The ligand-binding site of bovine beta-lactoglobulin: evidence for a function?

Effects of high pressure processing on structural changes, aggregation, and binding mechanisms of β-Lactoglobulin with typical polyphenols

The binding capacity of β-Lactoglobulin (BLG) is crucial for delivering polyphenols, influenced by structural changes. High pressure processing (HPP) has the potential to modify BLG's structure and aggregation, but its specific impact on BLG-polyphenol interactions is uncertain. This study used circular dichroism spectroscopy and molecular dynamics simulations to reveal HPP-induced structural changes in BLG, supported by particle size analysis indicating aggregation. Seven structurally diverse polyphenols (quercetin-QR, hesperetin-HSP, dihydromyricetin-DHM, gallic acid-GA, (-)-epicatechin-EC, resveratrol-RES, and secoisolariciresinol diglucoside-SDG) were investigated to comprehensively analyze their binding patterns using fluorescence spectroscopy and molecular docking. HPP reduced BLG's ordered structure and increased its aggregation. Binding affinities peaked at 400 MPa for DHM, QR, HSP, GA, and RES, while SDG and EC exhibited maximum affinities at atmospheric pressure and 600 MPa, respectively. Elevated pressures enhanced BLG-polyphenol interactions, particularly at residues 44GLU and 160CYS, with van der Waals forces dominating the binding free energy.

Interfacial Interactions between Nanoplastics and Biological Systems: toward an Atomic and Molecular Understanding of Plastics-Driven Biological Dyshomeostasis

Micro- and nano-plastics (NPs) are found in human milk, blood, tissues, and organs and associate with aberrant health outcomes including inflammation, genotoxicity, developmental disorders, onset of chronic diseases, and autoimmune disorders. Yet, interfacial interactions between plastics and biomolecular systems remain underexplored. Here, we have examined experimentally, in vitro, in vivo, and by computation, the impact of polystyrene (PS) NPs on a host of biomolecular systems and assemblies. Our results reveal that PS NPs essentially abolished the helix-content of the milk protein β-lactoglobulin (BLG) in a dose-dependent manner. Helix loss is corelated with the near stoichiometric formation of β-sheet elements in the protein. Structural alterations in BLG are also likely responsible for the nanoparticle-dependent attrition in binding affinity and weaker on-rate constant of retinol, its physiological ligand (compromising its nutritional role). PS NP-driven helix-to-sheet conversion was also observed in the amyloid-forming trajectory of hen egg-white lysozyme (accelerated fibril formation and reduced helical content in fibrils). Caenorhabditis elegans exposed to PS NPs exhibited a decrease in the fluorescence of green fluorescent protein-tagged dopaminergic neurons and locomotory deficits (akin to the neurotoxin paraquat exposure). Finally, in silico analyses revealed that the most favorable PS/BLG docking score and binding energies corresponded to a pose near the hydrophobic ligand binding pocket (calyx) of the protein where the NP fragment was found to make nonpolar contacts with side-chain residues via the hydrophobic effect and van der Waals forces, compromising side chain/retinol contacts. Binding energetics indicate that PS/BLG interactions destabilize the binding of retinol to the protein and can potentially displace retinol from the calyx region of BLG, thereby impairing its biological function. Collectively, the experimental and high-resolution in silico data provide new insights into the mechanism(s) by which PS NPs corrupt the bimolecular structure and function, induce amyloidosis and onset neuronal injury, and drive aberrant physiological and behavioral outcomes.

Binding of Per- and Polyfluoroalkyl Substances to β-Lactoglobulin from Bovine Milk

Per- and polyfluoroalkyl substances (PFAS) are known for their high environmental persistence and potential toxicity. The presence of PFAS has been reported in many dairy products. However, the mechanisms underlying the accumulation of PFAS in these products remain unclear. Here, we used native mass spectrometry and molecular dynamics simulations to probe the interactions between 19 PFAS of environmental concern and two isoforms of the major bovine whey protein β-lactoglobulin (β-LG). We observed that six of these PFAS bound to both protein isoforms with low- to mid-micromolar dissociation constants. Based on quantitative, competitive binding experiments with endogenous ligands, PFAS can bind orthosterically and preferentially to β-LG's hydrophobic ligand-binding calyx. β-Cyclodextrin can also suppress binding of PFAS to β-LG owing to the ability of β-cyclodextrin to directly sequester PFAS from solution. This research sheds light on PFAS-β-LG binding, suggesting that such interactions could impact lipid-fatty acid transport in bovine mammary glands at high PFAS concentrations. Furthermore, our results highlight the potential use of β-cyclodextrin in mitigating PFAS binding, providing insights toward the development of strategies to reduce PFAS accumulation in dairy products and other biological systems.

An Atomic and Molecular Insight into How PFOA Reduces α-Helicity, Compromises Substrate Binding, and Creates Binding Pockets in a Model Globular Protein

Per- and polyfluoroalkyl substances (PFAS) pose significant health risks due to their widespread presence in various environmental and biological matrices. However, the molecular-level mechanisms underlying the interactions between PFAS and biological constituents, including proteins, carbohydrates, lipids, and DNA, remain poorly understood. Here, we investigate the interactions between a legacy PFAS, viz. perfluorooctanoic acid (PFOA), and the milk protein β-lactoglobulin (BLG) obtained using a combination of experimental and computational techniques. Circular dichroism studies reveal that PFOA perturbs the secondary structure of BLG, by driving a dose-dependent loss of α-helicity and alterations in its β-sheet content. Furthermore, exposure of the protein to PFOA attenuates the on-rate constant for the binding of the hydrophobic probe 8-anilino-1-naphthalene sulfonic acid (ANS), suggesting potential functional impairment of BLG by PFOA. Steered molecular dynamics and umbrella sampling calculations reveal that PFOA binding leads to the formation of an energetically favorable novel binding pocket within the protein, when residues 129-142 are steered to unfold from their initial α-helical structure, wherein a host of intermolecular interactions between PFOA and BLG's residues serve to insert the PFOA into the region between the unfolded helix and beta-sheets. Together, the data provide a novel understanding of the atomic and molecular mechanism(s) by which PFAS modulates structure and function in a globular protein, leading to a beginning of our understanding of altered biological outcomes.

