Effects of pH and salt environment on the association of beta-lactoglobulin revealed by intrinsic fluorescence studies

Bovine β-lactoglobulin/fatty acid complexes: binding, structural, and biological properties

Ligand-binding properties of β-lactoglobulin (β-lg) are well documented, but the subsequent biological functions are still unclear. Focusing on fatty acids/β-lg complexes, the structure-function relationships are reviewed in the light of the structural state of the protein (native versus non-native aggregated proteins). After a brief description of β-lg native structure, the review takes an interest in the binding properties of native β-lg (localization of binding sites, stoichiometry, and affinity) and the way the interaction affects the biological properties of the protein and the ligand. The binding properties of non-native aggregated forms of β-lg that are classically generated during industrial processing are also related. Structural changes modify the stoichiometry and the affinity of β-lg for fatty acids and consequently the biological functions of the complex. Finally, the fatty acid-binding properties of other whey proteins (α-lactalbumin, bovine serum albumin) and some biological properties of the complexes are also addressed. These proteins affect β-lg/fatty acids complex in whey given their competition with β-lg for fatty acids.

pH-Dependent aggregation and disaggregation of native β-lactoglobulin in low salt

The aggregation of β-lactoglobulin (BLG) near its isoelectric point was studied as a function of ionic strength and pH. We compared the behavior of native BLG with those of its two isoforms, BLG-A and BLG-B, and with that of a protein with a very similar pI, bovine serum albumin (BSA). Rates of aggregation were obtained through a highly precise and convenient pH/turbidimetric titration that measures transmittance to ±0.05 %T. A comparison of BLG and BSA suggests that the difference between pHmax (the pH of the maximum aggregation rate) and pI is systematically related to the nature of protein charge asymmetry, as further supported by the effect of localized charge density on the dramatically different aggregation rates of the two BLG isoforms. Kinetic measurements including very short time periods show well-differentiated first and second steps. BLG was analyzed by light scattering under conditions corresponding to maxima in the first and second steps. Dynamic light scattering (DLS) was used to monitor the kinetics, and static light scattering (SLS) was used to evaluate the aggregate structure fractal dimensions at different quench points. The rate of the first step is relatively symmetrical around pHmax and is attributed to the local charges within the negative domain of the free protein. In contrast, the remarkably linear pH dependence of the second step is related to the uniform reduction in global protein charge with increasing pH below pI, accompanied by an attractive force due to surface charge fluctuations.

Effect of protein-surfactant interactions on aggregation of β-lactoglobulin

The milk protein β-lactoglobulin (βLG) dominates the properties of whey aggregates in food products. Here we use spectroscopic and calorimetric techniques to elucidate how anionic, cationic and non-ionic surfactants interact with bovine βLG and modulate its heat-induced aggregation. Alkyl trimethyl ammonium chlorides (xTAC) strongly promote aggregation, while sodium alkyl sulfates (SxS) and alkyl maltopyranosides (xM) reduce aggregation. Sodium dodecyl sulfate (SDS) binds to non-aggregated βLG in several steps, but reduction of aggregation was associated with the first binding step, which occurs far below the critical micelle concentration. In contrast, micellar concentrations of xMs are required to reduce aggregation. The ranking order for reduction of aggregation (normalized to their tendency to self-associate) was C10-C12>C8>C14 for SxS and C8>C10>C12>C14>C16 for xM. xTAC promote aggregation in the same ranking order as xM reduce it. We conclude that SxS reduce aggregation by stabilizing the protein's ligand-bound state (the melting temperature t(m) increases by up to 10°C) and altering its charge potential. xM monomers also stabilize the protein's ligand-bound state (increasing t(m) up to 6°C) but in the absence of charged head groups this is not sufficient by itself to prevent aggregation. Although micelles of both anionic and non-ionic surfactants destabilize βLG, they also solubilize unfolded protein monomers, leaving them unavailable for protein-protein association and thus inhibiting aggregation. Cationic surfactants promote aggregation by a combination of destabilization and charge neutralization. The food compatible surfactant sodium dodecanoate also inhibited aggregation well below the cmc, suggesting that surfactants may be a practical way to modulate whey protein properties.

