The pressure-temperature phase diagram of hen lysozyme at low pH

Heat and cold denaturation of yeast frataxin: The effect of pressure

Yfh1 is a yeast protein with the peculiar characteristic to undergo, in the absence of salt, cold denaturation at temperatures above the water freezing point. This feature makes the protein particularly interesting for studies aiming at understanding the rules that determine protein fold stability. Here, we present the phase diagram of Yfh1 unfolding as a function of pressure (0.1-500 MPa) and temperature 278-313 K (5-40°C) both in the absence and in the presence of stabilizers using Trp fluorescence as a monitor. The protein showed a remarkable sensitivity to pressure: at 293 K, pressures around 10 MPa are sufficient to cause 50% of unfolding. Higher pressures were required for the unfolding of the protein in the presence of stabilizers. The phase diagram on the pressure-temperature plane together with a critical comparison between our results and those found in the literature allowed us to draw conclusions on the mechanism of the unfolding process under different environmental conditions.

In Situ Monitoring of Protein Unfolding/Structural States under Cold High-Pressure Stress

Biopharmaceutical formulations may be compromised by freezing, which has been attributed to protein conformational changes at a low temperature, and adsorption to ice-liquid interfaces. However, direct measurements of unfolding/conformational changes in sub-0 °C environments are limited because at ambient pressure, freezing of water can occur, which limits the applicability of otherwise commonly used analytical techniques without specifically tailored instrumentation. In this report, small-angle neutron scattering (SANS) and intrinsic fluorescence (FL) were used to provide in situ analysis of protein tertiary structure/folding at temperatures as low as -15 °C utilizing a high-pressure (HP) environment (up to 3 kbar) that prevents water from freezing. The results show that the α-chymotrypsinogen A (aCgn) structure is reasonably maintained under acidic pH (and corresponding pD) for all conditions of pressure and temperature tested. On the other hand, reversible structural changes and formation of oligomeric species were detected near -10 °C via HP-SANS for ovalbumin under neutral pD conditions. This was found to be related to the proximity of the temperature of cold denaturation of ovalbumin (T_CD ∼ -17 °C; calculated via isothermal chemical denaturation and Gibbs-Helmholtz extrapolation) rather than a pressure effect. Significant structural changes were also observed for a monoclonal antibody, anti-streptavidin IgG1 (AS-IgG1), under acidic conditions near -5 °C and a pressure of ∼2 kbar. The conformational perturbation detected for AS-IgG1 is proposed to be consistent with the formation of unfolding intermediates such as molten globule states. Overall, the in situ approaches described here offer a means to characterize the conformational stability of biopharmaceuticals and proteins more generally under cold-temperature stress by the assessment of structural alteration, self-association, and reversibility of each process. This offers an alternative to current ex situ methods that are based on higher temperatures and subsequent extrapolation of the data and interpretations to the cold-temperature regime.

Enhanced accessibility and hydrophobicity of amyloidogenic intermediates of the β2-microglobulin D76N mutant revealed by high-pressure experiments

β2-Microglobulin (β2m) is the causative protein of dialysis-related amyloidosis. Its unfolding mainly proceeds along the pathway of N_C →U_C ⇄ U_T, whereas refolding follows the U_T → I_T (→N_T) →N_C pathway, in which N, I, and U are the native, intermediate, and unfolded states, respectively, with the Pro32 peptidyl-prolyl bond in cis or trans conformation as indicated by the subscript. It is noted that the I_T state is a putative amyloidogenic precursor state. Several aggregation-prone variants of β2m have been reported to date. One of these variants is D76N β2m, which is a naturally occurring amyloidogenic mutant. To elucidate the molecular mechanisms contributing to the enhanced amyloidogenicity of the mutant, we investigated the equilibrium and kinetic transitions of pressure-induced folding/unfolding equilibria in the wild type and D76N mutant by monitoring intrinsic tryptophan and 1-anilino-8-naphthalene sulfonate fluorescence. An analysis of kinetic data revealed that the different folding/unfolding behaviors of the wild type and D76N mutant were due to differences in the activation energy between the unfolded and the intermediate states as well as stability of the native state, leading to more rapid accumulation of I_T state for D76N in the refolding process. In addition, the I_T state was found to assume more hydrophobic nature. These changes induced the enhanced amyloidogenicity of the D76N mutant and the distinct pathogenic symptoms of patients. Our results suggest that the stabilization of the native state will be an effective approach for suppressing amyloid fibril formation of this mutant.

