Pressure-induced aggregation of β-lactoglobulin in ph 7.0 buffers

Comparison of the Effects of High Hydrostatic Pressure and Pasteurization on Quality of Milk during Storage

High hydrostatic pressure (HHP, 600 MPa/15 min), pasteurization (72 °C/15 s) and pasteurization-HHP (72 °C/15 s + 600 MPa/15 min) processing of milk were comparatively evaluated by examining their effects on microorganisms and quality during 30 days of storage at 4 °C. The counts of total aerobic bacteria in HHP-treated milk were less than 2.22 lgCFU/mL during storage, while they exceeded 5.00 lgCFU/mL in other treated milk. Although HHP changed the color, it had more advantages in maintaining the nutrient (fat, calcium and β-lactoglobulin) properties of milk during storage. Moreover, the viscosity and particle size of HHP-treated milk were more similar to the untreated milk during storage. However, consumer habits towards heat-treated milk have led to poor acceptance of HHP-treated milk, resulting in a low sensory score. In sum, compared with pasteurization- and pasteurization-HHP-treated milk, HHP-treated milk showed longer shelf life and better nutritional quality, but lower sensory acceptance.

Structure and adsorption behavior of high hydrostatic pressure-treated β-lactoglobulin

HYPOTHESIS

High hydrostatic pressure treatment causes structural changes in interfacial-active β-lactoglobulin (β-lg). We hypothesized that the pressure-induced structural changes affect the intra- and intermolecular interactions which determine the interfacial activity of β-lg. The conducted experimental and numerical investigations could contribute to the mechanistic understanding of the adsorption behavior of proteins in food-related emulsions.

EXPERIMENTS

We treated β-lg in water at pH 7 with high hydrostatic pressures up to 600 MPa for 10 min at 20 °C. The secondary structure was characterized with Fourier-transform infrared spectroscopy (FTIR) and circular dichroism (CD), the surface hydrophobicity and charge with fluorescence-spectroscopy and ζ-potential, and the quaternary structure with membrane-osmometry, analytical ultracentrifugation (AUC) and mass spectrometry (MS). Experimental analyses were supported through molecular dynamic (MD) simulations. The adsorption behavior was investigated with pendant drop analysis.

FINDINGS

MD simulation revealed a pressure-induced molten globule state of β-lg, confirmed by an unfolding of β-sheets with FTIR, a stabilization of α-helices with CD and loss in tertiary structure induced by an increase in surface hydrophobicity. Membrane-osmometry, AUC and MS indicated the formation of non-covalently linked dimers that migrated slower through the water phase, adsorbed more quickly due to hydrophobic interactions with the oil, and lowered the interfacial tension more strongly than reference β-lg.

Inactivation kinetics, structural, and morphological modification of mango soluble acid invertase by high pressure processing combined with mild temperatures

The activity, structure and morphology of mango soluble acid invertase (SAI) were investigated after high pressure processing (HPP) combined with mild temperature at 50-600MPa and 40-50°C. The activity of mango SAI was efficiently reduced by HPP at 50MPa/45 and 50°C, or 600MPa/40, 45 and 50°C, while it was increased by 10-30% after HPP at 50-200MPa/40°C. Significant antagonistic effect of pressure and temperature on the activity of SAI was observed at 50-400MPa/50°C. The secondary structure of SAI was not influenced by HPP. However, its tertiary structure was modified by HPP, and severer modification occurred with higher pressure, higher temperature, and longer treatment time. Results of atomic force microscope suggested that HPP at 400MPa/50°C for 2.5min induced dissociation of SAI, and HPP at 600MPa/50°C for 30min resulted aggregation of SAI.

Effect of high hydrostatic pressure on the enzymatic hydrolysis of bovine serum albumin

BACKGROUND

The extent of enzymatic proteolysis mainly depends on accessibility of the peptide bonds, which stabilize the protein structure. The high hydrostatic pressure (HHP) process is able to induce, at certain operating conditions, protein displacement, thus suggesting that this technology can be used to modify protein resistance to the enzymatic attack. This work aims at investigating the mechanism of enzymatic hydrolysis assisted by HHP performed under different processing conditions (pressure level, treatment time). Bovine serum albumin was selected for the experiments, solubilized in sodium phosphate buffer (25 mg mL^-1 , pH 7.5) with α-chymotrypsin or trypsin (E/S ratio = 1/10) and HPP treatment (100-500 MPa, 15-25 min).

