Pressure-exploration of the 33-kDa protein from the spinach photosystem II particle

Equilibria of oligomeric proteins under high pressure - A theoretical description

High pressure methods have become a useful tool for studying protein structure and stability. Using them, various physico-chemical processes including protein unfolding, aggregation, oligomer dissociation or enzyme-activity decrease were studied on many different proteins. Oligomeric protein dissociation is a process that can perfectly utilize the potential of high-pressure techniques, as the high pressure shifts the equilibria to higher concentrations making them better observable by spectroscopic methods. This can be especially useful when the oligomeric form is highly stable at atmospheric pressure. These applications may be, however, hindered by less intensive experimental response as well as interference of the oligomerization equilibria with unfolding or aggregation of the subunits, but also by more complex theoretical description. In this study we develop mathematical models describing different kinds of oligomerization equilibria, both closed (equilibrium of monomer and the highest possible oligomer without any intermediates) and consecutive. Closed homooligomer equilibria are discussed for any oligomerization degree, while the more complex heterooligomer equilibria and the consecutive equilibria in both homo- and heterooligomers are taken into account only for dimers and trimers. In all the cases, fractions of all the relevant forms are evaluated as functions of pressure and concentration. Significant points (inflection points and extremes) of the resulting transition curves, that can be determined experimentally, are evaluated as functions of pressure and/or concentration. These functions can be further used in order to evaluate the thermodynamic parameters of the system, i.e. atmospheric-pressure equilibrium constants and volume changes of the individual steps of the oligomer-dissociation processes.

High-pressure fluorescence applications

Fluorescence is the most widely used technique to study the effect of pressure on biochemical systems. The use of pressure as a physical variable sheds light into volumetric characteristics of reactions. Here we focus on the effect of pressure on protein solutions using a simple unfolding example in order to illustrate the applications of the methodology. Topics covered in this review include the relationships between practical aspects and technical limitations; the effect of pressure and the study of protein cavities; the interpretation of thermodynamic and relaxation kinetics; and the study of relaxation amplitudes. Finally, we discuss the insights available from the combination of fluorescence and other methods adapted to high pressure, such as SAXS or NMR. Because of the simplicity and accessibility of high-pressure fluorescence, the technique is a starting point that complements appropriately multi-methodological approaches related to understanding protein function, disfunction, and folding from the volumetric point of view.

Studying pressure denaturation of a protein by molecular dynamics simulations

Many globular proteins unfold when subjected to several kilobars of hydrostatic pressure. This "unfolding-up-on-squeezing" is counter-intuitive in that one expects mechanical compression of proteins with increasing pressure. Molecular simulations have the potential to provide fundamental understanding of pressure effects on proteins. However, the slow kinetics of unfolding, especially at high pressures, eliminates the possibility of its direct observation by molecular dynamics (MD) simulations. Motivated by experimental results-that pressure denatured states are water-swollen, and theoretical results-that water transfer into hydrophobic contacts becomes favorable with increasing pressure, we employ a water insertion method to generate unfolded states of the protein Staphylococcal Nuclease (Snase). Structural characteristics of these unfolded states-their water-swollen nature, retention of secondary structure, and overall compactness-mimic those observed in experiments. Using conformations of folded and unfolded states, we calculate their partial molar volumes in MD simulations and estimate the pressure-dependent free energy of unfolding. The volume of unfolding of Snase is negative (approximately -60 mL/mol at 1 bar) and is relatively insensitive to pressure, leading to its unfolding in the pressure range of 1500-2000 bars. Interestingly, once the protein is sufficiently water swollen, the partial molar volume of the protein appears to be insensitive to further conformational expansion or unfolding. Specifically, water-swollen structures with relatively low radii of gyration have partial molar volume that are similar to that of significantly more unfolded states. We find that the compressibility change on unfolding is negligible, consistent with experiments. We also analyze hydration shell fluctuations to comment on the hydration contributions to protein compressibility. Our study demonstrates the utility of molecular simulations in estimating volumetric properties and pressure stability of proteins, and can be potentially extended for applications to protein complexes and assemblies.

