Metal ions modulate thermal aggregation of beta-lactoglobulin: A joint chemical and physical characterization

Impact of Physicochemical Modifications in Casein Promoted by UV-C on the Peptide Profile of Gastric Digestion and the Transepithelial Transport of Peptides

Caseins are the main proteins in milk, and their structure and spatial conformation are responsible for their slow digestion rate. The release of bioactive and β-casomorphin peptides from casein digestion may induce allergic responses during consumption. Spectroscopic techniques were used to observe the structural changes in casein conformation induced by Ultraviolet light irradiation (UV-C). Raman spectroscopy results showed more pronounced peaks at 618 and 640 cm^-1 for phenylalanine and tyrosine moieties of the photolyzed micellar casein, respectively, suggesting changes in the micelle structure. The decrease in the intensity of Raman signals for tryptophan and tyrosine corroborates to the UV-C-induced modifications of the micelle structure. Particle size distribution showed a decrease in the average micelle size after 15 min of UV-C exposure, while low-temperature, long-time (LTLT) pasteurization led to the formation of large aggregates, as observed by atomic force microscopy. UV-C did not impact the formation or transport of peptides, as observed by using the Caco-2 cell as a model for peptide absorption. However, the absence of the opioid peptide SRYPSY from κ-casein and only 20% of the concentration of opioid peptide RYLGY were noted. This work demonstrated that UV-C can be utilized to induce the physicochemical modification of dairy products, promoting a higher digestion rate and reducing allergenicity.

Insights into the trace Sr2+ impact on the gel properties and spatial structure of mutton myofibrillar proteins

Myofibrillar proteins (MPs) and the quality of meat strongly depend on the properties of MP gels, which in turn depend on several parameters that include the thermal history and the concentration of metal ions. Strontium element (Sr) widely exists in mineral water and is found as strontium ions (Sr²⁺), which is an essential trace element for humans. This study investigated the effects of trace Sr²⁺ on the structure-function relationship of mutton MPs, as well as their gels with water. Trace concentrations of Sr²⁺ were found to significantly alter the conformation of the MPs. An increase in Sr²⁺ concentration was associated with a reduction in the tightness and strength of the gel and a significant increase in its water-holding capacity As compared to the untreated control sample, the solubility, particle size, and the magnitude of the Zeta potential of the gels increased by 13.03 %, 12.62 %, and 19.73 %, respectively, whereas the water retention capacity and the gel strength increased by 23.13 % and 21.90 %, at a Sr²⁺ concentration of 5.0 mg/L. Molecular docking predicted an increase in ionic bonds and disulfide bonds because Sr²⁺ had a strong interaction with hydrophilic amino acids and acidic amino acids. The analysis of molecular forces further verified the significant facilitation of interactions between MP molecules with the induction of Sr²⁺. As compare to the untreated control group, the ionic and disulfide bonds increased by 141.17 % and 66.94 %, when treated with 5.0 mg/L Sr²⁺. These changes were likely due to the enhancement of protein-protein interactions caused by Sr²⁺, which could induce MP molecules to properly unfold and aggregate in gel formation. The results could provide a basis for improving the texture and the quality of meat and meat products.

Analysis of copper-induced protein precipitation across the E. coli proteome

Metal cations have been exploited for their precipitation properties in a wide variety of studies, ranging from differentiating proteins from serum and blood to identifying the protein targets of drugs. Despite widespread recognition of this phenomenon, the mechanisms of metal-induced protein aggregation have not been fully elucidated. Recent studies have suggested that copper's (Cu) ability to induce protein aggregation may be a main contributor to Cu-induced cell death. Here, we provide the first proteome-wide analysis of the relative sensitivities of proteins across the Escherichia coli proteome to Cu-induced aggregation. We utilize a metal-induced protein precipitation (MiPP) methodology that relies on quantitative bottom-up proteomics to define the metal concentration-dependent precipitation properties of proteins on a proteomic scale. Our results establish that Cu far surpasses other metals in promoting protein aggregation and that the protein aggregation is reversible upon metal chelation. The bulk of the Cu bound in the protein aggregates is Cu1+, regardless of the Cu2+ source. Analysis of our MiPP data allows us to investigate underlying biophysical characteristics that determine a protein's sensitivity to Cu-induced aggregation, which is independent of the relative concentration of protein in the lysate. Overall, this analysis provides new insights into the mechanism behind Cu cytotoxicity, as well as metal cation-induced protein aggregation.