RESULTS

HHP treatment enhanced the extent of the hydrolysis reaction of globular proteins, being more effective than conventional hydrolysis. At HHP treatment conditions maximizing the protein unfolding, the hydrolysis degree of proteins was increased as a consequence of the increased exposure of peptide bonds to the attack of proteolytic enzymes. The maximum hydrolysis degree (10% and 7% respectively for the samples hydrolyzed with α-chymotrypsin and trypsin) was observed for the samples processed at 400 MPa for 25 min. At pressure levels higher than 400 MPa the formation of aggregates was likely to occur; thus the degree of hydrolysis decreased.

CONCLUSION

Protein unfolding represents the key factor controlling the efficiency of HHP-assisted hydrolysis treatments. The peptide produced under high pressure showed lower dimensions and a different structure with respect to those of the hydrolysates obtained when the hydrolysis was carried out at atmospheric pressure, thus opening new frontiers of application in food science and nutrition. © 2016 Society of Chemical Industry.

Review : High-pressure, microbial inactivation and food preservation

High pressure (1 to 10 kbars, i.e. 100-1000 MPa) affects biological constituents and systems. Several physicochemical properties of water are modified, such as the density, the ionic dissociation (and pH), and the melting point of ice. Pressure-induced unfolding, aggregation, and gelation of food proteins mainly depend on the effects of pressure on various noncovalent bonds and interactions. Enzyme inactivation (e.g., of ATPases) also results from similar effects, but some enzymes, including oxidative enzymes from fruits and vegetables, are strongly baroresistant. Chemical reactions, macromolecular transconformations, changes in membrane structure, or changes in crystal form and melting point that are accompanied by a decrease in volume are enhanced under pressure (and vice versa). Several of these phenomena, still poorly identified, are involved in the high inactivation ratio (5–6 logarithmic cycles) of most vegetative microbial cells: gram-negative bacteria, yeasts, complex viruses, molds, and gram-positive bacteria, in this decreasing order of sensitivity to pressure. Much variability is noted in the baroresistance of microorganisms, even within one single species or genus. Other parameters influence this resistance: pressure level, holding time (a two-phase kinetics of inactivation is often observed that prevents the calculation of decimal reduction times), temperature of pressure processing (temperatures above 50°C or between –30 and +5°C enhancing inactivation), composition of the medium or of the food (the pH having apparently little influence, but high salt or sugar concentrations, and low water contents, exerting very strong baroprotective effects).

Taking into account the baroprotective effects of some food constituents and the strong resistance of some microbial strains, recent research aims at combined processes in which high pressure is associated with moderate temperature, CO2, other bacteriostatic agents, or to nonthermal physical processes such as ultrasounds, alternative currents, high-voltage electric pulses, and so forth. The safety and refrigerated shelf life of pressurized foods could be maintained or extended, while the sensorial quality should improve due to the reduced severity of thermal processing. Further research is, however, needed for the regulatory authorities to assess and accept these novel foods and processes.

High-Pressure Inactivation of Enzymes: A Review on Its Recent Applications on Fruit Purees and Juices

In the last 2 decades high-pressure processing (HPP) has established itself as one of the most suitable nonthermal technologies applied to fruit products for the extension of shelf-life. Several oxidative and pectic enzymes are responsible for deterioration in color, flavor, and texture in fruit purees and juices (FP&J). The effect of HPP on the activities of polyphenoloxidase, peroxidase, β-glucosidase, pectinmethylesterase, polygalacturonase, lipoxygenase, amylase, and hydroperoxide lyase specific to FP&J have been studied by several researchers. In most of the cases, partial inactivation of the target enzymes was possible under the experimental domain, although their pressure sensitivity largely depended on the origin and their microenvironmental condition. The variable sensitivity of different enzymes also reflects on their kinetics. Several empirical models have been established to describe the kinetics of an enzyme specific to a FP&J. The scientific literature in the last decade illustrating the effects of HPP on enzymes in FP&J, enzymatic action on those products, mechanism of enzyme inactivation during high pressure, their inactivation kinetics, and several intrinsic and extrinsic factors influencing the efficacy of HPP is critically reviewed in this article. In addition, process optimization of HPP targeting specific enzymes is of great interest from an industrial approach. This review will give a fair idea about the target enzymes specific to FP&J and the optimum conditions needed to achieve sufficient inactivation during HPP treatment.