Cosolvent and Crowding Effects on the Temperature- and Pressure-Dependent Dissociation Process of the α/β-Tubulin Heterodimer

Tubulin is one of the main components of the cytoskeleton of eukaryotic cells. The formation of microtubules depends strongly on environmental and solution conditions, and has been found to be among the most pressure sensitive processes in vivo. We explored the effects of different types of cosolvents, such as trimethylamine-N-oxide (TMAO), sucrose and urea, and crowding agents to mimic cell-like conditions, on the temperature and pressure stability of the building block of microtubules, i. e. the α/β-tubulin heterodimer. To this end, fluorescence and FTIR spectroscopy, differential scanning and pressure perturbation calorimetry as well as fluorescence anisotropy and correlation spectroscopies were applied. The pressure and temperature of dissociation of α/β-tubulin as well as the underlying thermodynamic parameters upon dissociation, such as volume and enthalpy changes, have been determined for the different solution conditions. The temperature and pressure of dissociation of the α/β-tubulin heterodimer and hence its stability increases dramatically in the presence of TMAO and the nanocrowder sucrose. We show that by adjusting the levels of compatible cosolutes and crowders, cells are able to withstand deteriorating effects of pressure even up to the kbar-range.

Hyperbolic Pressure-Temperature Phase Diagram of the Zinc-Finger Protein apoKti11 Detected by NMR Spectroscopy

For a comprehensive understanding of the thermodynamic state functions describing the stability of a protein, the influence of the intensive properties of temperature and pressure has to be known. With the zinc-finger-containing Kti11, we found a suitable protein for this purpose because folding and unfolding transitions occur at an experimentally accessible temperature (280-330 °K) and pressure (0.1-240 MPa) range. We solved the crystal structure of the apo form of Kti11 to reveal two disulfide bonds at the metal-binding site, which seals off a cavity in the β-barrel part of the protein. From a generally applicable proton NMR approach, we could determine the populations of folded and unfolded chains under all conditions, leading to a hyperbolic pressure-temperature phase diagram rarely observed for proteins. A global fit of a two-state model to all derived populations disclosed reliable values for the change in Gibbs free energy, volume, entropy, heat capacity, compressibility, and thermal expansion upon unfolding. The unfolded state of apoKti11 has a lower compressibility compared to the native state and a smaller volume at ambient pressure. Therefore, a pressure increase up to 200 MPa reduces the population of the native state, and above this value, the native population increases again. Pressure-induced chemical-shift changes in two-dimensional ¹H-¹⁵N NMR spectra could be employed for a molecular interpretation of the thermodynamic properties of apoKti11.

Monitoring protein folding through high pressure NMR spectroscopy

High-pressure is a well-known perturbation method used to destabilize globular proteins. It is perfectly reversible, which is essential for a proper thermodynamic characterization of a protein equilibrium. In contrast to other perturbation methods such as heat or chemical denaturant that destabilize protein structures uniformly, pressure exerts local effects on regions or domains of a protein containing internal cavities. When combined with NMR spectroscopy, hydrostatic pressure offers the possibility to monitor at a residue level the structural transitions occurring upon unfolding and to determine the kinetic properties of the process. High-pressure NMR experiments can now be routinely performed, owing to the recent development of commercially available high-pressure sample cells. This review summarizes recent advances and some future directions of high-pressure NMR techniques for the characterization at atomic resolution of the energy landscape of protein folding.

High Pressure NMR Spectroscopy

The combination of high-resolution NMR spectroscopy with pressure perturbation, known as variable-pressure NMR spectroscopy or simply high pressure NMR spectroscopy, is a relatively recent accomplishment, but is a technique expanding rapidly with high promise in future. The importance of the method is that it allows, for the first time in history, a systematic means of detecting and analyzing the structures and thermodynamic stability of high-energy sub-states in proteins. High-energy sub-states have been only vaguely known so far, as normally their populations are too low to be detected by conventional spectroscopic techniques including NMR spectroscopy. By now, however, high pressure NMR spectroscopy has established unequivocally that high-energy conformers are universally present in proteins in equilibrium with their stable folded counterparts. This chapter describes briefly the techniques of high pressure NMR spectroscopy and its unique and novel aspects as a method to explore protein structure in its high-energy paradigm with illustrative examples. It is now well established that high pressure NMR spectroscopy is a method to study intrinsic fluctuations of proteins, rather than those forced by pressure, by detecting structural changes amplified by pressure. Extension of the method to other bio-macromolecular systems is considered fairly straightforward.

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.

High-Pressure Fluorescence Spectroscopy

The combination of fluorescence and pressure perturbation is a widely used technique to study the effect of pressure on a protein system to obtain thermodynamic, structural and kinetic information on proteins. However, we often encounter the situation where the available pressure range up to 400 MPa of most commercial high-pressure fluorescence spectrometers is insufficient for studying highly pressure-stable proteins like inhibitors and allergenic proteins. To overcome the difficulty, we have recently developed a new high-pressure fluorescence system that allows fluorescence measurements up to 700 MPa. Here we describe the basic design of the apparatus and its application to study structural and thermodynamic properties of a couple of highly stable allergenic proteins, hen lysozyme and ovomucoid, using Tryptophan and Tyrosine/Tyrosinate fluorescence, respectively. Finally, we discuss the utility and the limitation of Trp and Tyr fluorescence. We discuss pitfalls of fluorescence technique and importance of simultaneous use of other high-pressure spectroscopy, particularly high-pressure NMR spectroscopy.