Effect of disulfide interactions and hydrolysis on the thermal aggregation of β-lactoglobulin

The roles of sulfhydryl/disulfide interactions and acid/pepsin hydrolysis on β-lactoglobulin (β-lg) thermal aggregation at acidic pH 3.35 and 2 were studied using rheology, sodium dodecyl sulfate-polyacrylamide gel electrophoresis (SDS-PAGE), transmission electron microscopy (TEM), and Western blotting. Pepsin promoted additional hydrolysis compared to the acid-hydrolyzed control sample based on a 12% increase in free amino groups. Hydrolysis with pepsin also resulted in an increase in the apparent viscosity by 2 logs upon heating 8% β-lg solutions at pH 3.35. Seemingly, hydrolysis promoted thermal aggregation of β-lg, correlating well with viscosity increases. Large microgels were observed in heated pepsin hydrolysates using TEM, supporting the increased viscosities of these dispersions. During thermal aggregation (85 °C, 3 h) of β-lg at pH 3.35, beyond the existence of limited disulfide interactions, acid hydrolysis and noncovalent interactions more likely play a crucial role in defining the functionality of acidified powdered modified whey ingredients.

Changes in solvent exposure reveal the kinetics and equilibria of adsorbed protein unfolding in hydrophobic interaction chromatography

Hydrogen exchange has been a useful technique for studying the conformational state of proteins, both in bulk solution and at interfaces, for several decades. Here, we propose a physically based model of simultaneous protein adsorption, unfolding and hydrogen exchange in HIC. An accompanying experimental protocol, utilizing mass spectrometry to quantify deuterium labeling, enables the determination of both the equilibrium partitioning between conformational states and pseudo-first order rate constants for folding and unfolding of adsorbed protein. Unlike chromatographic techniques, which rely on the interpretation of bulk phase behavior, this methodology utilizes the measurement of a molecular property (solvent exposure) and provides insight into the nature of the unfolded conformation in the adsorbed phase. Three model proteins of varying conformational stability, alpha-chymotrypsinogen A, beta-lactoglobulin B, and holo alpha-lactalbumin, are studied on Sepharose HIC resins possessing assorted ligand chemistries and densities. alpha-Chymotrypsinogen, conformationally the most stable protein in the set, exhibits no change in solvent exposure at all the conditions studied, even when isocratic pulse-response chromatography suggests nearly irreversible adsorption. Apparent unfolding energies of adsorbed beta-lactoglobulin B and holo alpha-lactalbumin range from -4 to 3 kJ/mol and are dependent on resin properties and salt concentration. Characteristic pseudo-first order rate constants for surface-induced unfolding are 0.2-0.9 min(-1). While poor protein recovery in HIC is often associated with irreversible unfolding, this study documents that non-eluting behavior can occur when surface unfolding is reversible or does not occur at all. Further, this hydrogen exchange technique can be used to assess the conformation of adsorbed protein under conditions where the protein is non-eluting and chromatographic methods are not applicable.

Binding properties of the N-acetylglucosamine and high-mannose N-glycan PP2-A1 phloem lectin in Arabidopsis