Secretory protein beta-lactoglobulin in cattle stable dust may contribute to the allergy-protective farm effect

BACKGROUND

Growing up on a cattle farm and consuming raw cow's milk protects against asthma and allergies. We expect a cattle-specific protein as active component in this farm effect.

METHODS

Dust was collected from cattle and poultry stables and from mattresses of households. Urine was obtained from cattle, and ambient aerosols were sampled. Samples were analysed for BLG by SDS PAGE/immunoblot and mass spectrometry, and for association with metals by SEC-ICP-MS. PBMC of healthy donors were incubated with BLG +/- zinc, and proliferation and cytokines determined. BALB/c mice were pre-treated intranasally with stable dust extract containing BLG or depleted of BLG, and subsequent allergy response after sensitization was evaluated on antibody and symptom level.

RESULTS

A major protein in dust from cattle farms and ambient air was identified as BLG. Urine from female and male cattle is a major source of BLG. In dust samples, BLG was associated with zinc. In vitro, zinc-BLG provoked significantly lower proliferation of CD4+ and CD8+ cells while inducing significantly higher levels of IFN-γ and IL-6 than the apo-BLG devoid of zinc. In vivo, pre-treatment of mice with dust extract containing BLG resulted in lower allergy symptom scores to BLG and unrelated Bet v 1 than pre-treatment with extract depleted of BLG. These in vitro and in vivo effects were independent of endotoxin.

CONCLUSION

The lipocalin BLG is found in large amounts in cattle urine, accumulates in bovine dust samples and is aerosolized around farms. Its association with zinc favorably shapes the human cellular immune response towards Th1-cytokines in vitro. BLG together with zinc in stable dust protects mice from allergic sensitization. BLG with its associated ligands may in an innate manner contribute to the allergy-protective farm effect.

Effect of different cooking water on flavor characteristics of mutton soup

The mutton flavor is affected by cooking water significantly, and the flavor of mutton is delicious and widely loved by consumers through an extremely simple processing in northwest China, such as Inner Mongolia, Ningxia, and Xinjiang. The flavor shows obvious changes if get out of these areas even use the same raw meat, which may be caused by different cooking water. To determine whether and how the cooking water affect the mutton soup flavor, the elements in water, the flavor was studied by inductively coupled plasma mass spectrometry (ICP-MS), amino acid analyzer, and thermal desorption (TDS)-gas chromatography-mass spectrometry (GC-MS). Specifically, three water samples from different sources, Ningxia (NXW), Beijing (BJW), and ultrapure water from the laboratory (PUW), were used for cooking with Tan sheep's ribs to get different mutton soups. The inductively coupled plasma mass spectrometry (ICP-MS) results showed that the elements and the concentration of solutes in different water sources were significantly different. The NXW batch had the highest Na, Mg, K, and Sr concentrations, and Na in NXW water reached to 50.60 mg/L, which existed as Na+, significantly (p < .05) higher than BJW (8.63 mg/L) and PUW, which were important to the flavor of mutton soup. The PUW batch had the highest content of free amino acids, and the content of glutamic acid (Glu) reached to 17.89 μg/mL. The NXW batch had the highest content of taste nucleotides, and the content of 5´-IMP reached to 68.68 μg/ml. The volatiles of the three batches had significant differences, and only 40 volatiles were detected in all batches. Further flavor studies using electronic nose and electronic tongue showed significant differences in overall aroma and overall taste, especially bitterness, saltiness, and astringency. The results could provide a basis for improving the flavor quality for the mutton soup.

Effect of L-calcium lactate, zinc lactate, and ferric sodium EDTA on the physicochemical and functional properties of liquid whole egg