Temperature and pressure dependence of azurin stability as monitored by tryptophan fluorescence and phosphorescence. The case of F29A mutant

The effects of a single-point, F29A, cavity-forming mutation on the unfolding thermodynamic parameters of azurin from Pseudomonas aeruginosa and on the internal dynamics of the protein fold under pressure were probed by the fluorescence and phosphorescence emission of Trp48, deeply buried in the compact hydrophobic core of the macromolecule. Pressure-induced unfolding, monitored by the shift in the fluorescence spectrum, led to a volume change of 70-90mlmol(-1). The difference in the unfolding volume between F29A and wild type azurin was smaller than the volume of the space theoretically created in the mutant, indicating that the cavity is, at least partially, filled with water molecules. The complex temperature dependence of the unfolding volume, for temperatures up to 20°C, suggests the formation of an expanded form of the protein and highlights how the packing efficiency of azurin appears to contribute to the magnitude of internal void volume at any given temperature. Changes in flexibility of the protein matrix around the chromophore were monitored by the intrinsic phosphorescence lifetime. At 40°C the application of pressure in the predenaturation range initially decreases the internal flexibility of azurin, the trend eventually reverting on approaching unfolding. The main difference between wild type and the cavity mutant is the inversion point which happens at 300MPa for wild type and at 150MPa for F29A. This suggests that, for the cavity mutant, pressure-induced internal hydration is more dominant than any compaction of the globular fold at relatively low pressures.

Extensively hydrated but folded: a novel state of globular proteins stabilized at high pressure and low temperature

We studied conformational fluctuations of the transcription factor c-Myb R2 subdomain (52 residues with three Trp) at high pressure and low temperature (5°C) using two different spectroscopic methods, Trp fluorescence and (1)H NMR, on its chemically stable mutant C130I (pseudo-wild-type (WT(S))), which has a large internal cavity. As pressure was increased from 3 to 300 MPa, the Trp fluorescence λ(max) of WT(S) shifted from 342 to ∼355 nm, clearly showing that the three Trp rings become fully exposed to the polar environment, which usually is taken to indicate that the protein underwent unfolding. In contrast, as pressure was increased from 3 to 300 MPa, the high-field-shifted (1)H NMR signals characteristic of the folded state showed a still higher-field shift, but no significant changes in their intensity. The last result unequivocally shows that the protein remains largely folded at 300 MPa. The apparent discrepancy between the two predictions would only be solved if one were to postulate the existence of an extensively hydrated but folded state in WT(S). Intriguingly, such a state was not found in a cavity-filling mutant of WT(S), C130I/V103L, suggesting that this state is mediated by cavity hydration. The generality and significance of this state in proteins are discussed.

Pressure-accelerated dissociation of amyloid fibrils in wild-type hen lysozyme

The dynamics of amyloid fibrils, including their formation and dissociation, could be of vital importance in life. We studied the kinetics of dissociation of the amyloid fibrils from wild-type hen lysozyme at 25°C in vitro as a function of pressure using Trp fluorescence as a probe. Upon 100-fold dilution of 8 mg ml(-1) fibril solution in 80 mM NaCl, pH 2.2, no immediate change occurred in Trp fluorescence, but at pressures of 50-450 MPa the fluorescence intensity decreased rapidly with time (k(obs) = 0.00193 min(-1) at 0.1 MPa, 0.0348 min(-1) at 400 MPa). This phenomenon is attributable to the pressure-accelerated dissociation of amyloid fibrils into monomeric hen lysozyme. From the pressure dependence of the rates, which reaches a plateau at ~450 MPa, we determined the activation volume ΔV(0‡) = -32.9 ± 1.7 ml mol(monomer)(-1) and the activation compressibility Δκ(‡) = -0.0075 ± 0.0006 ml mol(monomer)(-1) bar(-1) for the dissociation reaction. The negative ΔV(0‡) and Δκ(‡) values are consistent with the notion that the amyloid fibril from wild-type hen lysozyme is in a high-volume and high-compressibility state, and the transition state for dissociation is coupled with a partial hydration of the fibril.

Ceramic cells for high pressure NMR spectroscopy of proteins

Application of high pressure to biological macromolecules can be used to find new structural states with a smaller specific volume of the system. High pressure NMR spectroscopy is a most promising analytical tool for the study of these states at atomic resolution. High pressure quartz cells are difficult to handle, high quality sapphire high pressure cells are difficult to obtain commercially. In this work, we describe the use of high pressure ceramic cells produced from yttrium stabilized ZrO(2) that are capable of resisting pressures up to 200 MPa. Since the new cells should also be usable in the easily damageable cryoprobes a completely new autoclave for these cells has been constructed, including an improved method for pressure transmission, an integrated safety jacket, a displacement body, and a fast self-closing emergency valve.