Molecular modeling and DFT studies on the antioxidant activity of Centaurea scoparia flavonoids and molecular dynamics simulation of their interaction with β-lactoglobulin

Plants of the genus Centaurea have been widely used as natural therapeutics in different countries. This study investigated the antioxidant-structure activity relationship of eight flavonoids isolated from Centaurea scoparia using DFT studies and in vitro radical scavenging and xanthine oxidase (XO) inhibition assays, and to correlate the theoretical values with the experimental findings. Docking analysis was carried out to explore the binding modes of the isolated phytochemicals with XO and bovine β-lactoglobulin (BLG). Interactions of the isolated compounds with BLG were studied using molecular dynamics (MD) simulations which revealed the involvement of hydrogen bonding. The root-mean-square deviation (RMSD) of BLG and BLG-flavonoid complexes reached equilibrium and fluctuated during the 10 ns MD simulations. The radius of gyration (Rg) and solvent accessible surface area (SASA) revealed that various systems were stabilized at approximately 2500 ps. In addition, the RMS fluctuations profile indicated that the ligand's active site exerted rigidity behavior during the simulation. The hydrogen atom transfer (HAT) and the energies of hydrogen abstractions were estimated by calculating the bond dissociation enthalpy (BDE) of O-H in gas phase and water. The isolated compounds showed radical scavenging and XO inhibitory activities along with binding affinity with XO as revealed in silico. The BDE was linked to the radical scavenging processes occurring in polar solvents. These processes are single electron transfer followed by proton transfer (SET-PT) and sequential proton loss electron transfer (SPLET). Our calculations indicated the agreement between the calculated results and the experimentally measured antioxidant activity of the flavonoids isolated from C. scoparia.

Characterization of the interaction of metal-protoporphyrins photosensitizers with β- lactoglobulin

We investigated the interaction of a series of metal-protoporphyrins (PPIXs) with bovine β- lactoglobulin (BLG) using a combination of optical spectroscopy and computational simulations. Unlike other studies, the simulations were not merely used to rationalize the experimental data but were employed to refine the experimental data itself. The study was carried out at two pH values, 5 and 9, where BLG is known to have different conformation dictated by the so-called Tanford transition which occurs near pH 7.5. The transition is postulated to regulate access to the interior binding cavity of the protein, thus the pH variation was used as a parameter to investigate whether PPIXs access the central cavity of BLG. The results of our study show that indeed binding increases significantly at alkaline pH, however, the increased affinity is not due to the accessibility of the central cavity. Instead, binding appears to be determined by the tendency of PPIXs to form large inhomogeneous aggregates at acidic pH which hinders interactions with proteins. The binding site determined through a combination of experimental and computational methods is located at the interface between two BLG monomers where the long α-helix segment of the protein face each other. This region is rich in positively charged Lys residues that interact with the propionic acid chains of the protoporphyrins. Establishing the modality of binding between protoporphyrins and BLG would have important consequences for the use of BLG:PPIX complexes in applications such as artificial photoreceptors, artificial metallo-enzymes, delivery of photosensitizers for phototherapy and even solar energy conversion.

Experimental and computational studies on the mechanism of the β-lactoglobulin-derived peptide inhibiting the antigenicity of β-lactoglobulin

In this study, through a combined simulated enzymolysis-molecular docking-molecular simulation-activity determination-action mechanism strategy, we screened a β-LG-derived peptide (VAGTWYSL) to inhibit the antigenicity of β-LG and explored its mechanism of action. Our results indicate that the inhibitory effect of the peptide on the antigenicity of β-LG is affected by different experimental conditions, including pH, reaction time and concentration. Three factors may contribute to the reduced allergenicity of β-LG. First, there must be sufficient forces between the peptide and β-LG, as a result, hydrophobic forces and hydrogen bonds are the main forces to maintain the structural stability of the complex. Second, the binding of the peptide changes the secondary structure of β-LG, especially with an increase in α-helices and a decrease in β-turns. Third, the peptide binds to the hydrophobic region of β-LG, involving the antigenic epitope region Val41-Lys60, which may reduce the antigenicity.

Vitamin A fortification: Recent advances in encapsulation technologies

Vitamin A is an essential micronutrient whose deficiency is still a major health concern in many regions of the world. It plays an essential role in human growth and development, immunity, and vision, but may also help prevent several other chronic diseases. The total amount of vitamin A in the human diet often falls below the recommended dietary allowance of approximately 900-1000 μ $ \umu $ g/day for a healthy adult. Moreover, a significant proportion of vitamin A may be degraded during food processing, storage, and distribution, thereby reducing its bioactivity. Finally, the vitamin A in some foods has a relatively low bioavailability, which further reduces its efficacy. The World Health Organization has recommended fortification of foods and beverages as a safe and cost-effective means of addressing vitamin A deficiency. However, there are several factors that must be overcome before effective fortified foods can be developed, including the low solubility, chemical stability, and bioavailability of this oil-soluble vitamin. Consequently, strategies are required to evenly disperse the vitamin throughout food matrices, to inhibit its chemical degradation, to avoid any adverse interactions with any other food components, to ensure the food is palatable, and to increase its bioavailability. In this review article, we discuss the chemical, physical, and nutritional attributes of vitamin A, its main dietary sources, the factors contributing to its current deficiency, and various strategies to address these deficiencies, including diet diversification, biofortification, and food fortification.

