Heat Treatment of Bovine α-Lactalbumin Results in Partially Folded, Disulfide Bond Shuffled States with Enhanced Surface Activity

Digestion-Resistant Linear Epitopes as Dominant Contributors to Strong Allergenicity of Tropomyosin in Antarctic Krill (Euphausia superba)

Although tropomyosin has been identified as a major allergen in Antarctic krill, the digestive fate of Antarctic krill tropomyosin and its relationship with allergenicity are unknown. In this study, Antarctic krill tropomyosin was administered to BALB/c mice via both gavage and intraperitoneal injection to explore its sensitizing and eliciting capacity, and its digestion products were analyzed for structural changes and digestion-resistant linear epitopes. Mice gavaged with tropomyosin exhibited lower levels of specific IgE and IgG1, mast cell degranulation, vascular permeability, and anaphylaxis symptoms than those in the intraperitoneal injection group. This may be due to the destruction of macromolecular aggregates, loose expansion of the tertiary structure, complete disappearance of α-helix, and significant changes in molecular force upon the digestion of tropomyosin. Nevertheless, the intragastric administration of Antarctic krill tropomyosin still triggered strong allergic reactions, which was attributed to the existence of seven digestion-resistant linear epitopes (Glu26-His44, Thr111-Arg125, Glu157-Glu164, Glu177-Gly186, Val209-Ile225, Arg244-Arg255, and Val261-Ile270).

Correlation between interfacial layer properties and physical stability of food emulsions: current trends, challenges, strategies, and further perspectives

Emulsions are thermodynamically unstable systems that tend to separate into two immiscible phases over time. The interfacial layer formed by the emulsifiers adsorbed at the oil-water interface plays an important role in the emulsion stability. The interfacial layer properties of emulsion droplets have been considered the cutting-in points that influence emulsion stability, a traditional motif of physical chemistry and colloid chemistry of particular significance in relation to the food science and technology sector. Although many attempts have shown that high interfacial viscoelasticity may contribute to long-term emulsion stability, a universal relationship for all cases between the interfacial layer features at the microscopic scale and the bulk physical stability of the emulsion at the macroscopic scale remains to be established. Not only that, but integrating the cognition from different scales of emulsions and establishing a unified single model to fill the gap in awareness between scales also remain challenging. In this review, we present a comprehensive overview of recent progress in the general science of emulsion stability with a peculiar focus on interfacial layer characteristics in relation to the formation and stabilization of food emulsions, where the natural origin and edible safety of emulsifiers and stabilizers are highly requested. This review begins with a general overview of the construction and destruction of interfacial layers in emulsions to highlight the most important physicochemical characteristics of interfacial layers (formation kinetics, surface load, interactions among adsorbed emulsifiers, thickness and structure, and shear and dilatational rheology), and their roles in controlling emulsion stability. Subsequently, the structural effects of a series of typically dietary emulsifiers (small-molecule surfactants,proteins, polysaccharides, protein-polysaccharide complexes, and particles) on oil-water interfaces in food emulsions are emphasized. Finally, the main protocols developed for modifying the structural characteristics of adsorbed emulsifiers at multiple scales and improving the stability of emulsions are highlighted. Overall, this paper aims to comprehensively study the literature findings in the past decade and find out the commonality of multi-scale structures of emulsifiers, so as to deeply understand the common characteristics and emulsification stability behaviour of adsorption emulsifiers with different interfacial layer structures. It is difficult to say that there has been significant progress in the underlying principles and technologies in the general science of emulsion stability over the last decade or two. However, the correlation between interfacial layer properties and physical stability of food emulsions promotes revealing the role of interfacial rheological properties in emulsion stability, providing guidance on controlling the bulk properties by tuning the interfacial layer functionality.