Phloem Protein2 (PP2) is a component of the phloem protein bodies found in sieve elements. We describe here the lectin properties of the Arabidopsis (Arabidopsis thaliana) PP2-A1. Using a recombinant protein produced in Escherichia coli, we demonstrated binding to N-acetylglucosamine oligomers. Glycan array screening showed that PP2-A1 also bound to high-mannose N-glycans and 9-acyl-N-acetylneuraminic sialic acid. Fluorescence spectroscopy-based titration experiments revealed that PP2-A1 had two classes of binding site for N,N',N''-triacetylchitotriose, a low-affinity site and a high-affinity site, promoting the formation of protein dimers. A search for structural similarities revealed that PP2-A1 aligned with the Cbm4 and Cbm22-2 carbohydrate-binding modules, leading to the prediction of a beta-strand structure for its conserved domain. We investigated whether PP2-A1 interacted with phloem sap glycoproteins by first characterizing abundant Arabidopsis phloem sap proteins by liquid chromatography-tandem mass spectrometry. Then we demonstrated that PP2-A1 bound to several phloem sap proteins and that this binding was not completely abolished by glycosidase treatment. As many plant lectins have insecticidal activity, we also assessed the effect of PP2-A1 on weight gain and survival in aphids. Unlike other mannose-binding lectins, when added to an artificial diet, recombinant PP2-A1 had no insecticidal properties against Acyrthosiphon pisum and Myzus persicae. However, at mid-range concentrations, the protein affected weight gain in insect nymphs. These results indicate the presence in PP2-A1 of several carbohydrate-binding sites, with potentially different functions in the trafficking of endogenous proteins or in interactions with phloem-feeding insects.

Inhaled Anesthetics Promote Albumin Dimerization through Reciprocal Exchange of Subdomains

Inhaled anesthetics affect protein-protein interaction, but the mechanisms underlying these effects are still poorly understood. We examined the impact of sevoflurane and isoflurane on the dimerization of human serum albumin (HSA), a protein with anesthetic binding sites that are well characterized. Intrinsic fluorescence emission was analyzed for spectral shifting and self-quenching, and control first derivatives (spectral responses to changes in HSA concentration) were compared against those obtained from samples treated with sevoflurane or isoflurane. Sevoflurane increased dimer-dependent self-quenching and both decreased oligomer-dependent spectral shifting, suggesting that inhaled anesthetics promoted HSA dimerization. Size exclusion chromatography and polarization data were consistent with these observations. The data support the proposed model of a reciprocal exchange of subdomains to form an HSA dimer. The open-ended exchange of subdomains, which we propose occuring in HSA oligomers, was inhibited by sevoflurane and isoflurane.

Effects of caseins on thermal stability of bovine beta-lactoglobulin

Casein fractions have been shown to act as molecular chaperones and inhibit aggregation of whey proteins in dilute solutions (< or =1% w/v). We evaluated if this approach would stabilize protein solutions at higher concentration and thermal processing temperatures desired for beverage applications. Mixtures of beta-lactoglobulin (BLG) (6% w/v) with either beta-casein (BCN) (0.01-2% w/v) or alpha s-casein (ACN) (2% w/v) were adjusted to pH 6.0 and heated (70-90 degrees C) for 20 min, cooled, and then analyzed to determine the degree of aggregation. Aggregation was determined by solution turbidity as optical density (OD) at 400 or 600 nm. The addition of 0.05% (w/v) BCN or greater caused a drop in turbidity for solutions heated at 70-90 degrees C. In contrast, inhibition was observed in BLG-ACN mixtures at 70 degrees C but not at > or =75 degrees C. Moreover, prolonged heating (90 min) of BLG with 2% (w/v) BCN (pH 6.0) at 90 degrees C produced a clear solution while BLG-ACN solutions formed translucent gels after heating for 15 min. The weight-averaged molar mass and root-mean-square (rms) radius of soluble aggregates were determined by size exclusion chromatography in conjunction with multiangle laser light scattering (SEC-MALS). SEC-MALS confirmed the turbidity results by showing that the BLG-BCN mixture (8% w/v protein) produced aggregates with lower molar mass and smaller rms radius (majority 20-40 nm). These results showed that BCN is a feasible component to stabilize higher concentrations of whey proteins in beverages.