The study aimed to evaluate the physicochemical and functional properties of liquid whole egg (LWE) with L-calcium lactate (L-Ca), zinc lactate (L-Zn), and sodium ferric EDTA (NaFeEDTA), and to compare with NaCl addition to determine the application potential of these mineral supplements. Results showed that salts addition significantly influenced the foaming, emulsifying, and gelling properties of LWE, which was possible through affecting the pH, particle size, surface hydrophobicity, apparent viscosity, and solubility. The addition of all the four salts reduced pH but increased the d_4,3 diameter of LWE. Additionally, the addition of 200 mM L-Ca and 6 mM L-Zn significantly improved the emulsifying capacity by 41.73% and 13.6%, the foaming capacity by 26.57% and 10%, and the protein solubility by 13.89% and 12.70%, respectively. In the meantime, mineral supplements tend to produce lower hardness gel, especially with 25 mM L-Ca and 8 mM L-Zn, and the hardness was decreased from 2401.13 to 1138.29 and 1175.59 g, respectively. A relative decrease in hardness was desirable in gelled egg products. Moreover, the addition of NaCl and L-Ca showed a higher redness and yellowness, but the addition of NaFeEDTA showed an undesirable color in dark brown, which may be not accepted by the public. In summary, L-Ca and L-Zn had great potential for application in LWE, which was more appropriate than adding NaCl. This study provides a basis for improving the functional properties of LWE products in the future. PRACTICAL APPLICATION: The addition of L-Ca and L-Zn to liquid whole egg (LWE) could improve the foaming and emulsifying capacity of LWE as well as produce a lower hardness gel, which may be more conducive to the production of cake, custards, and meat products. Meantime, it is more in line with people's pursuit of a healthy diet.

Change in conformational, digestive and immunological characteristics of bovine allergen β-lactoglobulin induced by metal ions in combination with heating

Aggregation of bovine β-lactoglobulin is affected easily by external factors. In this study, effects of metal ions combining with temperature on aggregation of β-lactoglobulin were explored. The conformational characteristics of aggregates were detected by environment scanning electron microscope, CD spectrum and free sulfhydryl group, respectively. Digestive and immunological characteristics were assessed by simulated digestion in vitro and ELISA respectively. The results showed that the morphology of β-lactoglobulin aggregates became more amorphous in Cu²⁺ and Mg²⁺ treated samples and more constricted in Zu²⁺-induced protein. Among them, Cu²⁺ altered the secondary structure of β-lactoglobulin aggregates and free sulfhydryl content most as well as that in gastric digestion. However, all ion-treated groups had similar digestive stability in intestinal digestion. Specially, Ca²⁺ and Mg²⁺ made the antigenicity and potential allergenicity of β-lactoglobulin aggregates decrease, which helps us understand the role of metal ions in immunological characteristics.

Formation and Stabilization of Gold Nanoparticles in Bovine Serum Albumin Solution

The formation and growth of gold nanoparticles (AuNPs) were investigated in pH 7 buffer solution of bovine serum albumin (BSA) at room temperature. The processes were monitored by UV-Vis, circular dichroism, Raman and electron paramagnetic resonance (EPR) spectroscopies. TEM microscopy and dynamic light scattering (DLS) measurements were used to evidence changes in particle size during nanoparticle formation and growth. The formation of AuNPs at pH 7 in the absence of BSA was not observed, which proves that the albumin is involved in the first step of Au(III) reduction. Changes in the EPR spectral features of two spin probes, CAT16 and DIS3, with affinity for BSA and AuNPs, respectively, allowed us to monitor the particle growth and to demonstrate the protective role of BSA for AuNPs. The size of AuNPs formed in BSA solution increases slowly with time, resulting in nanoparticles of different morphologies, as revealed by TEM. Raman spectra of BSA indicate the interaction of albumin with AuNPs through sulfur-containing amino acid residues. This study shows that albumins act as both reducing agents and protective corona of AuNPs.

Biophysical approaches for the study of metal-protein interactions

Protein-protein interactions play important roles for a variety of cell functions, often involving metal ions; in fact, metal-ion binding mediates and regulates the activity of a wide range of biomolecules. Enlightening all of the specific features of metal-protein and metal-mediated protein-protein interactions can be a very challenging task; a detailed knowledge of the thermodynamic and spectroscopic parameters and the structural changes of the protein is normally required. For this purpose, many experimental techniques are employed, embracing all fields of Analytical and Bioinorganic Chemistry. In addition, the use of peptide models, reproducing the primary sequence of the metal-binding sites, is also proved to be useful. In this paper, a review of the most useful techniques for studying ligand-protein interactions with a special emphasis on metal-protein interactions is provided, with a critical summary of their strengths and limitations.