A Double Payload Complex between Hypericin and All-Trans Retinoic Acid in the β-Lactoglobulin Protein

Combined therapies are usually used to treat acne vulgaris since this approach can tackle various foci simultaneously. Using a combination of spectroscopic, computational, and microbiological techniques and methods, herein we report on the use of β-lactoglobulin as a double payload carrier of hypericin (an antimicrobial photodynamic agent) and all-trans retinoic acid (an anti-inflammatory drug) for S. aureus in vitro photodynamic inactivation. The addition of all-trans retinoic acid to hypericin-β-lactoglobulin complex renders a photochemically safe vehicle due to the photophysical quenching of hypericin, which recovers its photodynamic activity when in contact with bacteria. The ability of hypericin to photoinactivate S. aureus was not affected by retinoic acid. β-Lactoglobulin is a novel biocompatible and photochemically safe nanovehicle with strong potential for the treatment of acne.

Beta-Lactoglobulin as a Model Food Protein: How to Promote, Prevent, and Exploit Its Unfolding Processes

Bovine milk beta-lactoglobulin (BLG) is a small whey protein that is a common ingredient in many foods. Many of the properties of BLG relevant to the food industry are related to its unfolding processes induced by physical or chemical treatments. Unfolding occurs through a number of individual steps, generating transient intermediates through reversible and irreversible modifications. The rate of formation of these intermediates and of their further evolution into different structures often dictates the outcome of a given process. This report addresses the main structural features of the BLG unfolding intermediates under conditions that may facilitate or impair their formation in response to chemical or physical denaturing agents. In consideration of the short lifespan of the transient species generated upon unfolding, this review also discusses how various methodological approaches may be adapted in exploring the process-dependent structural modifications of BLG from a kinetic and/or a thermodynamic standpoint. Some of the conceptual and methodological approaches presented and discussed in this review can provide hints for improving the understanding of transient conformers formation by proteins present in other food systems, as well as when other physical or chemical denaturing agents are acting on proteins much different from BLG in complex food systems.

A 200 nanoseconds all-atom simulation of the pH-dependent EF loop transition in bovine β-lactoglobulin. The role of the orientation of the E89 side chain

In silico molecular dynamics (MD) using crystallographic and NMR data was used to simulate the effects of the protonation state of E89 on the pH-dependent conformational rearrangement of the EF loop, also known as the Tanford transition, in a series of apo-β-lactoglobulin (BLG) structures. Compared to existing studies these simulations were carried out over a much longer time scale (200 ns where the stability of the transition can be evaluated) and used an explicit water model. We considered eight different entries from the Brookhaven Protein Data Bank (PDB) separated into two groups. We observed that fixing the protonation state of E89 prompts the transition of the EF loop only when its side chain is oriented under the loop and into the entrance of the interior cavity. The motion of the EF loop occurs mostly as a step-function and its timing varies greatly from ∼ 20 ns to ∼170 ns from the beginning of the simulation. Once the transition is completed, the protein appears to reach a stable conformation as in a true two-state transition. We also observed novel findings. When the transition occurs, the hydrogen bond between E89 and S116 is replaced with a salt bridge with Lys residues in the βC-CD loop-βD motif. This electrostatic interaction causes the distortion of this motif as well as the protrusion of the GH loop into the aperture of the cavity with the result of limiting the increase of its contour area despite the rotation of the EF loop.Communicated by Ramaswamy H. Sarma.

Interaction mechanisms and structure-affinity relationships between hyperoside and soybean β-conglycinin and glycinin

Hyperoside (HYP) is an important natural product that is widely distributed in fruits and whole grasses of various plants. It is also used by consumers as a healthy ingredient. This work explored the interaction mechanisms between HYP and two main soy proteins, namely, β-conglycinin (7S) and glycinin (11S), using computational simulation and multi-spectroscopic technology. In this study, the docking and dynamic simulation showed that HYP was stable in the hydrophobic pockets of the proteins. The conformation and microenvironment of 7S/11S also changed after binding to HYP. The binding of HYP to 7S/11S was a state quenching with a good affinity at 4 °C. This result was determined from the binding constant values of (1.995 ± 0.170) × 107 M-1 and (2.951 ± 0.109) × 107 M-1, respectively. The 7S/11S-HYP complex delineated here will provide a novel idea to construct an embedding and delivery system in improving the benefits of HYP for the development of high value-added food products.

β-Lactoglobulin and Glycodelin: Two Sides of the Same Coin?

The two lipocalins, β-lactoglobulin (βLg) and glycodelin (Gd), are possibly the most closely related members of the large and widely distributed lipocalin family, yet their functions appear to be substantially different. Indeed, the function of β-lactoglobulin, a major component of ruminant milk, is still unclear although neonatal nutrition is clearly important. On the other hand, glycodelin has several specific functions in reproduction conferred through distinct, tissue specific glycosylation of the polypeptide backbone. It is also associated with some cancer outcomes. The glycodelin gene, PAEP, reflecting one of its names, progestagen-associated endometrial protein, is expressed in many though not all primates, but the name has now also been adopted for the β-lactoglobulin gene (HGNC, www.genenames.org). After a general overview of the two proteins in the context of the lipocalin family, this review considers the properties of each in the light of their physiological functional significance, supplementing earlier reviews to include studies from the past decade. While the biological function of glycodelin is reasonably well defined, that of β-lactoglobulin remains elusive.