Combined effects of high-pressure and enzymatic treatments on the hydrolysis of chickpea protein isolates and antioxidant activity of the hydrolysates

A chickpea protein isolate (CPI) was pretreated before hydrolysis under a pressure that varied between 100 and 600MPa. The hydrolysis rate increased significantly with pressure above 300MPa. At 40min, the DH of the control was 15.3%, while the DH of the CPI treated at 300MPa was 18.5%, which reached 23.74% post treatment at 400MPa. The pretreatment of CPI above 300MPa enhanced the superoxide anion capturing rate of enzymatic hydrolysis. Pretreatment at 400MPa significantly reduced the hydrolysis time with the release of antioxidant peptides. While hydrolysis by Alcalase during treatment at high pressure (100-300MPa) significantly increased the degree of hydrolysis (DH), its maximum value peaked after hydrolysis at 200MPa for 30min. In addition, hydrolysates obtained at high pressure (100-300MPa) had a higher superoxide anion capturing rate. High-pressure treatment at 200MPa for 20min resulted in products with high antioxidative activity. The molecular-weight (MW) determination of the enzymatic hydrolysates indicated that hydrolysis at high pressure could significantly increase the amount of low-molecular-weight peptides.

Effect of pulsed-light treatment on milk proteins and lipids

Pulsed-light treatment offers the food industry a new technology for food preservation. It allows the inactivation of numerous micro-organisms including most infectious foodborne pathogens. In addition to microbial destruction, one can also question whether pulsed-light treatment induced conformational changes in food components. To investigate this question, the influence of pulsed-light treatment on protein components of milk was evaluated by using UV spectroscopy, spectrofluorometry, electrophoresis, and determination of amino acid composition. Pulsed-light treatment resulted in an increase of UV absorbance at 280 nm. The intrinsic tryptophan fluorescence of beta-lactoglobulin (BLG) showed a 7 nm red shift after 10 pulses. SDS-PAGE showed the formation of dimers after treatment of BLG by 5 pulses and more. No significant changes in the amino acid composition of proteins and lipid oxidation were observed after pulsed-light treatment. The obtained results indicated changes in the polarity of the tryptophanyl residue microenvironment of BLG solutions or changes in the tryptophan indole structure and some aggregation of studied proteins. Hence, pulsed-light treatment did not lead to very significant changes in protein components; consequently, it could be applied to process protein foods for their better preservation.

Functional Improvement of Milk Whey Proteins Induced by High Hydrostatic Pressure

High pressure is emerging as a new processing technology that produces particular changes in the molecular structure of proteins and thus gives rise to new properties inaccessible via conventional methods of protein modification. This review deals with the main effects of high hydrostatic pressure on the physicochemical characteristics of milk whey proteins and how modifications in their structural properties contribute to functionality. In this paper the mechanism underlying pressure-induced changes in ss-lactoglobulin, a-lactabumin, and bovine serum albumin is explained, and related to functional properties such as gel-forming ability, emulsifying activity, or foaming capacity. The possibility of using high pressures to favor chemical reactions of proteins with other food components, such as carbohydrates, to produce novel molecules with new food uses is also considered.

Protein conformation determines the sensibility to high pressure treatment of infectious scrapie prions

Application of high pressure can be used for gentle pasteurizing of food, minimizing undesirable alterations such as vitamin losses and changes in taste and color. In addition, pressure has become a useful tool for investigating structural changes in proteins. Treatments of proteins with high pressure can reveal conformations that are not obtainable by other physical variables like temperature, since pressure favors structural transitions accompanied with smaller volumes. Here, we discuss both the potential use of high pressure to inactivate infectious TSE material and the application of this thermodynamic parameter for the investigation of prion folding. This review summarizes our findings on the effects of pressure on the structure of native infectious scrapie prions in hamster brain homogenates and on the structure of infectious prion rods isolated from diseased hamsters brains. Native prions were found to be pressure sensitive, whereas isolated prions revealed an extreme pressure-resistant structure. The discussion will be focused on the different pressure behavior of these prion isoforms, which points out differences in the protein structure that have not been taken into consideration before.