Simulated in vitro digestion of α-lactalbumin modified by phosphorylation: Detection of digestive products and allergenicity

The effects of phosphorylation on the allergenicity of bovine α-lactalbumin (BLA) and digestive products were studied in vitro digestion. Two components with different molecular weight and conformation were obtained from natural and phosphorylated BLA. In vivo and in vitro assessment of allergenicity showed that phosphorylation prior to digestion significantly decreased the IgE/IgG binding capacity and allergic response in KU812 cells, and reduced the levels of IgG, IgE, IL-4 and histamine, with an increase in IFN-γ levels in mouse serum, depending on the changes in BLA structures, producing numerous small peptides. There were four phosphorylated sites (S22, T29, S47 and S70) in the high molecular weight components of phosphorylated BLA after digestion. These phosphorylated sites could mask the linear epitopes of digestive products, resulting in reduced allergic activity. Phosphorylation prior to digestion of dairy products can reduce the risk of anaphylaxis in patients with milk allergy to some extent.

Oral retention of thermally denatured whey protein: In vivo measurement and structural observations by CD and NMR

This study investigated structural changes and the in vivo retention in the oral cavity of heated whey protein concentrate (WPC). Heated WPC was shown to have both a higher retention time in the oral cavity compared to unheated whey protein up to 1 min post swallow, and a concomitant increase in free thiol concentration. Nuclear magnetic resonance and circular dichroism demonstrated structural changes in the secondary and tertiary structures of the WPC upon heating. Structural loss of the β-barrel was shown to increase during heating, leading to the exposure of hydrophobic regions. The increase in free thiols and hydrophobic regions are two factors which are known to increase mucoadhesive strength and hence increase oral retention of heated whey protein which may subsequently increase the perception of mouthdrying.

Immunological Analysis of Isothiocyanate-Modified α-Lactalbumin Using High-Performance Thin Layer Chromatography

Undirected modifications between food proteins and secondary plant metabolites can occur during food processing. The results of covalent interactions can alter the functional and biological properties of the proteins. The present work studied the extent of which covalent conjugation of the bioactive metabolite benzyl isothiocyanate (BITC; a glucosinolate breakdown product) to the whey protein α-lactalbumin affects the protein’s allergenicity. Additional to the immunological analysis of native untreated and BITC-modified α-lactalbumin, the analysis of antigenic properties of proteolytically digested protein derivatives was also performed by high performance thin layer chromatography and immunostaining. As a result of the chemical modifications, structural changes in the protein molecule affected the allergenic properties. In this process, epitopes are destroyed or inactivated, but at the same time, buried epitopes can be exposed or newly formed, so that the net effect was an increase in allergenicity, in this case. Results from the tryptic hydrolysis suggest that BITC conjugation sterically hindered the cleavage sites for the enzyme, resulting in reduced digestibility and allergenicity. Residual antigenicity can be still present as short peptide fragments that provide epitopes. The desire to make food safer for allergy sufferers and to protect sensitized individuals from an allergenic reaction makes it clear that the detection of food antigens is mandatory; especially by considering protein interactions.

Effect of Oxidation and Protein Unfolding on Cross-Linking of β-Lactoglobulin and α-Lactalbumin

Oxidation and heat treatment can initiate changes in the amino acid composition, structure, solubility, hydrophobicity, conformation, function, and susceptibility to proteolysis of proteins. These can result in adverse consequences for mammals, plants, foodstuffs, and pharmaceuticals. This study investigated whether and how individual or combined treatment with heat, a commonly encountered factor in industrial processing, and H₂O₂ alters the structure and composition of two major milk whey proteins, α-lactalbumin and β-lactoglobulin, and mixtures of these. Thermal treatment induced reducible cross-links in isolated β-lactoglobulin, but not isolated α-lactalbumin under the conditions employed. Cross-linking occurred at lower temperatures and to a greater extent in the presence of low concentrations of H₂O₂. H₂O₂ did not induce cross-linking in the absence of heat. Mixtures of α-lactalbumin and β-lactoglobulin showed similar behavior, except that mixed α-lactalbumin-β-lactoglobulin dimers were detected. Cross-linking was associated with formation of sulfenic acids (RS-OH species), oxidation of methionine residues, cleavage of disulfide bonds in α-lactalbumin, altered conformation of disulfide bonds in β-lactoglobulin, alterations in the fluorescence intensity and maximum emission wavelength of endogenous tryptophan residues, and binding of the hydrophobic probe 8-anilinonaphthalenesulfonate. These data are consistent with increased unfolding and subsequent aggregation of the protein, with these changes being maximized in the presence of both heat and H₂O₂. The enhanced aggregation detected with H₂O₂ is consistent with additional pathways to aggregation above that induced by heat alone. These mechanistic insights provide potential strategies for modulating the extent and nature of protein modification induced by thermal and oxidant treatment.