Thermodynamics and kinetics of the pressure unfolding of phosphoglycerate kinase

Due to the relationship between compressibility and volume fluctuations, high-pressure studies provide vital insight into protein dynamics and function. Most high-pressure experiments were performed on small and fast folding proteins or model peptides. Here we show that a detailed kinetic study is necessary to extract reliable information from the high-pressure-induced structural conversion of large, slowly folding proteins. The pressure-jump unfolding kinetics of yeast phosphoglycerate kinase was recorded at pressures between 50 and 150 MPa. The time dependence of the conformational state of the protein was followed by tryptophan fluorescence measurements from 30 s to 2 h. The observed changes were described by a three-state model, and the volume change and the activation volume as well as the midpoint pressure of the transitions between the folded, intermediate, and unfolded states were determined. An interesting feature of the pressure unfolding of phosphoglycerate kinase was that the unfolding process speeds up with increasing pressure, which is the consequence of negative activation volumes for the folded --> intermediate, intermediate --> unfolded, and unfolded --> intermediate transitions.

Theoretical study of the cosolvent effect on the partial molar volume change of staphylococcal nuclease associated with pressure denaturation

We explain the molecular mechanism of the effect of urea and glycerol cosolvents on the partial molar volume (PMV) change associated with the pressure denaturation of staphylococcal nuclease (SNase) protein recently observed in experiments. Native and denatured conformations of SNase are produced by using molecular dynamics simulations in water, and the PMV is obtained from the integral equation theory of molecular liquids called 3D-RISM, which is based on statistical mechanics. The PMV of the native SNase in water predicted by 3D-RISM theory is in good agreement with experiment. The PMV changes associated with pressure denaturation in water and in water-urea and water-glycerol mixtures are qualitatively reproduced. By analyzing the results obtained, we found two interesting cosolvent effects on the PMV: (1) both urea and glycerol cosolvents increase the PMVs of both native and denatured SNase compared to those in water and (2) both urea and glycerol cosolvents increase the PMV of denatured SNase more than that of native SNase. We also showed that these two observations can be explained in terms of the thermal volume, which is related to the packing effect of solvent molecules.

Folding studies of two hydrostatic pressure sensitive proteins

High hydrostatic pressure combined with various spectroscopies is a powerful technique to study protein folding. An ideal model system for protein folding studies should have the following characteristics. (1) The protein should be sensitive to pressure, so that the protein can be unfolded under mild pressure. (2) The folding process of the protein should be easily modulated by several chemical or physical factors. (3) The folding process should be easily monitored by some spectroscopic parameters. Here, we summarized the pressure induced folding studies of two proteins isolated from spinach photosystem II, namely the 23-kDa and the 33-kDa protein. They have all the characteristics mention above and might be an ideal model protein system for pressure studies.

The use of pressure-jump relaxation kinetics to study protein folding landscapes

Pressure-jump induced relaxation kinetics can be used to study both protein unfolding and refolding. These processes can be initiated by upward and downward pressure-jumps of amplitudes of a few 10 to 100 MPa, with a dead-time on the order of milliseconds. In many cases, the relaxation times can be easily determined when the pressure cell is connected to a spectroscopic detection device, such as a spectrofluorimeter. Adiabatic heating or cooling can be limited by small pressure-jump amplitudes and a special design of the sample cell. Here, we discuss the application of this method to four proteins: 33-kDa and 23-kDa proteins from photo-system II, a variant of the green fluorescent protein, and a fluorescent variant of ribonuclease A. The thermodynamically predicted equivalency of upward and downward pressure-jump induced protein relaxation kinetics for typical two-state folders was observed for the 33-kDa protein, only. In contrast, the three other proteins showed significantly different kinetics for pressure-jumps in opposite directions. These results cannot be explained by sequential reaction schemes. Instead, they are in line with a more complex free energy landscape involving multiple pathways.