Structural characterization of bovine beta-lactoglobulin-galactose/tagatose Maillard complexes by electrophoretic, chromatographic, and spectroscopic methods

To investigate the influence of the type of carbonyl group of the sugar on the structural changes of proteins during glycation, an exhaustive structural characterization of glycated beta-lactoglobulin with galactose (aldose) and tagatose (ketose) has been carried out. Conjugates were prepared via Maillard reaction at 40 and 50 degrees C, pH 7, and a w = 0.44. The progress of the Maillard reaction was followed by indirect formation of Amadori and Heyns compounds, advanced glycation end products, and brown polymers. The structural characterization of glycoconjugates was conducted by using a number of analytical techniques such as RP-HPLC, isoelectric focusing, MALDI-ToF, SDS-PAGE, size exclusion chromatography, and spectrofluorimetry (tryptophan fluorescence). In addition, the surface hydrophobicity of the beta-lactoglobulin glycoconjugates was also assessed. The results showed a higher reactivity of galactose than tagatose to form the glycoconjugates, probably due to the higher electrophilicity of the aldehyde group. At 40 degrees C, more aggregation was produced when beta-lactoglobulin was conjugated with tagatose as compared to galactose. However, at 50 degrees C hardly any difference was observed in the aggregation produced by galactose and tagatose. These results afford more insight into the importance of the functional group of the carbohydrate moiety during the formation of protein-carbohydrate conjugates via Maillard reaction.

A novel method for study of the aggregation of protein induced by metal ion aluminum(III) using resonance Rayleigh scattering technique

We present a novel method for the study of the aggregation of protein induced by metal ion aluminum(III) using resonance Rayleigh scattering (RRS) technique. In neutral Tris-HCl medium, the effect of this aggregation of protein results in the enhancement of RRS intensity and the relationship between the enhancement of the RRS signal and the Al concentration is nonlinear. On this basis, we established a new method for the determination of the critical induced-aggregation concentrations (C(CIAC)) of metal ion Al(III) inducing the protein aggregation. Our results show that many factors, such as, pH value, anions, salts, temperature and solvents have obvious effects. We also studied the extent of aggregation and structural changes using ultra-violet spectrometry, protein intrinsic fluorescence and circular dichroism to further understand the exact mechanisms of the aggregation characteristics of proteins induced by metal ion Al(III) at the molecular level, to help us to develop effective methods to investigate the toxicity of metal ion Al, and to provide theoretical and quantitative evidences for the development of appropriate treatments for neurodementia such as Parkinson's disease, Alzheimer's disease and dementia related to dialysis.

Effects of salt concentration on formation and dissociation of beta-lactoglobulin/pectin complexes

The formation and dissociation of beta-lactoglobulin/pectin complexes at various sodium chloride concentrations (CNaCl) have been studied by turbidimetric titration. An increase of CNaCl up to 0.1 M shifts the critical pHphi1, which designates the formation of beta-lactoglobulin/pectin coacervates, to higher pH values, whereas further increase of CNaCl from 0.1 to 0.8 M decreases pHphi1 values. These salt effects can be explained in terms of a salt-enhanced effect at lower salt concentrations or a salt-reduced effect at higher salt concentrations, respectively. On the other hand, the value of pHphi2, which corresponds to the dissociation of beta-lactoglobulin/pectin coacervates, tends to have smaller pH values when CNaCl increases from 0.1 to 0.3 M. No observable pHphi2 values are found at CNaCl higher than 0.3 M. The disappearance of pHphi2 is mainly attributed to the strong self-aggregation capability of beta-lactoglobulin at higher CNaCl. The aggregation of beta-lactoglobulin at high CNaCl is reversible, as suggested by the atomic force microscopy results.