Evaluation of irreversible protein thermal inactivation caused by breakage of disulphide bonds using methanethiosulphonate

Many extracellular globular proteins have evolved to possess disulphide bonds in their native conformations, which aids in thermodynamic stabilisation. However, disulphide bond breakage by heating leads to irreversible protein denaturation through disulphide-thiol exchange reactions. In this study, we demonstrate that methanethiosulphonate (MTS) specifically suppresses the heat-induced disulphide-thiol exchange reaction, thus improving the heat-resistance of proteins. In the presence of MTS, small globular proteins that contain disulphides can spontaneously refold from heat-denatured states, maintaining wild-type disulphide pairing. Because the disulphide-thiol exchange reaction is triggered by the generation of catalytic amounts of perthiol or thiol, rapid and specific perthiol/thiol protection by MTS reagents prevents irreversible denaturation. Combining MTS reagents with another additive that suppresses chemical modifications, glycinamide, further enhanced protein stabilisation. In the presence of these additives, reliable remnant activities were observed even after autoclaving. However, immunoglobulin G and biotin-binding protein, which are both composed of tetrameric quaternary structures, failed to refold from heat-denatured states, presumably due to chaperon requirements. Elucidation of the chemical modifications involved in irreversible thermoinactivation is useful for the development of preservation buffers with optimum constitutions for specific proteins. In addition, the impact of disulphide bond breakage on the thermoinactivation of proteins can be evaluated using MTS reagents.

Impact of the environmental conditions and substrate pre-treatment on whey protein hydrolysis: A review

Proteins in solution are subject to myriad forces stemming from interactions with each other as well as with the solvent media. The role of the environmental conditions, namely pH, temperature, ionic strength remains under-estimated yet it impacts protein conformations and consequently its interaction with, and susceptibility to, the enzyme. Enzymes, being proteins are also amenable to the environmental conditions because they are either activated or denatured depending on the choice of the conditions. Furthermore, enzyme specificity is restricted to a narrow regime of optimal conditions while opportunities outside the optimum conditions remain untapped. In addition, the composition of protein substrate (whether mixed or single purified) have been underestimated in previous studies. In addition, protein pre-treatment methods like heat denaturation prior to hydrolysis is a complex phenomenon whose progression is influenced by the environmental conditions including the presence or absence of sugars like lactose, ionic strength, purity of the protein, and the molecular structure of the mixed proteins particularly presence of free thiol groups. In this review, we revisit protein hydrolysis with a focus on the impact of the hydrolysis environment and show that preference of peptide bonds and/or one protein over another during hydrolysis is driven by the environmental conditions. Likewise, heat-denaturing is a process which is dependent on not only the environment but the presence or absence of other proteins.

Conformations of Myoglobin-Derived Peptides at the Air-Water Interface

The conformational change exhibited by proteins at liquid interfaces, such as the air-water and oil-water interfaces, has long been of interest both for understanding protein structure outside of native environments and for applications in areas including food technology and pharmaceuticals. Using molecular simulation, this article studies the conformations of two peptides derived from myoglobin, for which the emulsification behavior has been studied. Both peptides were found to readily adsorb onto the air-water interface, with one of these (experimentally, the more effective stabilizer) adopting a flat, extended conformation and the other peptide remaining close to its solution conformation.