Binding Sites for Oligosaccharide Repeats from Lactic Acid Bacteria Exopolysaccharides on Bovine β-Lactoglobulin Identified by NMR Spectroscopy

Lactic acid bacterial exopolysaccharides (EPS) are used in the food industry to improve the stability and rheological properties of fermented dairy products. β-Lactoglobulin (BLG), the dominant whey protein in bovine milk, is well known to bind small molecules such as fatty acids, vitamins, and flavors, and to interact with neutral and anionic polysaccharides used in food and pharmaceuticals. While sparse data are available on the affinity of EPS-milk protein interactions, structural information on BLG-EPS complexes, including the EPS binding sites, is completely lacking. Here, binding sites on BLG variant A (BLGA), for oligosaccharides prepared by mild acid hydrolysis of two EPS produced by Streptococcus thermophilus LY03 and Lactobacillus delbrueckii ssp. bulgaricus CNRZ 1187, respectively, are identified by NMR spectroscopy and supplemented by isothermal titration calorimetry (ITC) and molecular docking of complexes. Evidence of two binding sites (site 1 and site 2) on the surface of BLGA is achieved for both oligosaccharides (LY03-OS and 1187-OS) through NMR chemical shift perturbations, revealing multivalency of BLGA for EPS. The affinities of LY03-OS and 1187-OS for BLGA gave K D values in the mM range obtained by both NMR (pH 2.65) and ITC (pH 4.0). Molecular docking suggested that the BLGA and EPS complexes depend on hydrogen bonds and hydrophobic interactions. The findings provide insights into how BLGA engages structurally different EPS-derived oligosaccharides, which may facilitate the design of BLG-EPS complexation, of relevance for formulation of dairy products and improve understanding of BLGA coacervation.

β-Lactoglobulin-gold nanoparticles interface and its interaction with some anticancer drugs - an approach for targeted drug delivery

The protein-nanoparticle interface plays a crucial role in drug binding and stability, in turn enhancing efficacy in targeted drug delivery. In the present study, whey protein β-lactoglobulin (BLG) is conjugated with gold nanoparticles (AuNP) and its interaction with curcumin (CUR) and gemcitabine (GEM) has been explored. Further, AuNP-BLG conjugate interactions with anticancer drugs were characterized using dynamic light scattering (DLS), zeta potential, UV-visible, Raman spectroscopy, fluorescence, circular dichroism along with molecular dynamics simulation. The cytotoxicity studies were performed using breast cancer cell lines (MCF-7). ∼8 µM of BLG resides on AuNP (∼29 nm) surface revealed by DLS. Raman scattering of AuNP-BLG conjugate showed orientation of the central calyx of BLG towards solvent. BLG fluorescence confirmed the interaction between AuNP-BLG conjugate with drugs and indicated strong binding and affinity (for CUR K_D = 3.71 x 10⁸ M ^-1, n = 1.83, and for GEM K_D = 3.78 x 10³ M ^-1, n = 0.94), enhanced in the presence of AuNP. CD and Raman analysis exhibited selective hydrophilic and hydrophobic conformations induced by drug binding. Computational studies on BLG-drug complexes revealed that the residues Pro38, Leu39 and Met107 are largely associated with CUR binding, while GEM interaction is via hydrophilic contacts which significantly matches with spectroscopic investigation. IC₅₀ values were calculated for all components of this loading system on MCF-7. The possible mechanisms of interaction between AuNP-BLG with anticancer drugs has been explored at the molecular level. We believe that these conjugates could be considered in the targeted drug delivery studies for cancer research.Communicated by Ramaswamy H. Sarma.

Effect of Ligands on HP-Induced Unfolding and Oligomerization of β-Lactoglobulin

To probe intermediate states during unfolding and oligomerization of proteins remains a major challenge. High pressure (HP) is a powerful tool for studying these problems, revealing subtle structural changes in proteins not accessible by other means of denaturation. Bovine β-lactoglobulin (BLG), the main whey protein, has a strong propensity to bind various bioactive molecules such as retinol and resveratrol, two ligands with different affinity and binding sites. By combining in situ HP-small-angle neutron scattering (SANS) and HP-ultraviolet/visible absorption spectroscopy, we report the specific effects of these ligands on three-dimensional conformational and local changes in BLG induced by HP. Depending on BLG concentration, two different unfolding mechanisms are observed in situ under pressures up to ∼300 MPa: either a complete protein unfolding, from native dimers to Gaussian chains, or a partial unfolding with oligomerization in tetramers mediated by disulfide bridges. Retinol, which has a high affinity for the BLG hydrophobic cavity, significantly stabilizes BLG both in three-dimensional and local environments by shifting the onset of protein unfolding by ∼100 MPa. Increasing temperature from 30 to 37°C enhances the hydrophobic stabilization effects of retinol. In contrast, resveratrol, which has a low binding affinity for site(s) on the surface of the BLG, does not induce any significant effect on the structural changes of BLG due to pressure. HP treatment back and forth up to ∼300 MPa causes irreversible covalent oligomerization of BLG. Ab initio modeling of SANS shows that the oligomers formed from the BLG-retinol complex are smaller and more elongated compared to BLG without ligand or in the presence of resveratrol. By combining HP-SANS and HP-ultraviolet/visible absorption spectroscopy, our strategy highlights the crucial role of BLG hydrophobic cavity and opens up new possibilities for the structural determination of HP-induced protein folding intermediates and irreversible oligomerization.

