and de novo designed λ Cro (SN4m), which are α + β proteins. The sequence of SN4m is 75% different from that of natural Cro, but the structures are almost identical. Consequently, a comparison of the folding properties of these proteins is of interest. Pressure- and temperature-variable Fourier transform infrared spectroscopic analyses revealed that the α-helices and β-sheets of natural Cro are cooperatively and reversibly unfolded by pressure and temperature, whereas those of SN4m are not cooperatively unfolded by pressure; i.e., the α-helices of SN4m unfold at significantly higher pressures than the β-sheets and irreversibly unfold with increases in temperature. The higher unfolding pressure for the α-helices of SN4m indicates the presence of an intermediate structure of SN4m that does not retain β-sheet structure but does preserve the α-helices. These results demonstrate that the α-helices of natural Cro are stabilized by global tertiary contacts among the α-helices and the β-sheets, whereas the α-helices of SN4m are stabilized by local tertiary contacts between the α-helices.

Analysis of the mechanism of lysozyme pressure denaturation from Raman spectroscopy investigations, and comparison with thermal denaturation

Pressure denaturation of lysozyme dissolved in H(2)O and D(2)O was analyzed using Raman investigations in a wide frequency range. The simultaneous analysis of regions corresponding to the molecular fingerprint of the protein (500-1800 cm(-1)), and the low- (50-450 cm(-1)) and high- (2600-3800 cm(-1)) frequency spectra, allow us to probe protein denaturation and the organization of water molecules. The pressure- and heat-induced transformations are compared. Both pressure- and heat-denatured states are obtained through an intermediate state characterized by intact secondary structure and enhanced water penetration in the tertiary structure. As a consequence of a weaker penetration upon pressurizing, it was found that the pressure-denatured state was partially unfolded compared with the heat-denatured state. The mechanism of pressure denaturation was related to the disruption of the hydrogen-bond network of water onto a set of clusters characterized by strengthened O - H interactions, inducing a hardening of protein dynamics. The mechanism is opposite to that observed upon heating, i.e., the softening of the hydrogen bond network of water inducing a softer protein dynamics. The analysis of the intramolecular O-H stretching reveals that pressurizing lysozyme aqueous solution favors the development of low-density water from the protein surface to the bulk, contrasting to the compression of pure water leading to crystallization of high-density ice-VI.

Size and sequence and the volume change of protein folding

The application of hydrostatic pressure generally leads to protein unfolding, implying, in accordance with Le Chatelier's principle, that the unfolded state has a smaller molar volume than the folded state. However, the origin of the volume change upon unfolding, ΔV(u), has yet to be determined. We have examined systematically the effects of protein size and sequence on the value of ΔV(u) using as a model system a series of deletion variants of the ankyrin repeat domain of the Notch receptor. The results provide strong evidence in support of the notion that the major contributing factor to pressure effects on proteins is their imperfect internal packing in the folded state. These packing defects appear to be specifically localized in the 3D structure, in contrast to the uniformly distributed effects of temperature and denaturants that depend upon hydration of exposed surface area upon unfolding. Given its local nature, the extent to which pressure globally affects protein structure can inform on the degree of cooperativity and long-range coupling intrinsic to the folded state. We also show that the energetics of the protein's conformations can significantly modulate their volumetric properties, providing further insight into protein stability.

Complexation of lysozyme with poly(sodium(sulfamate-carboxylate)isoprene)

The complexation between hen egg white lysozyme (HEWL) and a novel pH-sensitive and intrinsically hydrophobic polyelectrolyte poly(sodium(sulfamate-carboxylate)isoprene) (SCPI), was investigated by means of dynamic, static, and electrophoretic light scattering and isothermal titration calorimetry measurements. The complexation process was studied at both pH 7 and 3 (high and low charge density of the SCPI, respectively) and under low ionic strength conditions for two polyelectrolyte samples of different molecular weights. The solution behavior, structure, and effective charge of the formed complexes proved to be dependent on the pH, the [-]/[+] charge ratio, and the molecular weight of the polyelectrolyte. Increasing the ionic strength of the solution led to vast aggregation and eventually precipitation of the complexes. The interaction between HEWL and SCPI was found to be mainly electrostatic, associated with an exothermic enthalpy change. The structural investigation of the complexed protein by fluorescence, infrared, circular dichroism spectroscopic, and differential scanning calorimetric measurements revealed no signs of denaturation upon complexation.