Inhibition of α-amylase Activity by Zn2+: Insights from Spectroscopy and Molecular Dynamics Simulations

Background:

Inhibition of α-amylase activity is an important strategy in the treatment of diabetes mellitus. An important treatment for diabetes mellitus is to reduce the digestion of carbohydrates and blood glucose concentrations. Inhibiting the activity of carbohydrate-degrading enzymes such as α-amylase and glucosidase significantly decreases the blood glucose level. Most inhibitors of α-amylase have serious adverse effects, and the α-amylase inactivation mechanisms for the design of safer inhibitors are yet to be revealed.

Objective:

In this study, we focused on the inhibitory effect of Zn2+ on the structure and dynamic characteristics of α-amylase from Anoxybacillus sp. GXS-BL (AGXA), which shares the same catalytic residues and similar structures as human pancreatic and salivary α-amylase (HPA and HSA, respectively).

Methods:

Circular dichroism (CD) spectra of the protein (AGXA) in the absence and presence of Zn2+ were recorded on a Chirascan instrument. The content of different secondary structures of AGXA in the absence and presence of Zn2+ was analyzed using the online SELCON3 program. An AGXA amino acid sequence similarity search was performed on the BLAST online server to find the most similar protein sequence to use as a template for homology modeling. The pocket volume measurer (POVME) program 3.0 was applied to calculate the active site pocket shape and volume, and molecular dynamics simulations were performed with the Amber14 software package.

Results:

According to circular dichroism experiments, upon Zn2+ binding, the protein secondary structure changed obviously, with the α-helix content decreasing and β-sheet, β-turn and randomcoil content increasing. The structural model of AGXA showed that His217 was near the active site pocket and that Phe178 was at the outer rim of the pocket. Based on the molecular dynamics trajectories, in the free AGXA model, the dihedral angle of C-CA-CB-CG displayed both acute and planar orientations, which corresponded to the open and closed states of the active site pocket, respectively. In the AGXA-Zn model, the dihedral angle of C-CA-CB-CG only showed the planar orientation. As Zn2+ was introduced, the metal center formed a coordination interaction with H217, a cation-π interaction with W244, a coordination interaction with E242 and a cation-π interaction with F178, which prevented F178 from easily rotating to the open state and inhibited the activity of the enzyme.

Conclusion:

This research may have uncovered a subtle mechanism for inhibiting the activity of α-amylase with transition metal ions, and this finding will help to design more potent and specific inhibitors of α-amylases.

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.

In vivo effects of metal ions on conformation and mechanical performance of silkworm silks

BACKGROUND

The mechanism of silk fiber formation is of particular interest. Although in vitro evidence has shown that metal ions affect conformational transitions of silks, the in vivo effects of metal ions on silk conformations and mechanical performance are still unclear.

METHODS

This study explored the effects of metal ions on silk conformations and mechanical properties of silk fibers by adding K⁺ and Cu²⁺ into the silk fibroin solutions or injecting them into the silkworms. Aimed by CD analysis, FTIR analysis, and mechanical testing, the conformational and mechanical changes of the silks were estimated. By using BION Web Server, the interactions of K⁺ and N-terminal of silk fibroin were also simulated.

RESULTS

We presented that K⁺ and Cu²⁺ induced the conformational transitions of silk fibroin by forming β-sheet structures. Moreover, the mechanical parameters of silk fibers, such as strength, toughness and Young's modulus, were also improved after K⁺ or Cu²⁺ injection. Using BION Web Server, we found that potassium ions may have strong electrostatic interactions with the negatively charged residues.

CONCLUSION

We suggest that K⁺ and Cu²⁺ play crucial roles in the conformation and mechanical performances of silks and they are involved in the silk fiber formation in vivo.

GENERAL SIGNIFICANCE

Our results are helpful for clarifying the mechanism of silk fiber formation, and provide insights for modifying the mechanical properties of silk fibers.

Peroxidase-like Catalytic Activity of Copper-Mediated Protein-Inorganic Hybrid Nanoflowers and Nanofibers of β-Lactoglobulin and α-Lactalbumin: Synthesis, Spectral Characterization, Microscopic Features, and Catalytic Activity