Structural mechanism of the Tanford transition of bovine β-lactoglobulin through microsecond molecular dynamics simulations

X-ray analysis has provided structural data about a pH-driven conformational change in β-lactoglobulin (BLG) known as the Tanford transition, which occurs at around pH 7 and involves the EF loop, which acts as a lid closing the internal cavity of the protein below pH 7 and opening it above pH 7. NMR studies using wild-type BLG have encountered problems trying to explain the Tanford transition, however, they have provided important insight using a dimeric BLG mutant, revealing that the opening and closure of the EF loop consists of two types of motions in the microsecond and milliseconds timescales. This provides valuable information indicating that the dimeric state is a good model to study the Tanford transition, although the understanding of this structural change is still lacking at the atomic level. We performed microsecond molecular dynamics (MD) simulations starting from different conformations of BLG in the monomeric and dimeric state, with protonated and deprotonated E89, in order to explore the Tanford transition. Our results provide structural information for the transition from the closed to the open conformation in BLG and show it occurs in the dimeric state in the microsecond timescale, in line with the fast motion observed through NMR experiments. In addition, MD simulations coupled to MMGBSA approach indicated that the most populated conformer of BLG in the open state is able to bind ligands with similar affinity to that of BLG at neutral pH obtained through crystallographic experiments.Communicated by Ramaswamy H. Sarma.

On the utility of fluorescence-detection analytical ultracentrifugation in probing biomolecular interactions in complex solutions: a case study in milk

β-Lactoglobulin is the most abundant protein in the whey fraction of ruminant milks, yet is absent in human milk. It has been studied intensively due to its impact on the processing and allergenic properties of ruminant milk products. However, the physiological function of β-lactoglobulin remains unclear. Using the fluorescence-detection system within the analytical ultracentrifuge, we observed an interaction involving fluorescently labelled β-lactoglobulin in its native environment, i.e. cow and goat milk, for the first time. Co-elution experiments support that these β-lactoglobulin interactions occur naturally in milk and provide evidence that the interacting partners are immunoglobulins, while further sedimentation velocity experiments confirm that an interaction occurs between these molecules. The identification of these interactions, made possible through the use of fluorescence-detected analytical ultracentrifugation, provides possible clues to the long debated physiological function of this abundant milk protein.

Locating the binding sites of citric acid and gallic acid on milk β-lactoglobulin

β-Lactoglobulin (β-LG) is a member of lipocalin superfamily of transporters for small hydrophobic and hydrophilic molecules. We located the binding sites of citric acid and gallic acid on β-lactoglobulin (β-LG) in aqueous solution at physiological conditions, using spectroscopic methods, thermodynamic analysis and molecular modeling. Thermodynamic parameters ΔH⁰ -9.5 to -6.9 (kJ mol^-1), ΔS⁰ 23.9 to 13.6 (J mol^-1K^-1) and ΔG⁰ -14.5 to -13.6 (kJ mol^-1) showed that acid binds protein via ionic contacts with gallic acid forming stronger protein conjugates consistent with theoretical modeling. Different amino acids are involved in gallic acid and citric acid complexation. Protein conformation was altered with reduction of β-sheet from 58% (free protein) to 46-43% and a major increase in α-helix from 11% (free protein) to 29-23% and random coil structure in the acid-protein, indicating a partial protein destabilization. Communicated by Ramaswamy H. Sarma.

Cow's milk protein β-lactoglobulin confers resilience against allergy by targeting complexed iron into immune cells

BACKGROUND

Beta-lactoglobulin (BLG) is a bovine lipocalin in milk with an innate defense function. The circumstances under which BLG is associated with tolerance of or allergy to milk are not understood.

OBJECTIVE

Our aims were to assess the capacity of ligand-free apoBLG versus loaded BLG (holoBLG) to protect mice against allergy by using an iron-quercetin complex as an exemplary ligand and to study the molecular mechanisms of this protection.

METHODS

Binding of iron-quercetin to BLG was modeled and confirmed by spectroscopy and docking calculations. Serum IgE binding to apoBLG and holoBLG in children allergic to milk and children tolerant of milk was assessed. Mice were intranasally treated with apoBLG versus holoBLG and analyzed immunologically after systemic challenge. Aryl hydrocarbon receptor (AhR) activation was evaluated with reporter cells and Cyp1A1 expression. Treated human PBMCs and human mast cells were assessed by fluorescence-activated cell sorting and degranulation, respectively.

RESULTS

Modeling predicted masking of major IgE and T-cell epitopes of BLG by ligand binding. In line with this modeling, IgE binding in children allergic to milk was reduced toward holoBLG, which also impaired degranulation of mast cells. In mice, only treatments with holoBLG prevented allergic sensitization and anaphylaxis, while sustaining regulatory T cells. BLG facilitated quercetin-dependent AhR activation and, downstream of AhR, lung Cyp1A1 expression. HoloBLG shuttled iron into monocytic cells and impaired their antigen presentation.

CONCLUSION

The cargo of holoBLG is decisive in preventing allergy in vivo. BLG without cargo acted as an allergen in vivo and further primed human mast cells for degranulation in an antigen-independent fashion. Our data provide a mechanistic explanation why the same proteins can act either as tolerogens or as allergens.