Energetics of protein homodimerization: Effects of water sequestering on the formation of β-lactoglobulin dimer

Transient protein-protein interactions are functionally relevant as a control mechanism in a variety of biological processes. Analysis of the 3D structure of protein-protein complexes indicates that water molecules trapped at the interface are very common; however, their role in the stability and specificity of protein homodimer interactions has been not addressed yet. To provide new insights into the energetic bases that govern the formation of highly hydrated interfaces, the dissociation process of bovine beta lg variant A at a neutral pH was characterized here thermodynamically by conducting dilution experiments with an isothermal titration calorimeter. Association was enthalpically driven throughout the temperature range spanned. DeltaH and deltaC(p) were significantly more negative than estimates based on surface area changes, suggesting the occurrence of effects additional to the dehydration of the contact surfaces between subunits. Near-UV CD spectra proved to be independent of protein concentration, indicating a rigid body-like association. Furthermore, the process proved not to be coupled to significant changes in the protonation state of ionizable groups or counterion exchange. In contrast, both osmotic stress experiments and a computational analysis of the dimer's 3D structure indicated that a large number of water molecules are incorporated into the interface upon association. Numerical estimates considering the contributions of interface area desolvation and water immobilization accounted satisfactorily for the experimental deltaC(p). Thus, our study highlights the importance of explicitly considering the effects of water sequestering to perform a proper quantitative analysis of the formation of homodimers with highly hydrated interfaces.

Unfolding kinetics of beta-lactoglobulin induced by surfactant and denaturant: a stopped-flow/fluorescence study

The beta-->alpha transition of beta-lactoglobulin, a globular protein abundant in the milk of several mammals, is investigated in this work. This transition, induced by the cationic surfactant dodecyltrimethylammonium chloride (DTAC), is accompanied by partial unfolding of the protein. In this work, unfolding of bovine beta-lactoglobulin in DTAC is compared with its unfolding induced by the chemical denaturant guanidine hydrochloride (GnHCl). The final protein states attained in the two media have quite different secondary structure: in DTAC the alpha-helical content increases, leading to the so-called alpha-state; in GnHCl the amount of ordered secondary-structure decreases, resulting in a random coil-rich final state (denatured, or D, state). To obtain information on both mechanistic routes, in DTAC and GnHCl, and to characterize intermediates, the kinetics of unfolding were investigated in the two media. Equilibrium and kinetic data show the partial accumulation of an on-pathway intermediate in each unfolding route: in DTAC, an intermediate (I(1)) with mostly native secondary structure but loose tertiary structure appears between the native (beta) and alpha-states; in GnHCl, another intermediate (I(2)) appears between states beta and D. Kinetic rate constants follow a linear Chevron-plot representation in GnHCl, but show a more complex mechanism in DTAC, which acts like a stronger binding species.

The pH Threshold in the Dissolution of β-Lactoglobulin Gels and Aggregates in Alkali

The existence of a practical minimum pH for the dissolution of heat-induced whey gels in alkaline solutions has been studied using beta-lactoglobulin (betaLg) as a model protein. A sharp transition in solubility was observed between pH 11 and 12; this transition shifts to higher pHs for gels formed at higher temperatures and for longer gelling times. The breakdown reactions of heat-induced aggregates in alkali were monitored with size exclusion chromatography. The destruction of large aggregates was faster at higher pH and also showed a transition between pH 11 and 12. Using tryptophan fluorescence and near- and far-UV circular dichroism, this transition was assigned to the base-induced denaturation observed in solutions of aggregates (pK 11.53). It is suggested that the high protein repulsion caused by the large number of charges at pH > 11.5 drives the unfolding of the protein and the disruption of the intermolecular noncovalent bonds. Concentrated urea and GuHCl were found to be less effective than a pH 12 solution in destroying large aggregates. Aggregates formed for a long time (80 degrees C for 24 h) contained a larger number of intermolecular disulfide bonds that hinder the dissolution process. Gels formed at low temperatures (65 degrees C for 60 min), with fewer intermolecular noncovalent bonds, showed a similar solubility-pH profile to that observed for the base-induced denaturation of unheated beta-lactoglobulin (betaLg) (pK 10.63).