A free Cys-SH-containing protein, β-lactoglobulin (β-LG), and another protein not possessing the same, viz., apo-α-lactoglobulin (apo-α-LA), were used in studies to demonstrate the role of this amino acid, along with its secondary structure, in the formation of a protein dimer and a protein-inorganic hybrid nanoflower and in the creation of the peroxidase-like activity of the nanomaterials produced when the proteins were treated with varying Cu(2+) concentration under different pH conditions. An increase in the pH as well as the Cu(2+) mole ratio results in increasing dimer formation in case of β-LG due to the presence of free Cys121-SH, while the dimer is not formed in case of apo-α-LA under the same conditions. The role of Cys in the dimer formation has been demonstrated both by MALDI and sodium dodecyl sulfate-polyacrylamide gel electrophoresis studies. Both of the proteins exhibited changes in their secondary structures to different extents as a function of pH, and the structures were stabilized by Cu(2+) interactions, as studied by CD and fluorescence spectroscopy. The small and spherical nanoparticles formed at pH 7 with lower equivalents of Cu(2+) join together to form larger aggregates at higher equivalents of Cu(2+). For the same concentration at pH 9, both the aggregates and the nanoflowers were noticed. However, at pH 12, the Cu(2+) binding induces the formation of fibers along with the flowers. Both the nanoflowers and nanofibers exhibited peroxidase-like activity in a catalytic manner. Nanoflowers were also shown to detect phenol in the concentration range from 10 to 200 μM. The copper-induced nanobiomaterial obtained in the case of apo-α-LA also exhibited peroxidase-like activity. Thus, this paper deals with the green synthesis of copper-induced protein (β-LG/apo-α-LA)-inorganic hybrid nanomaterials that are important due to their applications as nanobiomaterials.

Simultaneous acquisition of infrared, fluorescence and light scattering spectra of proteins: direct evidence for pre-fibrillar species in amyloid fibril formation

Different spectroscopic approaches are often used to probe specific aspects of amyloid fibril formation but are usually performed separately and under different conditions. This makes it problematic to relate different aspects of the aggregation process when these are monitored by different methods. We report on a multispectral approach for simultaneous acquisition of infrared, fluorescence and light scattering spectra of proteins undergoing aggregation. We have applied our approach to study β-lactoglobulin, a milk protein known to form amyloid fibrils under well-established conditions. Our real-time multispectral measurements show that unfolding of this protein is followed by formation of early aggregates consisting of intermolecular β-sheets with a typical infrared absorption at ∼1619 cm(-1) in (2)H2O. These aggregates, which lead to an increase in the light scattering signal, do not bind the amyloid-specific fluorophore ThT and therefore consist of oligomers or protofibrils. Fibril growth is then observed as a sigmoidal increase in ThT fluorescence. After ∼25 h, a plateau is observed in the intensities of ThT emission and of the band at 1619 cm(-1), indicating that no new fibrils are forming. However, a second phase in the light scattering signal taking place after ∼25 h suggests that the fibrils are assembling into larger structures, known as mature fibrils. This is associated with an upshift of the main β-sheet band in the infrared spectrum. TEM analyses confirmed the existence of thick fibrils comprising 3-5 filaments.

Curcumin inhibits the Al(iii) and Zn(ii) induced amyloid fibrillation of β-lactoglobulin in vitro

Accumulation of ordered protein aggregates (or amyloids) is responsible for several neurodegenerative diseases.

Ferric Ions Inhibit the Amyloid Fibrillation of β-Lactoglobulin at High Temperature

The energetics of amyloid fibrillar aggregation of β-lactoglobulin (βLG) following incubation at high temperature and acid pH was studied by differential scanning calorimetry in the presence of Cu(2+) or Fe(3+) cations, and without any metal. Cu(2+) and metal-free protein solutions showed a distinct exothermic response that disappeared almost completely when the Fe(3+) molar concentration was ten times greater than the βLG concentration. Thioflavin T fluorescence studies in solution and atomic force microscopy analysis of the deposit left on flat mica substrates by heat-incubated βLG solutions correlated the absence of exothermic response of Fe(3+)-βLG solutions with a lack of fibril production. In contrast, abundant fibril deposits were observed for Cu(2+)-βLG solutions, with a rich polymorphism of multistrand fibrillar structures. Electron paramagnetic resonance revealed that Fe(3+) permanently binds to βLG in the aggregate state whereas Cu(2+) plays a catalytic role without binding to the protein. We propose that Fe(3+) inhibits fibril production after binding to a key region of the protein sequence, possibly interfering with the nucleation step of the fibrillation process and opening a nonfibrillar aggregation pathway. These findings suggest that transition metal ions can be utilized to effectively modulate protein self-assembly into a variety of structures with distinct morphologies at the nanoscale level.