Pressure-induced unfolding and aggregation of the proteins in whey protein concentrate solutions

Whey protein concentrate solutions (12% w/v, pH 6.65 +/- 0.05) were pressure treated at 800 MPa for 20-120 min and then examined using size exclusion chromatography (SEC), small deformation rheology, transmission electron microscopy, and various types of one-dimensional (1D) and two-dimensional (2D) polyacrylamide gel electrophoresis (PAGE). The pressure-treated samples showed a time-dependent loss of native whey proteins by SEC and 1D PAGE and a corresponding increase in non-native proteins and protein aggregates of different sizes. These aggregates altered the viscosity and opacity of the samples and were shown to be cross-linked by intermolecular disulfide bonds and by noncovalent interactions using 1D PAGE [alkaline (or native), sodium dodecyl sulfate (SDS), and SDS of reduced samples (SDS(R))] and 2D PAGE (native:SDS and SDS:SDS(R)). The sensitivity of the major whey proteins to pressure was in the order beta-lactoglobulin B (beta-LG B) > beta-LG A > bovine serum albumin (BSA) > alpha-lactalbumin (alpha-LA), and the large internal hydrophobic cavity of beta-LG may have been partially responsible for its sensitivity to high-pressure treatments. It seemed likely that, at 800 MPa, the formation of a beta-LG disulfide-bonded network preceded the formation of disulfide bonds between alpha-LA or BSA and beta-LG to form multiprotein aggregates, possibly because the disulfide bonds of alpha-LA and BSA are less exposed than those of beta-LG either during or after pressure treatment. It may be possible that intermolecular disulfide bond formation occurred at high pressure and that hydrophobic association became important after the high-pressure treatment.

Influence of binding of sodium dodecyl sulfate, all-trans-retinol, and 8-anilino-1-naphthalenesulfonate on the high-pressure-induced unfolding and aggregation of beta-lactoglobulin B

Bovine beta-lactoglobulin B (beta-LG) is susceptible to pressure treatment, which unfolds it, allowing thiol-catalyzed disulfide bond interchange to occur, facilitating intermolecular bonding (both noncovalent and disulfide). In the present study, beta-LG was mixed with sodium dodecyl sulfate (SDS), all-trans-retinol (retinol), or 8-anilino-1-naphthalenesulfonate (ANS) on a 1:1.1 molar basis, and aliquots were held at pressures between 50 and 800 MPa for 30 min at pH 7.2 and 20 degrees C. Polyacrylamide gel electrophoresis (PAGE) showed that beta-LG alone (control) was converted into a non-native monomer and a series of dimers, trimers, etc., at pressures beyond 100 MPa; SDS inhibited the formation of non-native species up to 200 MPa, and neither retinol nor ANS inhibited the formation of the non-native species as effectively as SDS. At pressures beyond 350 MPa, SDS ceased to have any inhibitory effect, but both ANS and retinol showed significant inhibition. The near- and far-UV CD patterns and the ANS fluorescent data were consistent with the PAGE data, but the retinol fluorescent data did not show sufficient change to interpret. The results suggested that there were three discernible structural stages. In Stage I (0.1-150 MPa), the native structure is stable; in Stage II (200-450 MPa), the native monomer is reversibly interchanging with non-native monomers and disulfide-bonded dimers; and in Stage III (>500 MPa), the free CysH in non-native monomer and dimer interacts with -S-S- bonds to produce high molecular weight aggregates of beta-LG. SDS inhibited the Stage I to Stage II transition at 200 MPa, and ANS and retinol inhibited the Stage II to Stage III transition at 600 MPa.