A Tryptophan Rotamer Located in a Polar Environment Probes pH-Dependent Conformational Changes in Bovine β-Lactoglobulin A

Bovine beta-lactoglobulin A (BLGA) is a well characterized globular protein whose tertiary structure has been investigated in detail. BLGA undergoes a pH-dependent conformational change which X-ray data described as involving mostly the loop connecting strands E and F and the deprotonation of a glutamic acid residue (Glu89). These structural changes have been investigated using, among other techniques, fluorescence spectroscopy. The intrinsic fluorescence of BLGA is dominated by two Trp residues. These residues are located far from the EF loop and would not be expected to probe the pH-induced conformational change of the protein. Trp19 is located at the bottom of the interior beta-barrel, whereas Trp61 is located at the aperture of the barrel near the CD loop and is "silent" in the emission of native BLGA because of the proximity of a disulfide moiety. Our study suggests that, surprisingly, the fluorescence of Trp19 has the characteristic of a more polar environment than structural models from X-ray data would suggest and that at least two distinct conformations (or rotamers) of Trp19 contribute to the fluorescence of the protein. The less populated rotamer (relative amplitude (alpha) approximately 20%, tau approximately 3 ns) probes a more polar environment and a pH-dependent conformational change of BLGA in the region of Trp19 which X-ray data do not detect. Finally, our study provides the estimate of the fluorescence lifetime of Trp61 in the "unquenched" form.

Composition and Rheological Properties of β-Lactoglobulin/Pectin Coacervates: Effects of Salt Concentration and Initial Protein/Polysaccharide Ratio

The composition and rheological properties of beta-lactoglobulin/pectin coacervates have shown significant correlations with sodium chloride concentration (C(NaCl)) and initial protein/polysaccharide ratio (r). An increase of C(NaCl) from 0.01 to 0.21 M at r = 5:1 leads to the increase in both beta-lactoglobulin and pectin contents in the coacervates, which can be explained in terms of salt-enhanced effect at lower salt concentrations. Further increase of C(NaCl) from 0.21 to 0.41 M decreases the proportions of these two biopolymers in the coacervates, exhibiting salt-reduced effect at higher salt concentrations. Moreover, the stronger self-aggregation of beta-lactoglobulin with increasing salt concentration gives rise to a decreasing actual protein/polysaccharide ratio in the coacervates at 0.01-0.21 M C(NaCl) and r = 5:1. An increase of r from 5:1 to 40:1 often increases the actual amount of pectin chains in beta-lactoglobulin/pectin coacervates, but it exhibits a maximum in beta-lactoglobulin content at r = 20:1. A much higher storage modulus (G') than loss modulus (G' ') for all beta-lactoglobulin/pectin coacervates suggests the formation of highly interconnected gel-like structure. The values of G' increase as C(NaCl) increases from 0.01 to 0.21 M, whereas a further increase of C(NaCl) from 0.21 to 0.41 M causes G' values to decrease to much lower values. These results further disclose the salt-enhanced effect and the salt-reduced effect at low and high salt concentrations, respectively. On the other hand, increasing r from 5:1 to 40:1 favors the formation of stronger gel-like beta-lactoglobulin/pectin coacervates, which mainly originates from the higher actual amount of pectin chains in beta-lactoglobulin/pectin coacervates at higher r values.