Insights into the role of hydration in protein structure and stability obtained through hydrostatic pressure studies

A thorough understanding of protein structure and stability requires that we elucidate the molecular basis for the effects of both temperature and pressure on protein conformational transitions. While temperature effects are relatively well understood and the change in heat capacity upon unfolding has been reasonably well parameterized, the state of understanding of pressure effects is much less advanced. Ultimately, a quantitative parameterization of the volume changes (at the basis of pressure effects) accompanying protein conformational transitions will be required. The present report introduces a qualitative hypothesis based on available model compound data for the molecular basis of volume change upon protein unfolding and its dependence on temperature.

Protein volume changes on cosolvent denaturation

A thermodynamic relationship is obtained which links the effect of a cosolvent on the denaturation equilibrium of a protein to the effect of the cosolvent on the change in partial molar volume (pmv) of a protein on denaturation. The relationship uses the concept of preferential interactions and is exact for an infinitely dilute protein. Analysis of the literature data on protein volume changes suggests that many of the observed volume changes are thermodynamically inconsistent with the corresponding free energy changes, especially at low cosolvent concentrations. It is argued that the most reasonable explanation for this involves cosolvent induced changes in the degree of protein-protein association.

Acetonitrile-induced unfolding of the photosystem II manganese-stabilizing protein studied by electrospray mass spectrometry

In this paper an acetonitrile-induced unfolding of the manganese-stabilizing protein (MSP) of photosystem II was discovered. More distinct unfolding states of MSP were identified than previously by using mainly electrospray ionization mass spectrometry (ESI-MS), together with fluorescence spectra and far-UV circular dichroism (CD) at pH 2.0, 6.2 or 11.6, and with acetonitrile concentrations from 0 to 50%. At pH 6.2 with acetonitrile concentration changing from 0 to 10%, relatively broad charge-state distributions and poor intensity were observed in ESI-MS, indicating the presence of coexisting conformers. It was concluded that the structure of the MSP protein is unlikely to be a tightly folded form. When the concentration of acetonitrile was 20-40%, simulating the state in the biological membrane, changes in the state of unfolding of MSP were observed to a certain extent using ESI-MS, fluorescence and CD spectroscopy. The charge-state distribution in ESI-MS was found to move toward high states (from 13+ to 27+ to 15+ to 31+) with increasing acetonitrile concentration. At pH 2.0, the MSP structure is rearranged into an unfolded state, and at pH 11.6 the MSP structure is induced to assume another unordered state by deprotonation of appropriate residues. An interesting observation was that a second peak envelope emerged with 20-50% acetonitrile in the medium at pH 11.6.

Pressure equilibrium and jump study on unfolding of 23-kDa protein from spinach photosystem II

Pressure-induced unfolding of 23-kDa protein from spinach photosystem II has been systematically investigated at various experimental conditions. Thermodynamic equilibrium studies indicate that the protein is very sensitive to pressure. At 20 degrees C and pH 5.5, 23-kDa protein shows a reversible two-state unfolding transition under pressure with a midpoint near 160 MPa, which is much lower than most natural proteins studied to date. The free energy (DeltaG(u)) and volume change (DeltaV(u)) for the unfolding are 5.9 kcal/mol and -160 ml/mol, respectively. It was found that NaCl and sucrose significantly stabilize the protein from unfolding and the stabilization is associated not only with an increase in DeltaG(u) but also with a decrease in DeltaV(u). The pressure-jump studies of 23-kDa protein reveal a negative activation volume for unfolding (-66.2 ml/mol) and a positive activation volume for refolding (84.1 ml/mol), indicating that, in terms of system volume, the protein transition state lies between the folded and unfolded states. Examination of the temperature effect on the unfolding kinetics indicates that the thermal expansibility of the transition state and the unfolded state of 23-kDa protein are closer to each other and they are larger than that of the native state. The diverse pressure-refolding pathways of 23-kDa protein in some conditions were revealed in pressure-jump kinetics.