Development of a real-time quantitative PCR assay using a TaqMan minor groove binder probe for the detection of α-lactalbumin in food

Here, we report the development of a real-time PCR assay using a TaqMan minor groove binder (MGB, Genecore, NCBI: AF249896.1, 806-820) probe and primer sets designed to recognize the α-lactalbumin gene from the cow (Bos taurus). We evaluated the efficacy of this assay for detecting and quantifying cow α-lactalbumin in commercial foods. Our results demonstrated that the developed method was highly sensitive and showed high specificity for cow milk, with consistent detection of 0.05 ng of bovine DNA. We tested 42 commercial food samples with or without cow milk listed as an ingredient by using the developed assay. Among the 42 samples, 26 products that listed milk as an ingredient and 3 products might contain milk showed positive signals, whereas the other 9 products that did not contain milk and 4 products that might contain milk tested negative. Therefore, this method could be widely used for the detection of cow milk allergens in food.

Adsorption and dilatational rheology of heat-treated soy protein at the oil-water interface: relationship to structural properties

We evaluated the influence of heat treatment on interfacial properties (adsorption at the oil-water interface and dilatational rheology of interfacial layers) of soy protein isolate. The related structural properties of protein affecting these interfacial behaviors, including protein unfolding and aggregation, surface hydrophobicity, and the state of sulfhydryl group, were also investigated. The structural and interfacial properties of soy protein depended strongly on heating temperature (90 and 120 °C). Heat treatment at 90 °C induced an increase in surface hydrophobicity due to partial unfolding of protein, accompanied by the formation of aggregates linked by disulfide bond, and lower surface pressure at long-term adsorption and similar dynamic interfacial rheology were observed as compared to native protein. Contrastingly, heat treatment at 120 °C led to a higher surface activity of the protein and rapid development of intermolecular interactions in the adsorbed layer, as evidenced by a faster increase of surface pressure and dilatational modulus. The interfacial behaviors of this heated protein may be mainly associated with more flexible conformation and high free sulfhydryl group, even if some exposed hydrophobic groups are involved in the formation of aggregates. These results would be useful to better understand the structure dependence of protein interfacial behaviors and to expand utilization of heat-treated protein in the formulation and production of emulsions.

Conformational changes of α-lactalbumin adsorbed at oil-water interfaces: interplay between protein structure and emulsion stability

The conformation and structural dimensions of α-lactalbumin (α-La) both in solution and adsorbed at oil-water interfaces of emulsions were investigated using synchrotron radiation circular dichroism (SRCD) spectroscopy, front-face tryptophan fluorescence (FFTF) spectroscopy, and dual polarization interferometry (DPI). The near-UV SRCD and the FFTF results demonstrated that the hydrophobic environment of the aromatic residues located in the hydrophobic core of native α-La was significantly altered upon adsorption, indicating the unfolding of the hydrophobic core of α-La upon adsorption. The far-UV SRCD results showed that adsorption of α-La at oil-water interfaces created a new non-native secondary structure that was more stable to thermally induced conformational changes. Specifically, the α-helical conformation increased from 29.9% in solution to 45.8% at the tricaprylin-water interface and to 58.5% at the hexadecane-water interface. However, the β-sheet structure decreased from 18.0% in solution to less than 10% at both oil-water interfaces. The DPI study showed that adsorption of α-La to a hydrophobic C18-water surface caused a change in the dimensions of α-La from the native globule-like shape (2.5-3.7 nm) to a compact/dense layer approximately 1.1 nm thick. Analysis of the colloidal stability of α-La stabilized emulsions showed that these emulsions were physically stable against droplet flocculation at elevated temperatures both in the absence and in the presence of 120 mM NaCl. In the absence of salt, the thermal stability of emulsions was due to the strong electrostatic repulsion provided by the adsorbed α-La layer, which was formed after the adsorption and structural rearrangement. In the presence of salt, although the electrostatic repulsion was reduced via electrostatic screening, heating did not induce strong and permanent droplet flocculation. The thermal stability of α-La stabilized emulsions in the presence of salt is a combined effect of the electrostatic repulsion and the lack of covalent disulfide interchange reactions. This study reports new information on the secondary and tertiary structural changes of α-La upon adsorption to oil-water interfaces. It also presents new results on the physical stability of α-La stabilized emulsions during heating and at moderate ionic strength (120 mM NaCl). The results broaden our understanding of the factors controlling protein structural change at emulsion interfaces and how this affects emulsion stability.