Effects of Polar Head Nature and Tail Length of Single-Chain Lipids on the Conformational Stability of β-Lactoglobulin

Interaction between β-lactoglobulin and single-chain lipids, differing for either the length of the aliphatic chain or the molecular properties of the headgroup, was investigated at neutral and acidic pH to determine the impact on the thermal stability of the protein. Differential scanning calorimetry results with different fatty acids (from C10:0 to C18:0) show a correlation of both melting temperature and unfolding enthalpy of the protein with the ligand binding affinity, and the maximum effect was found for palmitic acid (PLM). The influence of the lipid polar head was investigated by comparing PLM with lyso-palmitoylphosphatidylcholine (LPC), which possesses the same aliphatic chain. At neutral pH, the stabilizing effect of LPC is less favorable compared to PLM. However, fluorescence results revealed that LPC can bind into the protein calyx even at acidic pH, at variance with fatty acids. Molecular dynamics simulations indicated that this difference is due to the ability of the polar head of LPC to interact with the protein loop that regulates the shift (Tanford transition) between open and closed state of the binding site of β-lactoglobulin. The results provide a rationale for how a ligand has the ability to access the protein active site at acidic conditions by overcoming the Tanford transition, and they demonstrate that β-lactoglobulin can deliver ligands with tailored properties of the polar head in a wide pH range.

Use of acid whey protein concentrate as an ingredient in nonfat cup set-style yogurt

Acid whey resulting from the production of soft cheeses is a disposal problem for the dairy industry. Few uses have been found for acid whey because of its high ash content, low pH, and high organic acid content. The objective of this study was to explore the potential of recovery of whey protein from cottage cheese acid whey for use in yogurt. Cottage cheese acid whey and Cheddar cheese whey were produced from standard cottage cheese and Cheddar cheese-making procedures, respectively. The whey was separated and pasteurized by high temperature, short time pasteurization and stored at 4°C. Food-grade ammonium hydroxide was used to neutralize the acid whey to a pH of 6.4. The whey was heated to 50°C and concentrated using ultrafiltration and diafiltration with 11 polyethersulfone cartridge membrane filters (10,000-kDa cutoff) to 25% total solids and 80% protein. Skim milk was concentrated to 6% total protein. Nonfat, unflavored set-style yogurts (6.0 ± 0.1% protein, 15 ± 1.0% solids) were made from skim milk with added acid whey protein concentrate, skim milk with added sweet whey protein concentrate, or skim milk concentrate. Yogurt mixes were standardized to lactose and fat of 6.50% and 0.10%, respectively. Yogurt was fermented at 43°C to pH 4.6 and stored at 4°C. The experiment was replicated in triplicate. Titratable acidity, pH, whey separation, color, and gel strength were measured weekly in yogurts through 8 wk. Trained panel profiling was conducted on 0, 14, 28, and 56 d. Fat-free yogurts produced with added neutralized fresh liquid acid whey protein concentrate had flavor attributes similar those with added fresh liquid sweet whey protein but had lower gel strength attributes, which translated to differences in trained panel texture attributes and lower consumer liking scores for fat-free yogurt made with added acid whey protein ingredient. Difference in pH was the main contributor to texture differences, as higher pH in acid whey protein yogurts changed gel structure formation and water-holding capacity of the yogurt gel. In a second part of the study, the yogurt mix was reformulated to address texture differences. The reformulated yogurt mix at 2% milkfat and using a lower level of sweet and acid whey ingredient performed at parity with control yogurts in consumer sensory trials. Fresh liquid acid whey protein concentrates from cottage cheese manufacture can be used as a liquid protein ingredient source for manufacture of yogurt in the same factory.

Effect of thermal and microwave processing on secondary structure of bovine β-lactoglobulin: A molecular modeling study

Milk allergy is known to cause severe allergic reactions in hypersensitive patients, especially in infants and children. β-Lactoglobulin is one of the major allergens in bovine milk. The influence of thermal and microwave processing on the structural deviations of β-lactoglobulin protein have been studied using molecular modeling techniques. The structural deviations are studied using root mean square deviations, radius of gyration, dipole moment, and solvent accessible surface area. STRIDE analysis showed significant changes in the β-lactoglobulin, especially when oscillating electric fields were applied along with heat. Root mean square fluctuations (RMSF) has been assessed for known epitopes in the β-lactoglobulin molecule. This showed that when the protein is exposed to certain thermal stress, it compacts by burying hydrophobic residues in the core. However, few allergic epitope residues also exhibit increased RMSF leading to higher reactive sites on the surface of the protein molecule. PRACTICAL APPLICATIONS: This study showed that molecular modeling can be used to gain valuable insights regarding the structural changes during processing. In the future, with more computational capacity, it can be used to make comparison between results obtained from simulations and real-time experiments. The current techniques used in food industries such as Nuclear Magnetic Resonance Imaging, Fourier Transformation Infrared Spectroscopy, X-ray diffraction can analyze pre- and post-processing effects. Hence, it become necessary to understand the changes that takes place during the processing techniques. Molecular dynamic simulation could be a useful technique in analyzing the changes occurring during the processing.