The 33 kDa protein of photosystem II is a low-affinity calcium- and lanthanide-binding protein

We have shown that the isolated 33 kDa protein of photosystem II contains one calcium and one lanthanide low-affinity binding site with binding constants (K(D)) on the order of 10(-5) M. Binding of calcium or lanthanides to this site induces conformational changes in the protein that manifest in fluorescence emission spectra of the protein, circular dichroism spectra, and calorimetric thermograms where the phase transitions are shifted to lower temperatures. The role of calcium binding to the 33 kDa protein in the attainment of its native structure and the significance of this interaction for the oxygen evolution process are discussed.

Effects of Chaotropic and Kosmotropic Cosolvents on the Pressure-Induced Unfolding and Denaturation of Proteins: An FT-IR Study on Staphylococcal Nuclease†

FT-IR spectroscopy was used to study the effects of various chaotropic and kosmotropic cosolvents (glycerol, sucrose, sorbitol, K(2)SO(4), CaCl(2), and urea) on the secondary structure and thermodynamic properties upon unfolding and denaturation of staphylococcal nuclease (Snase). The data show that the different cosolvents have a profound effect on the denaturation pressure and the Gibbs free energy (DeltaG(o)) and volume (DeltaV(o) change of unfolding. Moreover, by analysis of the amide I' infrared bands, conformational changes of the protein upon unfolding in the different cosolvents have been determined. An increase, a reduction, or an independence of the volume change of unfolding is observed, depending on the type of cosolvent, which can at least in part be attributed to the formation of a different unfolded state structure of the protein. The data are compared with the corresponding thermodynamic values of DeltaV(o) for the temperature-induced unfolding process of Snase as obtained by pressure perturbation calorimetry, and significant differences are observed and discussed.

The propeptide in the precursor form of carboxypeptidase Y ensures cooperative unfolding and the carbohydrate moiety exerts a protective effect against heat and pressure

The heat- and pressure-induced unfolding of the glycosylated and unglycosylated forms of mature carboxypeptidase Y and the precursor procarboxypeptidase Y were analysed by differential scanning calorimetry and/or by their intrinsic fluorescence in the temperature range of 20-75 degrees C or the pressure range of 0.1-700 MPa. Under all conditions, the precursor form showed a clear two-state transition from a folded to an unfolded state, regardless of the presence of the carbohydrate moiety. In contrast, the mature form, which lacks the propeptide composed of 91 amino acid residues, showed more complex behaviour: differential scanning calorimetry and pressure-induced changes in fluorescence were consistent with a three-step transition. These results show that carboxypeptidase Y is composed of two structural domains, which unfold independently but that procarboxypeptidase Y behaves as a single domain, thus ensuring cooperative unfolding. The carbohydrate moiety has a slightly protective role in heat-induced unfolding and a highly protective role in pressure-induced unfolding.

Calcium-Dependent Conformational Change and Thermal Stability of the Isolated PsbO Protein Detected by FTIR Spectroscopy

The structure and function of the photosystem II PsbO extrinsic protein is under intense research, being an essential part of the biomolecular engine that carries out water oxidation and oxygen production. This paper presents a structural analysis of the isolated PsbO protein by FTIR spectroscopy, reporting detailed secondary structure quantification and changes in the secondary structure content of the protein attributed to the effect of calcium (Ca(2+)). Measurements in H(2)O and D(2)O have allowed us to see the effect of calcium on the conformation of the protein. The results indicate that (i) the protein presents a major content of beta-structure (i.e., beta-sheet, beta-strands, beta-turns) as detected by the infrared bands at 1624-1625, 1678-1679, 1688-1689 cm(-1), which account for about 38% in water and 33% in heavy water, in the presence of calcium; and (ii) the amount of this beta-structure fraction increases 7-10% in the absence of calcium, with a concomitant decrease in loops and nonordered structure. The thermal denaturation profile of the protein in the presence of calcium showed low stability with T(m) approximately 56 degrees C. This profile also shows a second phase of denaturation above 60 degrees C and the appearance of aggregation signals above 70 degrees C. Our observations indicate that calcium is able to modify the conformation of the protein at least in solution and confirm that PsbO is mainly a beta-protein where beta-sheet is the major ordered secondary structure element of the protein core.