Some physico-chemical parameters that influence proteinase K resistance and the infectivity of PrP Sc after high pressure treatment

Crude brain homogenates of terminally diseased hamsters infected with the 263 K strain of scrapie (PrP Sc) were heated and/or pressurized at 800 MPa at 60 degrees C for different times (a few seconds or 5, 30, 120 min) in phosphate-buffered saline (PBS) of different pH and concentration. Prion proteins were analyzed on immunoblots for their proteinase K (PK) resistance, and in hamster bioassays for their infectivity. Samples pressurized under initially neutral conditions and containing native PrP Sc were negative on immunoblots after PK treatment, and a 6-7 log reduction of infectious units per gram was found when the samples were pressurized in PBS of pH 7.4 for 2 h. A pressure-induced change in the protein conformation of native PrP Sc may lead to less PK resistant and less infectious prions. However, opposite results were obtained after pressurizing native infectious prions at slightly acidic pH and in PBS of higher concentration. In this case an extensive fraction of native PrP Sc remained PK resistant after pressure treatment, indicating a protective effect possibly due to induced aggregation of prion proteins in such buffers.

Dual Nature of the Infectious Prion Protein Revealed by High Pressure

Crude brain homogenates of terminally diseased hamsters infected with the 263K strain of scrapie (PrP(Sc)) and purified prion fibrils were heated or pressurized at 800 megapascals and 60 degrees C for 2 h in different buffers and in water. Prion proteins (PrP) were analyzed for their proteinase K resistance in immunoblots and for their infectivity in hamster bioassays. A notable decrease in the proteinase K resistance of unpurified prion proteins, probably because of pressure-induced changes in the protein conformation of native PrP(Sc) or the N-truncated PrP-(27-30), could be demonstrated when pressurized at initially neutral conditions in several buffers and in water but not in a slightly acidic pH. A subsequent 6-7 log(10) reduction of infectious units/g in phosphate-buffered saline buffer, pH 7.4, was found. The proteinase K-resistant core was also not detectable after purification of prions extracted from pressurized samples, confirming pressure effects at the level of the secondary structure of prion proteins. However, opposite results were found after pressurizing purified prions, arguing for the existence of pressure-sensitive beta-structures (PrP(Sc)(DeltaPsen)) and extremely pressure-resistant beta-structures (PrP(Sc)(DeltaPres)). Remarkably, after the first centrifugation step at 540,000 x g during isolation, prions remained proteinase K-resistant when pressurized in all tested buffers and in water. It is known that purified fibrils retain infectivity, but the isolated protein (full and N-truncated) behaved differently from native PrP(Sc) under pressure, suggesting a kind of semicrystalline polymer structure.

Exploring the temperature-pressure configurational landscape of biomolecules: from lipid membranes to proteins

Hydrostatic pressure has been used as a physical parameter for studying the stability and energetics of biomolecular systems, such as lipid mesophases and proteins, but also because high pressure is an important feature of certain natural membrane environments and because the high-pressure phase behaviour of biomolecules is of biotechnological interest. By using spectroscopic and scattering techniques, the temperature- and pressure-dependent structure and phase behaviour of lipid systems, differing in chain configuration, headgroup structure and concentration, and proteins have been studied and are discussed. A thermodynamic approach is presented for studying the stability of proteins as a function of both temperature and pressure. The results demonstrate that combined temperature-pressure dependent studies can help delineate the free-energy landscape of proteins and hence help elucidate which features and thermodynamic parameters are essential in determining the stability of the native conformational state of proteins. We also introduce pressure as a kinetic variable. Applying the pressure jump relaxation technique in combination with time-resolved synchrotron X-ray diffraction and spectroscopic techniques, the kinetics of un/refolding of proteins has been studied. Finally, recent advances in using pressure for studying misfolding and aggregation of proteins will be discussed.