Fluorescence-based soft-sensor for monitoring β-lactoglobulin and α-lactalbumin solubility during thermal aggregation

A soft-sensor for monitoring solubility of native-like alpha-lactalbumin (alpha-LA) and beta-lactoglobulin (beta-LG) and their aggregation behavior following heat treatment of mixtures under different treatment conditions was developed using fluorescence spectroscopy data regressed with a multivariate Partial Least Squares (PLS) regression algorithm. PLS regression was used to correlate the concentrations of alpha-LA and beta-LG to the fluorescence spectra obtained for their mixtures. Data for the calibration and validation of the soft sensor was derived from fluorescence spectra. The process of thermal induced aggregation of beta-LG and alpha-LA protein in mixtures, which involves the disappearance of native-like proteins, was studied under various treatment conditions including different temperatures, pH, total initial protein concentration and proportions of alpha-LA and beta-LG. It was demonstrated that the multivariate regression models used could effectively deconvolute multi-wavelength fluorescence spectra collected under a variety of process conditions and provide a fairly accurate quantification of respective native-like proteins despite the significant overlapping between their emission profiles. It was also demonstrated that a PLS model can be used as a black-box prediction tool for estimating protein aggregation when combined with simple mass balances.

Microstructure of β-Lactoglobulin/Pectin Coacervates Studied by Small-Angle Neutron Scattering

Small-angle neutron scattering (SANS) has been used to investigate the microstructure of beta-lactoglobulin/pectin coacervates prepared by different initial protein/polysaccharide weight ratio (r), sodium chloride concentration (C(NaCl)), and pectin charge density. The higher r and higher pectin charge density lead to higher scattering intensity at small q range (0.007 Angstrom(-1) < q < 0.02 Angstrom(-1)), suggesting that the charges of pectin chains are screened significantly by the binding of oppositely charged protein molecules, leading to a tighter aggregation of pectin chains. On the other hand, the appearance of a shoulder peak at intermediate q range (0.04 Angstrom(-1) < q < 0.2 Angstrom(-1)) is used to interpret the formation of protein domains in beta-lactoglobulin/pectin coacervates. At C(NaCl) = 0.1 M, the coacervate of beta-lactoglobulin and pectin A does not show a shoulder peak at intermediate q range at r = 10:1, suggesting that protein molecules are separately bound on pectin chains. However, a shoulder peak appears at intermediate q range at r = 20:1 and 30:1, and the average protein domain size estimated from the shoulder peak position is 7.2 and 8.5 nm, respectively, for these two coacervates. When C(NaCl) increases from 0.05 to 0.2 M, the shoulder peak shifts toward smaller q and becomes broader, indicating that the addition of a higher amount of salt leads to a more heterogeneous coacervate structure. Pectin B with a lower linear charge density favors the formation of larger protein domains. The formation of protein domains in beta-lactoglobulin/pectin coacervates is partially ascribed to the self-aggregation of beta-lactoglobulin molecules. Two kinds of microstructures of beta-lactoglobulin/pectin coacervates with and without observable protein domains have been proposed.

Electrostatically driven protein aggregation: beta-lactoglobulin at low ionic strength

The aggregation of beta-lactoglobulin (BLG) at ambient temperature was studied using turbidimetry and dynamic light scattering in the range 3.8<pH<5.2 in 0.0045 M NaCl, and in the ionic strength range 0.0045-0.5 M at fixed pH=5.0. The initial rate of aggregation, taken as the initial slope of turbidity vs time, (dtau/dt)0, indicated maximum aggregation near pH 4.6 (below the isoelectric point of 5.2), but the dependence of the initial rate of aggregation on pH was highly asymmetric. At pH 5.0, (dtau/dt)0 strongly increased with a decrease in ionic strength I from 0.1 to 0.0045 M and was found to be nearly linear with 1/I. DLS measurements revealed an increase in particle size with time, with the appearance of bimodal distributions in which the fast and slow modes corresponded, respectively, to a BLG dimer and to larger aggregates in the 100-800 nm range. At conditions of slower aggregation, DLS revealed the consumption of dimers to form higher order aggregates with no intermediate species. Computer modeling (Delphi) was used to visualize the electrostatic potential around the dimer to elucidate the pH and ionic strength dependence of the initial aggregation rates. The aggregation process appears to comprise an initial fast consumption of the dimer, whose dependence on pH and I arises from the interaction of the positive and negative domains of interacting dimers, followed by the slow formation of much larger aggregates with relatively little sensitivity to pH and I. The open-ended nature of BLG aggregation is thought to arise from the asymmetry of the dimer charge distribution.