Structural instability and fibrillar aggregation of non-expanded human ataxin-3 revealed under high pressure and temperature

Protein misfolding and formation of structured aggregates are considered to be the earliest events in the development of neurodegenerative diseases, but the mechanism of these biological phenomena remains to be elucidated. Here, we report a study of heat- and pressure-induced unfolding of human Q26 and murine Q6 ataxin-3 using spectroscopic methods. UV absorbance and fluorescence revealed that heat and pressure induced a structural transition of both proteins to a molten globule conformation. The unfolding pathway was partly irreversible and led to a protein conformation where tryptophans were more exposed to water. Furthermore, the use of fluorescent probes (8-anilino-1-naphthalenesulfonate and thioflavin T) allowed the identification of different intermediates during the process of pressure-induced unfolding. At high temperature and pressure, human Q26, but not murine Q6, underwent concentration-dependent aggregation. Fourier transform infrared and circular dichroism spectroscopy revealed that these aggregates are characterized by an increased beta-sheet content. As revealed by electron microscopy, heat- and pressure-induced aggregates were different; high temperature treatment led to fibrillar microaggregates (8-10-nm length), whereas high pressure induced oligomeric structures of globular shape (100 nm in diameter), which sometimes aligned to higher order suprastructures. Several intermediate structures were detected in this process. Two factors appear to govern ataxin unfolding and aggregation, the length of the polyglutamine tract and its protein context.

The thermodynamic analysis of protein stabilization by sucrose and glycerol against pressure-induced unfolding

We have studied the reaction native left arrow over right arrow denatured for the 33-kDa protein isolated from photosystem II. Sucrose and glycerol have profound effects on pressure-induced unfolding. The additives shift the equilibrium to the left; they also cause a significant decrease in the standard volume change (DeltaV). The change in DeltaV was related to the sucrose and glycerol concentrations. The decrease in DeltaV varied with the additive: sucrose caused the largest effect, glycerol the smallest. The theoretical shift of the half-unfolding pressure (P1/2) calculated from the net increase in free energy by addition of sucrose and glycerol was lower than that obtained from experimental mea- surements. This indicates that the free energy change caused by preferential hydration of the protein is not the unique factor involved in the protein stabilization. The reduction in DeltaV showed a large contribution to the theoretical P1/2 shift, suggesting that the DeltaV change, caused by the sucrose or glycerol was associated with the protein stabilization. The origin of the DeltaV change is discussed. The rate of pressure-induced unfolding in the presence of sucrose or glycerol was slower than the refolding rate although both were significantly slower than that observed without any stabilizers.

Optimized overproduction, purification, characterization and high-pressure sensitivity of the prion protein in the native (PrP(C)-like) or amyloid (PrP(Sc)-like) conformation

Overproduction and purification of the prion protein is a major concern for biological or biophysical analysis as are the structural specificities of this protein in relation to infectivity. We have developed a method for the effective cloning, overexpression in Escherichia coli and purification to homogeneity of Syrian golden hamster prion protein (SHaPrP(90-231)). A high level of overexpression, resulting in the formation of inclusion bodies, was obtained under the control of the T7-inducible promoter of the pET15b plasmid. The protein required denaturation, reduction and refolding steps to become soluble and attain its native conformation. Purification was carried out by differential centrifugation, gel filtration and reverse phase chromatography. An improved cysteine oxidation protocol using oxidized glutathione under denaturing conditions, resulted in the recovery of a higher yield of chromatographically pure protein. About 10 mg of PrP protein per liter of bacterial culture was obtained. The recombinant protein was identified by monoclonal antibodies and its integrity was confirmed by electrospray mass spectrometry (ES/MS), whereas correct folding was assessed by circular dichroism (CD) spectroscopy. This protein had the structural characteristics of PrP(C) and could be converted to an amyloid structure sharing biophysical and biochemical properties of the pathologic form (PrP(Sc)). The sensitivity of these two forms to high pressure was investigated. We demonstrate the potential of using pressure as a thermodynamic parameter to rescue trapped aggregated prion conformations into a soluble state, and to explore new conformational coordinates of the prion protein conformational landscape.