Characterization of an alternative low energy fold for bovine α-lactalbumin formed by disulfide bond shuffling

Bovine α-lactalbumin (αLA) forms a misfolded disulfide bond shuffled isomer, X-αLA. This X-αLA isomer contains two native disulfide bridges (Cys 6-Cys 120 and Cys 28-Cys 111) and two non-native disulfide bridges (Cys 61-Cys 73 and Cys 77-Cys 91). MD simulations have been used to characterize the X-αLA isomer and its formation via disulfide bond shuffling and to compare it with the native fold of αLA. In the simulations of the X-αLA isomer the structure of the α-domain of native αLA is largely retained in agreement with experimental data. However, there are significant rearrangements in the β-domain, including the loss of the native β-sheet and calcium binding site. Interestingly, the energies of X-αLA and native αLA in simulations in the absence of calcium are closely similar. Thus, the X-αLA isomer represents a different low energy fold for the protein. Calcium binding to native αLA is shown to help preserve the structure of the β-domain of the protein limiting possibilities for disulfide bond shuffling. Hence, binding calcium plays an important role in both maintaining the native structure of αLA and providing a mechanism for distinguishing between folded and misfolded species.

Rapid fingerprinting of milk thermal processing history by intact protein mass spectrometry with nondenaturing chromatography

Thermal processing of foods results in proteins undergoing conformational changes, aggregation, and chemical modification notably with sugars via the Maillard reaction. This can impact their functional, nutritional, and allergenic properties. Native size-exclusion chromatography with online electrospray mass spectrometry (SEC-ESI-MS) was used to characterize processing-induced changes in milk proteins in a range of milk products. Milk products could be readily grouped into either pasteurized liquid milks, heavily processed milks, or milk powders by SEC behavior, particularly by aggregation of whey proteins by thermal processing. Maillard modification of all major milk proteins by lactose was observed by MS and was primarily present in milk powders. The method developed is a rapid tool for fingerprinting the processing history of milk and has potential as a quality control method for food ingredient manufacture. The method described here can profile milk protein oligomeric state, aggregation, and Maillard modification in a single shot, rapid analysis.

Photophysics, photochemistry and energetics of UV light induced disulphide bridge disruption in apo-α-lactalbumin

Continuous 295 nm excitation of whey protein bovine apo-α-lactalbumin (apo-bLA) results in an increase of tryptophan fluorescence emission intensity, in a progressive red-shift of tryptophan fluorescence emission, and breakage of disulphide bridges (SS), yielding free thiol groups. The increase in fluorescence emission intensity upon continuous UV-excitation is correlated with the increase in concentration of free thiol groups in apo-bLA. UV-excitation and consequent SS breakage induce conformational changes on apo-bLA molecules, which after prolonged illumination display molten globule spectral features. The rate of tryptophan fluorescence emission intensity increase at 340 nm with excitation time increases with temperature in the interval 9.3-29.9°C. The temperature-dependent 340 nm emission kinetic traces were fitted by a 1st order reaction model. Native apo-bLA molecules with intact SS bonds and low tryptophan emission intensity are gradually converted upon excitation into apo-bLA molecules with disrupted SS, molten-globule-like conformation, high tryptophan emission intensity and red-shifted tryptophan emission. Experimental Ahrrenius activation energy was 21.8 ± 2.3 kJ x mol(-1). Data suggests that tryptophan photoionization from the S(1) state is the likely pathway leading to photolysis of SS in apo-bLA. Photoionization mechanism(s) of tryptophan in proteins and in solution and the activation energy of tryptophan photoionization from S(1) leading to SS disruption in proteins are discussed. The observations present in this paper raise concern regarding UV-light pasteurization of milk products. Though UV-light pasteurization is a faster and cheaper method than traditional thermal denaturation, it may also lead to loss of structure and functionality of milk proteins.