Isolation and Characterization of Copolymers of β-Lactoglobulin, α-Lactalbumin, κ-Casein, and αs1-Casein Generated by Pressurization and Thermal Treatment of Raw Milk

Raw skim milk was submitted to high pressure (300 to 600 MPa) and temperature (4 to 70 degrees C) treatments for 2 or 5 min. The combined effects of pressure and temperature on milk proteins induced structural changes and polymer and copolymer formation characterized by anion-exchange and size-exclusion fast protein liquid chromatography and electrophoretic techniques. Approximately half of the beta-lactoglobulin formed polymers, and the other half formed large copolymers, mainly with kappa-casein, alpha-lactalbumin via intermolecular disulfide bond exchange, and alpha(s1)-casein via physicochemical interactions, in proportions of 1.0:0.7:0.3:0.1, respectively. Minor whey proteins (serum albumin, immunoglobulins, and lactoferrin) also participated in the formation of the copolymers but to a lesser extent. Two populations of the copolymers were found with apparent molecular masses ranging from 440 to 2000 kDa for the first and more than 2000 kDa for the second. On the contrary, for heated milks the aggregation kinetics obtained by combination of high pressure and thermal treatment were very fast, as no intermediates such as dimers and small size oligomers were observed after pressurization, whatever the temperature studied. Lactosylation of proteins as well as proteolysis were very limited. A beta-casein amino-terminal peptide of 22 kDa was specifically recovered in milk samples treated under the more drastic conditions (500 MPa/55 degrees C per 5 min and 600 MPa/70 degrees C per 5 min) and might have been generated by neutral proteases such as elastase released from somatic cells present in milk. No casein was released from the micelle whatever the combination of high pressure and temperature studied.

Physicochemical modifications of high-pressure-treated soybean protein isolates

Changes induced by high pressure (HP) treatment (200-600 MPa) on soybean protein isolates (SPI) at pH 3 (SPI3) and pH 8 (SPI8) were analyzed. Changes in protein solubility, surface hydrophobicity (Ho), and free sulfhydryl content (SH(F)) were determined. Protein aggregation and denaturation and changes in secondary structure were also studied. An increase in protein Ho and aggregation, a reduction of free SH, and a partial unfolding of 7S and 11S fractions were observed in HP-treated SPI8. Changes in secondary structure were also detected, which led to a more disordered structure. HP-treated SPI3 was partially denatured and presented insoluble aggregates. A major molecular unfolding, a decrease of thermal stability, and an increase of protein solubility and Ho were also detected. At 400 and 600 MPa, a decrease of the SH(F) and a total denaturation were observed.

High-pressure-induced rheological changes of low-methoxyl pectin plus micellar casein mixtures

The influence of high-pressure treatment (HPT) (200-800 MPa, 5 or 20 min, at 20 degrees C) on the rheological properties of solutions of amidated low-methoxyl pectin (LMP) and its mixtures with micellar casein (MC) has been investigated in the presence and absence of sucrose. The storage modulus G' of LMP gels containing 0-55 wt % sucrose and 0.1-1 wt % LMP was found to increase significantly following HPT at >or=400 MPa. Various concentrations of LMP in the presence of different amounts of MC (0.5-12 wt %) showed contrasting types of rheological behavior. In the presence of a low concentration of LMP (<0.3 wt %), HPT was found to induce a sol-gel transformation at relatively high LMP/MC molar ratios (<4 wt % MC), to reduce values of G' and the loss modulus G' ' at intermediate LMP/MC ratios (4-10 wt % MC), and to increase the values of G' and G' ' at low LMP/MC ratios (>10 wt % MC). In contrast, in the presence of a higher amount of LMP (>0.5 wt %), it was observed that HPT enhances the values of both the storage and the loss moduli over the whole range of MC concentrations.

High hydrostatic pressure as a tool to study protein aggregation and amyloidosis

Aggregation of proteins is a serious problem, affecting both industrial production of proteins and human health. Despite recent advances in the theories and experimental techniques available to address understanding of protein aggregation processes, mechanisms of aggregate formation have proved challenging to study. This is in part because the typical irreversibility of protein aggregation processes at atmospheric conditions complicates analysis of their kinetics and thermodynamics. Because high hydrostatic pressures act to disfavor the hydrophobic and electrostatic interactions that cause protein aggregation, studies conducted under high hydrostatic pressures may allow protein aggregates to be formed reversibly, enabling thermodynamic and kinetic parameters to be measured in greater detail. Although application of high hydrostatic pressures to protein aggregation problems is rather recent, a growing literature, reviewed herein, suggests that high pressure may be a useful tool for both understanding protein aggregation and reversing it in industrial applications.