Alternative prion structural changes revealed by high pressure

At high temperature, recombinant hamster prion protein (SHaPrP(90-231)) undergoes aggregation and changes from a predominantly alpha-helical to beta-sheet conformation. We then applied high pressure (200 MPa) to the beta-sheet-rich conformation. The aggregation was reversed, and the original tertiary and secondary structures were recovered at ambient pressure, after pressure release. The application of a pressure of 200 MPa thus allowed studying the heat-induced equilibrium refolding in the absence of protein aggregation. Prion protein unfolding as a function of high pressure was also investigated. Simple two-state, reversible unfolding transitions were observed, as monitored by spectral changes in the UV and fluorescence of the hydrophobic probe 8-anilino-1-naphthalene sulfonate. However, these heat- and pressure-induced conformers differed in their unfolding free energy. At pressures over 400 MPa, strong thioflavin-T binding was observed, suggesting a further structural change to a metastable oligomeric structure.

A rare protein fluorescence behavior where the emission is dominated by tyrosine: case of the 33-kDa protein from spinach photosystem II

An abnormal fluorescence emission of protein was observed in the 33-kDa protein which is one component of the three extrinsic proteins in spinach photosystem II particle (PS II). This protein contains one tryptophan and eight tyrosine residues, belonging to a "B type protein". It was found that the 33-kDa protein fluorescence is very different from most B type proteins containing both tryptophan and tyrosine residues. For most B type proteins studied so far, the fluorescence emission is dominated by the tryptophan emission, with the tyrosine emission hardly being detected when excited at 280 nm. However, for the present 33-kDa protein, both tyrosine and tryptophan fluorescence emissions were observed, the fluorescence emission being dominated by the tyrosine residue emission upon a 280 nm excitation. The maximum emission wavelength of the 33-kDa protein tryptophan fluorescence was at 317 nm, indicating that the single tryptophan residue is buried in a very strong hydrophobic region. Such a strong hydrophobic environment is rarely observed in proteins when using tryptophan fluorescence experiments. All parameters of the protein tryptophan fluorescence such as quantum yield, fluorescence decay, and absorption spectrum including the fourth derivative spectrum were explored both in the native and pressure-denatured forms.

Expanding the pressure technique: insights into protein folding from combined use of pressure and chemical denaturants

The fundamental principles derived from in vitro protein folding experiments have practical application in understanding the pathology of diseases of protein misfolding and for the development of industrial processes to produce proteins as pharmaceuticals and biotechnological reagents. High pressure as a tool to denature or disaggregate proteins offers a number of unique advantages. The emphasis of this review is on how low concentrations of chemical denaturants can be used in combination with high pressure to extend the range and scope of this useful technique. This approach has already been used in a number of studies, which are discussed here in the context of the questions they address. These include: the origin of the volume change observed on protein unfolding, pressure-induced formation of partially structured intermediates, pressure-induced dissociation of oligomeric and aggregated proteins, and the use of volume changes to probe the structure of the transition state. Wider use of hydrostatic pressure as a denaturation tool, facilitated by combination with chemical denaturants, is likely to bring significant advances to our understanding of protein structure, stability and folding, particularly in relation to proteins associated with the amyloid and prion diseases.

UV-visible derivative spectroscopy under high pressure

High hydrostatic pressure affects proteins, changing their intra- or intermolecular interactions, conformation and solvation. How to detect these changes? In this paper, via some selected examples, we show the potentiality (but also the limits) of the ultraviolet derivative spectroscopy specially adapted to high pressure experiments.