Impact of food processing on the structural and allergenic properties of food allergens

This article reviews recent studies that address one of the major unanswered questions in food allergy research: what attributes of food or food proteins contribute to or enhance food allergenicity?

Glycation and phosphorylation of alpha-lactalbumin by dry heating: effect on protein structure and physiological functions

Alpha-lactalbumin (alpha-LA) was glycated with maltopentaose (MP) through the Maillard reaction (MP-alpha-LA) and subsequently phosphorylated by dry heating in the presence of pyrophosphate to investigate its structure and physiological functions. Glycation occurred effectively, and the sugar content of alpha-LA increased by approximately 22.3% through the Maillard reaction. The phosphorylation of MP-alpha-LA was enhanced with an increase in the dry-heating time from 1 to 5 d, and the phosphorous content of MP-alpha-LA increased by approximately 1.01% by dry heating at pH 4.0 and 85 degrees C for 5 d in the presence of pyrophosphate. The electrophoretic mobility of alpha-LA increased with an increase in the phosphorylation level. The circular dichroism spectra showed that the change in the secondary structure of the alpha-LA molecule by glycation and subsequent phosphorylation was slight. However, the Trp fluorescence intensity was increased by phosphorylation after glycation. In addition, the differential scanning calorimetry thermograms of alpha-LA showed that the denaturation temperature of MP-alpha-LA was decreased by phosphorylation. These results indicated that molten (partially unfolded) conformations of alpha-LA were formed by dry heating in the presence of pyrophosphate after glycation. The anti-alpha-LA antibody response was significantly reduced by glycation and subsequent phosphorylation. The suppressive effect of alpha-LA on the production of proinflammatory cytokines such as IL-6 and tumor necrosis factor-alpha from THP-1 cells after stimulation with lipopolysaccharide was significantly enhanced by glycation with MP and was further enhanced by phosphorylation after glycation. The Ca phosphate-solubilizing ability of alpha-LA was enhanced by phosphorylation. The apoptotic activity of alpha-LA was reduced by glycation and subsequent phosphorylation. These results suggest that phosphorylation by dry heating in the presence of pyrophosphate after glycation with MP through the Maillard reaction is a useful method for improvement of the physiological functions of alpha-LA.

(15)N NMR relaxation data reveal significant chemical exchange broadening in the alpha-domain of human alpha-lactalbumin

Human alpha-lactalbumin (alpha-LA), a 123-residue calcium-binding protein, has been studied using (15)N NMR relaxation methods in order to characterize backbone dynamics of the native state at the level of individual residues. Relaxation data were collected at three magnetic field strengths and analyzed using the model-free formalism of Lipari and Szabo. The order parameters derived from this analysis are generally high, indicating a rigid backbone. A total of 46 residues required an exchange contribution to T(2); 43 of these residues are located in the alpha-domain of the protein. The largest exchange contributions are observed in the A-, B-, D-, and C-terminal 3(10)-helices of the alpha-domain; these residues have been shown previously to form a highly stable core in the alpha-LA molten globule. The observed exchange broadening, along with previous hydrogen/deuterium amide exchange data, suggests that this part of the alpha-domain may undergo a local structural transition between the well-ordered native structure and a less-ordered molten-globule-like structure.

Post-translational modification of barley LTP1b: The lipid adduct lies in the hydrophobic cavity and alters the protein dynamics

NMR techniques have been used to characterise the effects of a lipid-like post-translational modification on barley lipid transfer protein (LTP1b). NMR chemical shift data indicate that the lipid-like molecule lies in the hydrophobic cavity of LTP1b, with Tyr 79 being displaced to accommodate the ligand in the cavity. The modified protein has a reduced level of backbone amide hydrogen exchange protection, presumably reflecting increased dynamics in the protein. This may result from a loosening of the protein structure and may explain the enhanced surface properties observed for LTP1b.