Beta-lactoglobulin molten globule induced by high pressure

Beta-lactoglobulin (beta-LG) was treated with high hydrostatic pressure (HHP) at 600 MPa and 50 degrees C for selected times as long as 64 min. The intrinsic tryptophan fluorescence of beta-LG indicated that HHP treatment conditions induced a conformational change. HHP treatment conditions also promote a 3-fold increase in the extrinsic fluorescence of 1-anilinonaphthalene-8-sulfonate and a 2.6-fold decrease for cis-paraneric acid, suggesting an increase in accessible aromatic hydrophobicity and a decrease in aliphatic hydrophobicity. Far-ultraviolet circular dichroism (CD) spectra reveal that the secondary structure of beta-LG converts from native beta-sheets to non-native alpha-helices following HHP treatment, whereas near-ultraviolet CD spectra reveal that the native tertiary structure of beta-LG essentially disappears. Urea titrations reveal that native beta-LG unfolds cooperatively, but the pressure-treated molecule unfolds noncooperatively. The noncooperative state is stable for 3 months at 5 degrees C. The nonaccessible free thiol group of cysteine121 in native beta-LG became reactive to Ellman's reagent after adequate HHP treatment. Gel electrophoresis with and without beta-mercaptoethanol provided evidence that the exposed thiol group was lost concomitant with the formation of S-S-linked beta-LG dimers. Overall, these results suggest that HHP treatments induce beta-LG into hydrophobic molten globule structures that remain stable for at least 3 months.

A (1)H-NMR study on the effect of high pressures on beta-lactoglobulin

1H NMR was used to study the effect of high pressure on changes in the structure of beta-lactoglobulin (beta-Lg), particularly the strongly bonded regions, the "core". beta-Lg was exposed to pressures ranging from 100 to 400 MPa at neutral pH. After depressurization and acidification to pH 2.0, (1)H NMR spectra were taken. Pressure-induced unfolding was studied by deuterium exchange. Refolding was also evaluated. Our results showed that the core was unaltered at 100 MPa but increased its conformational flexibility at >/=200 MPa. Even though the core was highly flexible at 400 MPa, its structure was found to be identical to the native structure after equilibration back to atmospheric pressure. It is suggested that pressure-induced aggregates are formed by beta-Lg molecules maintaining most of their structure, and the intermolecular -SS- bonds, formed by -SH/-SS- exchange reaction, are likely to involve C(66)-C(160) rather than C(106)-C(119). In addition, the beta-Lg variants A and B could be distinguished in a (1)H NMR spectrum from a solution made with the AB mixed variant, by the differences in chemical shifts of M(107) and C(106); structural implications are discussed. Under pressure, the core of beta-Lg A seemed to unfold faster than that of beta-LgB. The structural recovery of the core was full for both variants.

Pressure-induced conformational changes of beta-lactoglobulin by variable-pressure Fourier transform infrared spectroscopy

Pressure-induced conformational changes in D(2)O solutions of the two genetic variants of beta-lactoglobulin A (beta-lg A) and beta-lactoglobulin B (beta-lg B) and an equal mixture of both variants (beta-lg A+B) were studied by employing variable-pressure Fourier transform infrared (VP-FTIR) spectroscopy. Changes in the secondary structure of beta-lg A were observed at lower pressure compared to beta-lg B, indicating that beta-lg A had a more flexible structure. During the decompression cycle beta-lg A showed protein aggregation, accompanied by an increase in alpha-helical conformation. The changes in the secondary structure of beta-lg B with the pressure were minor and for the most part reversible. Upon decompression no aggregation in beta-lg B was observed. Increasing the pressure from 0.01 to 12.0 kbar of a solution containing beta-lg A+B resulted in substantial broadening of all major amide I bands. This effect was partially reversed by decreasing the hydrostatic pressure. beta-lg A+B underwent less aggregate formation than beta-lg A, possibly as a result of protein-protein interactions between beta-lg A and beta-lg B. Hence, it is likely that the functional or biological attributes of beta-lg proteins may be affected in different ways by hydrostatic pressure.