Pressure-temperature phase diagrams of biomolecules

The pressure-temperature phase diagram of various biomolecules is reviewed. Special attention is focused on the elliptic phase diagram of proteins. The phenomenological thermodynamic theory describing this diagram explains the heat, cold and pressure denaturations in a unified picture. The limitations and possible developments of this theory are discussed as well. It is pointed out that a more complex diagram can be obtained when the intermolecular interactions are also taken into account. In this case metastable states appear on the pressure-temperature (p-T) diagram due to intermolecular interactions. Pressure-temperature phase diagrams of other biopolymers are also discussed. While the p-T diagrams of helix-coil transition of nucleic acids and of gel-liquid crystal transition of lipid bilayers are non-elliptical, those of gelatinization of starch and of phase separation of some synthetic polymers show an elliptic profile, similar to that of proteins. Finally, the p-T diagram of bacterial inactivation is shown to be elliptic. From the point of view of basic science, this fact shows that the key factor of inactivation should be the protein type, and from the viewpoint of practical applications, it serves as the theoretical basis of pressure treatment of biosystems.

High pressure static fluorescence to study macromolecular structure-function

Through some typical examples, the high pressure static fluorescence method is described. The potentiality of the intrinsic and extrinsic fluorescence probes are analyzed for structural characterizations. Special attention is given to the use of fluorescence to understand the behavior of enzymatic reactions under high pressure. The application of fluorescence polarization is also presented together with some relevant spectroscopic problems inherent in data interpretation.

Revisiting volume changes in pressure-induced protein unfolding

It has long been known that the application of hydrostatic pressure generally leads to the unfolding of proteins. Despite a relatively large number of reports in the literature over the past few decades, there has been great confusion over the sign and magnitude as well as the fundamental factors contributing to volume effects in protein conformational transitions. It is the goal of this review to present and discuss the results obtained concerning the sign and magnitude of the volume changes accompanying the unfolding of proteins. The vast majority of cases point to a significant decrease in volume upon unfolding. Nonetheless, there is evidence that, due to differences in the thermal expansivity of the folded and unfolded states of proteins reported in a half dozen manuscripts, that the sign of the volume change may become positive at higher temperatures.

Protein folding pathways of adenylate kinase from E. coli: hydrostatic pressure and stopped-flow studies

Adenylate kinase (AKe) from E. coli is a small, single-chain, monomeric enzyme with no tryptophan and a single cysteine residue. We have constructed six single-Trp mutants of AKe to facilitate optical studies of these proteins and to specifically examine the interrelationship between their structure, function, dynamics, and folding reactions. In this study, the effects of hydrostatic pressure on the folding reactions of AKe were studied. The native structure of AKe was transformed to a non-native, yet pressure stable, conformation by hydrostatic pressure of about 300 MPa. This pressure lability of AKe is rather low for a monomeric protein and presumably may be attributed to substantial conformational flexibility and a correspondingly large volume change. The refolding of AKe after pressure-induced denaturation was reversible under ambient conditions. At low temperature (near 0 degrees C), the refolding process of pressure-exposed AKe mutants displayed a significant hysteresis. The observation of a slow refolding rate in the 193 region and a faster folding rate around the active site (86, 41, 73 regions) leads us to suggest that in the folding process, priority is afforded to functional regions. The slow structural return of the 193 region apparently does not hinder the more rapid return of enzymatic activity of AKe. Circular dichroism studies on the pressure-denatured Y193W mutant show that the secondary structure (calculated from far-UV spectra) returned at a rapid rate, but the tertiary structure alignment (calculated from near-UV spectra) around the 193 region occurred more slowly at rates comparable to those detected by fluorescence intensity. Denaturation of AKe mutants by guanidine hydrochloride and subsequent refolding experiments were also consistent with a much slower refolding process around the 193 region than near the active site. Fast refolding kinetic traces were observed in F86W, S41W, and A73W mutants using a fluorescence detection stopped-flow rapid mixing device, while only a slow kinetic trace was observed for Y193W. The results suggest that the differences in regional folding rates of AKe are not derived from the specific denaturation methods, but rather are inherent in the structural organization of the protein.