Pressure and temperature stability of the main apple allergen Mal d1

High-pressure Fourier-transform infrared (FTIR) spectroscopy was used to determine the pressure and temperature stability of Mal d1. This study was triggered by contradictory results in the literature regarding the success of pressure treatment in the destruction of the allergen. The protein unfolded at 55°C when heated at normal atmospheric pressure. We also studied the effect exerted on pressure stability by environmental factors, which can be important for the stability of the protein in the apple. The pressure unfolding was measured under different pD conditions, and the effect of sugar mixture similar to that of the apple and the effect of ionic strength were also studied. In all cases the allergen unfolded with a transition midpoint in the range of 150-250 MPa. Unfolding was irreversible and was followed by aggregation of the unfolded protein. Lowering the pD destabilized the protein, while addition of sugar mixture and of KCl had stabilizing effect.

Unique features of the folding landscape of a repeat protein revealed by pressure perturbation

The volumetric properties of proteins yield information about the changes in packing and hydration between various states along the folding reaction coordinate and are also intimately linked to the energetics and dynamics of these conformations. These volumetric characteristics can be accessed via pressure perturbation methods. In this work, we report high-pressure unfolding studies of the ankyrin domain of the Notch receptor (Nank1-7) using fluorescence, small-angle x-ray scattering, and Fourier transform infrared spectroscopy. Both equilibrium and pressure-jump kinetic fluorescence experiments were consistent with a simple two-state folding/unfolding transition under pressure, with a rather small volume change for unfolding compared to proteins of similar molecular weight. High-pressure fluorescence, Fourier transform infrared spectroscopy, and small-angle x-ray scattering measurements revealed that increasing urea over a very small range leads to a more expanded pressure unfolded state with a significant decrease in helical content. These observations underscore the conformational diversity of the unfolded-state basin. The temperature dependence of pressure-jump fluorescence relaxation measurements demonstrated that at low temperatures, the folding transition state ensemble (TSE) lies close in volume to the folded state, consistent with significant dehydration at the barrier. In contrast, the thermal expansivity of the TSE was found to be equivalent to that of the unfolded state, indicating that the interactions that constrain the folded-state thermal expansivity have not been established at the folding barrier. This behavior reveals a high degree of plasticity of the TSE of Nank1-7.

Correlation Between Thermal Aggregation and Stability of Lysozyme with Salts Described by Molar Surface Tension Increment: An Exceptional Propensity of Ammonium Salts as Aggregation Suppressor

Protein aggregation is a critical problem for biotechnology and pharmaceutical industries. Despite the fact that soluble proteins have been used for many applications, our understanding of the effect of the solution chemistry on protein aggregation still remains to be elucidated. This paper investigates the process of thermal aggregation of lysozyme in the presence of various types of salts. The simple law was found; the aggregation rate of lysozyme increased with increasing melting temperature of the protein (T (m)) governed by chemical characteristics of additional salts. Ammonium salts were, however, ruled out; the aggregation rates of lysozyme in the presence of the ammonium salts were smaller than the ones estimated from T (m). Comparing with sodium salts, ammonium salts increased the solubility of the hydrophobic amino acids, indicating that ammonium salts adsorb the hydrophobic region of proteins, which leads to the decrease in aggregation more effectively than sodium salts. The positive relation between aggregation rate and T (m) was described by another factor such as the surface tension of salt solutions. Fourier transform infrared spectral analysis showed that the thermal aggregates were likely to form beta-sheet in solutions that give high molar surface tension increment. These results suggest that protein aggregation is attributed to the surface free energy of the solution.

Pressure-jump-induced kinetics reveals a hydration dependent folding/unfolding mechanism of ribonuclease A

Pressure-jump (p-jump)-induced relaxation kinetics was used to explore the energy landscape of protein folding/unfolding of Y115W, a fluorescent variant of ribonuclease A. Pressure-jumps of 40 MPa amplitude (5 ms dead-time) were conducted both to higher (unfolding) and to lower (folding) pressure, in the range from 100 to 500 MPa, between 30 and 50 degrees C. Significant deviations from the expected symmetrical protein relaxation kinetics were observed. Whereas downward p-jumps resulted always in single exponential kinetics, the kinetics induced by upward p-jumps were biphasic in the low pressure range and monophasic at higher pressures. The relative amplitude of the slow phase decreased as a function of both pressure and temperature. At 50 degrees C, only the fast phase remained. These results can be interpreted within the framework of a two-dimensional energy surface containing a pressure- and temperature-dependent barrier between two unfolded states differing in the isomeric state of the Asn-113-Pro-114 bond. Analysis of the activation volume of the fast kinetic phase revealed a temperature-dependent shift of the unfolding transition state to a larger volume. The observed compensation of this effect by glycerol offers an explanation for its protein stabilizing effect.