Interactions of flavonoids from yellow onion skins with whey proteins: Mechanisms of binding and microencapsulation with different combinations of polymers

The interaction of flavonoids extracted from yellow onion skins with whey proteins isolate was studied using fluorescence spectroscopy and simulation methods from the perspectives of microencapsulation. The fluorescence spectroscopy revealed a static quenching mechanism and the involvement of van der Waals and H bonding in complexes formation. The in silico methods suggested that the heat treatment of the major whey proteins affected the binding pockets and therefore the affinity for the main flavonoids. The interaction surface decreased and the interaction energy increased, suggesting lower binding strength. Further, the yellow onion skins extract was successfully encapsulated in whey proteins isolate and different combinations of polymers, including chitosan, maltodextrin and pectin by freeze drying. The resulted powder showed a total flavonoid content of 5.84 ± 0.23 mg quercetin equivalents/g DW in whey protein-chitosan combination and 104.97 ± 5.02 mg quercetin equivalents/g DW in whey protein-maltodextrin-pectin combinations, with antioxidant activity of 175.93 ± 1.50 mM mM Trolox/g DW and 269.20 ± 3.59 mM Trolox/g DW, respectively. The confocal microscopy indicated that the flavonoids aggregated inside the matrix formed between the whey proteins and various polymers and irregular and compact clusters. Therefore, a comprehensive approach involving the extraction of flavonoids from underutilized food by-products, such as yellow onion skins, evaluation of binding mechanisms with whey proteins, whereas tailoring their functional benefit through microencapsulation in order to obtain active ingredients are reported.

Tracking of β-Lactoglobulin Binding Compounds with Biofingerprinting Chromatogram Analysis of Natural Products

The addition of milk to tea has been shown to prevent the vascular protective effects of the tea. Thus we had an interest in evaluating the possible binding of compounds in extracts of black tea, Camellia sinensis (L.) O. Kunze, and nettle, Urtica dioica L, to β-lactoglobulin (βLG), which is the main proteinaceous component of whey. Since our previously miniaturised 96-well plate method based on fluorescence quenching is suitable only for pure compounds, we have developed a method of ultrafiltration sampling combined with HPLC to study the βLG-binding property of the extracts. The method development was made with extract of Piper nigrum L. (black pepper), because its major component is piperine, which has been earlier shown to bind to βLG. The results showed that piperine from P. nigrum was highly and rutin from U. dioica slightly bound to βLG. No components from C. sinensis were bound to βLG. We showed for the first time, that the ultrafiltration/HPLC system can be used in βLG-binding studies with plant extracts. Furthermore, we demonstrated that βLG could bind compounds from extracts and in this way either enhance, or decrease, their health benefits, for example in food preparations.

Genome-wide association study for αS1- and αS2-casein phosphorylation in Dutch Holstein Friesian

Phosphorylation of caseins (CN) is a crucial post-translational modification that allows caseins to form colloid particles known as casein micelles. Both α_S1- and α_S2-CN show varying degrees of phosphorylation (isoforms) in cow milk and were suggested to be more relevant for stabilizing internal micellar structure than β- and κ-CN. However, little is known about the genetic background of individual α_S2-CN phosphorylation isoforms and the phosphorylation degrees of α_S1- and α_S2-CN (α_S1-CN PD and α_S2-CN PD), defined as the proportion of isoforms with higher degrees of phosphorylation in total α_S1- and α_S2-CN, respectively. We aimed to identify genomic regions associated with these traits using 50K single nucleotide polymorphisms for 1,857 Dutch Holstein Friesian cows. A total of 10 quantitative trait loci (QTL) regions were identified for all studied traits on 10 Bos taurus autosomes (BTA1, 2, 6, 9, 11, 14, 15, 18, 24, and 28). Regions associated with multiple traits were found on BTA1, 6, 11, and 14. We showed 2 QTL regions on BTA1, one affecting α_S2-CN production and the other harboring the SLC37A1 gene, which encodes a phosphorus antiporter and affects α_S1- and α_S2-CN PD. The QTL on BTA6 harbors the casein gene cluster and affects individual α_S2-CN phosphorylation isoforms. The QTL on BTA11 harbors the PAEP gene that encodes for β-lactoglobulin and affects relative concentrations of α_S2-CN-10P and α_S2-CN-11P as well as α_S1-CN PD and α_S2-CN PD. The QTL on BTA14 harbors the DGAT1 gene and affects relative concentrations of α_S2-CN-10P and α_S2-CN-11P as well as α_S1-CN PD and α_S2-CN PD. Our results suggest that effects of identified genomic regions on phosphorylation of α_S1- and α_S2-CN are related to changes in milk synthesis and phosphorus secretion in milk. The actual roles of SLC37A1, PAEP, and DGAT1 in α_S1- and α_S2-CN phosphorylation in Dutch Holstein Friesian require further investigation.

β-Lactoglobulin as a potential carrier for bioactive molecules

In this study, the interaction and binding behavior of anesthetic tetracaine (TET) with bovine β-lactoglobulin (LGB) isoform A and a mixture of isoforms A and B were investigated under varying environmental conditions (pH, ionic strength, concentration, LGB-TET complex molar ratio). A wide range of analytical techniques (dynamic light scattering (DLS), electrophoretic mobility, UV-Vis spectroscopy, circular dichroism (CD), quartz crystal microbalance (QCM-D) were used to analyze the physicochemical properties of the complexes in bulk solution and on the surface of gold. The experiments revealed that TET, which is amphiphilic, could bind with LGB not only in the β-barrel but also onto the surface. The zeta potential of the LGB becomes more positively charged upon interaction with TET due to electrostatic interaction of the amino group present in the TET structure. Changes in the zeta potential values are mostly visible above pH 6 for all tested systems. CD spectra show that interaction with the ligand does not change the secondary structure of the protein. The physicochemical properties of LGB-TET complex were examined in an adsorbed state on a gold surface using the QCM-D method. Additionally, molecular docking was used to evaluate the most likely binding site for TET with LGB.