Interactions between bovine beta-lactoglobulin A and various bioactive peptides as studied by front-face fluorescence spectroscopy

Front-face fluorescence spectroscopy was used for the first time to study the interactions between bovine beta-lactoglobulin variant A (beta-Lg A) and various beta-Lg-derived bioactive peptides. Fluorescence spectra were recorded for beta-Lg A-peptide mixtures at 25 degrees C and pH 6.8 with an excitation wavelength of 290 nm to characterize the molecular environment of tryptophan (Trp) residues present in the protein but absent in the peptides. Spectra remained unchanged following addition of peptides beta-Lg f92-100 and beta-Lg f125-135, while Phe-Phe interaction between beta-Lg f69-83 molecules interfered with analysis. Addition of beta-Lg f102-105 produced a blue shift (3 nm) and a significant increase in fluorescence intensity, while addition of beta-Lg f142-148 also caused a significant increase in fluorescence intensity but accompanied by a red shift (3 nm). These results indicate that the polarity of the Trp environment in the beta-Lg A structure may be modified differently depending on the peptide added.

Comparison of the conformational stability of the non-native α-helical intermediate of thiol-modified β-lactoglobulin upon interaction with sodium n-alkyl sulfates at two different pH

Bovine beta-lactoglobulin assumes a dimeric native conformation at neutral pH, while the conformation at pH 2 is monomeric but still native. beta-lactoglobulin has a free thiol at Cys121, which is buried between the beta-barrel and the C-terminal major or alpha-helix. This thiol group was specifically reacted with DTNB (5,5'-dithiobis(2-nitrobenzoic acid)) at pH 7.5 and 2, producing a modified beta-lactoglobulin containing a mix disulfide bond with 5-thio-2-nitrobenzoic acid (TNB). beta-Lactoglobulin is a predominantly beta-sheet protein, although it has a markedly high intrinsic preference for alpha-helical structure. The formation of non-native alpha-helical intermediate of thiol modified beta-lactoglobulin (TNB-beta-LG) was induced by n-alkyl sulfates including sodium octyl sulfate, SOS; sodium decyl sulfate, SDeS; sodium dodecyl sulfate, SDS; and sodium tetradecyl sulfate, STS at pH 7.5 and 2. The conformation and stability of non-native alpha-helical intermediate (alphaI) state of TNB-beta-LG were studied by circular dichroism (CD), fluorescence and differential scanning calorimetry (DSC) techniques. The effect of n-alkyl sulfates on the structure of alphaI state at both pH was utilized to investigate the contribution of hydrophobic interactions to the stability of alphaI intermediate. The present results suggest that the folding reaction of beta-LG follows a non-hierarchical mechanism and hydrophobic interactions play important roles in stabilizing the native state of beta-LG at pH 2 with more positive charges repulsion than at pH 7.5. Then TNB-beta-LG will become a useful model to analyze the conformation and stability of the intermediate of protein folding.

Conformational changes of beta-lactoglobulin induced by anionic phospholipid

Conformational changes of beta-lactoglobulin (beta-LG) induced by anionic phospholipid (dimyristoylphosphatidylglycerol, DMPG) at physiological conditions (pH 7.0) have been investigated by UV-VIS, circular dichroism (CD) and fluorescence spectra. The experimental results suggest that beta-LG-DMPG interactions cause beta-LG a structural reorganization of the secondary structure elements accompanied by an increase in alpha-helical content, and a loosening of the protein tertiary structure. The interaction forces between beta-LG and DMPG are further evaluated by fluorescence spectra. The fluorescence spectral data show that conformational changes in the protein are driven by electrostatic interaction at first, then by hydrophobic interaction between a protein with a negative net charge and a negatively charged phospholipid.