Adsorption effectiveness of β-lactoglobulin onto gold surface determined by quartz crystal microbalance

Bovine β-lactoglobulin (LGB) is a transport protein that can bind to its structure hydrophobic bioactive molecules. Due to the lack of toxicity, high stability and pH-dependent molecular binding mechanism, lactoglobulin can be used as a carrier of sparingly soluble drugs. Dynamic light scattering has confirmed LGB's tendency to create oligomeric forms. The hydrodynamic diameter of LGB molecules varies from 4 nm to 6 nm in the pH range of 2-10 and ionic strength I = 0.001-0.15 M, which corresponds to the presence of mono or dimeric LGB forms. The LGB zeta potential varies from 26.5 mV to -33.3 mV for I = 0.01 M and from 13.3 mV to -16 mV for I = 0.15 M in the pH range of 2-10. The isoelectric point is at pH 4.8. As a result of strong surface charge compensation, the maximum effective ionization degree of the LGB molecule is 35% for ionic strength I = 0.01 M and 22% for I = 0.15 M. The effectiveness of adsorption is linked with the properties of the protein, as well as those of the adsorption surface. The functionalization of gold surfaces with β-lactoglobulin (LGB) was studied using a quartz crystal microbalance with energy dissipation monitoring (QCM-D). The effectiveness of LGB adsorption correlates strongly with a charge of gold surface and the zeta potential of the molecule. The greatest value of the adsorbed mass was observed in the pH range in which LGB has a positive zeta potential values, below pH 4.8. This observation shows that electrostatic interactions play a dominant role in LGB adsorption on gold surfaces. Based on the adsorbed mass, protein orientation on gold surfaces was determined. The preferential side-on orientation of LGB molecules observed in the adsorption layer is consistent with the direction of the molecule dipole momentum determined by molecular dynamics simulations of the protein (MD). The use of the QCM-D method also allowed us to determine the effectiveness of adsorption of LGB on gold surface. Knowing the mechanism of LGB adsorption is significant importance for determining the optimum conditions for immobilizing this protein on solid surfaces. As β-lactoglobulin is a protein that binds various ligands, the binding properties of immobilized β-lactoglobulin can be used to design controlled protein structures for biomedical applications.

Retinoic acid prevents immunogenicity of milk lipocalin Bos d 5 through binding to its immunodominant T-cell epitope

The major cow’s milk allergen Bos d 5 belongs to the lipocalin protein family, with an intramolecular pocket for hydrophobic ligands. We investigated whether Bos d 5 when loaded with the active vitamin A metabolite retinoic acid (RA), would elicit differential immune responses compared to the unloaded state. By in silico docking an affinity energy of −7.8 kcal/mol was calculated for RA into Bos d 5. Loading of RA to Bos d 5 could be achieved in vitro, as demonstrated by ANS displacement assay, but had no effect on serum IgE binding in tolerant or challenge-positive milk allergic children. Bioinformatic analysis revealed that RA binds to the immunodominant T-cell epitope region of Bos d 5. In accordance, Bos d 5 significantly suppressed the CD3+ CD4+ cell numbers, proliferative response and IL-10, IL-13 and IFN-γ secretion from stimulated human PBMCs only when complexed with RA. This phenomenon was neither associated with apoptosis of T-cells nor with the activation of Foxp3+ T-cells, but correlated likely with enhanced stability to lysosomal digestion due to a predicted overlap of Cathepsin S cleavage sites with the RA binding site. Taken together, proper loading of Bos d 5 with RA may suppress its immunogenicity and prevent its allergenicity.

Characterization of the binding of strychnine with bovine β-lactoglobulin and human lysozyme using spectroscopic, kinetic and molecular docking analysis

The binding interaction of a well known alkaloid strychnine (STN) with the mammalian milk protein β-lactoglobulin and human lysozyme has been explored by using several spectroscopic techniques along with computational studies.

Monitoring of nanoclay-protein adsorption isotherms via fluorescence techniques

The investigation of nanoparticles and their interaction with bio-macromolecules have become an important issue; the widely discussed protein corona around nanoparticles and their biological fate in general have drawn particular attention. Here, we focus on nanoclay dispersions and the use of solvatochromic fluorescent dyes (Dansyl and Coumarin 153) for monitoring the interaction with two model proteins, bovine serum albumin and β-lactoglobulin. On one hand, these dyes are poorly emissive in water, but experience a boost in their fluorescence when adsorbed into the hydrophobic domains of proteins. On the other hand, (nano)clays and clay minerals have previously been investigated in terms of their individual protein adsorption isotherms and their usefulness for the solubilization of water-insoluble dyes into an aqueous environment. In the following, we have combined all three individual parts (nanoclay, fluorophore and protein) in dispersions in a wide range of concentration ratios to systematically study the various adsorption processes via fluorescence techniques. In order to clarify the extent of dye diffusion and adsorption-desorption equilibria in the investigations, nanoclay hybrids with an adsorbed dye (Coumarin 153) and a covalently conjugated dye (Dansyl) were compared. The results suggest that the fluorescence progression of protein titration curves correlate with the amount of protein adsorbed, matching their reported adsorption isotherms on hectorite clays. Furthermore, experimental data on the protein monolayer formation around the nanoclays could be extracted due to only minor alterations of the dispersions' optical quality and transparency. In this manner, a fluorescence-based monitor for the formation of the globular protein layer around the nanoclay was realized.