High-pressure effects on beta-lactoglobulin interactions with ligands studied by fluorescence

Complexation between curcumin and walnut protein isolate modified by pH shifting combined with protein-glutaminase

The poor techno-functional properties of walnut protein isolate (WPI) limit its application as carrier to improve bioavailability of curcumin. In this study, WPI was modified by pH-shifting (PS) and protein-glutaminase (PG). Changes on the physicochemical and structural characteristics of WPI and effects on complexation with curcumin were investigated. Treatment of PS plus PG increased electrostatic repulsion of WPI with altered secondary and tertiary structure. Solubility of WPI was greatly improved from 18.09% to 52.90%. The increased flexibility resulted in reduced particle size and increased exposure of hydrophobic groups. The improved amphiphilicity of WPI provided more binding sites for complexation with curcumin. Encapsulation efficiency of curcumin was increased from 32.50% to 94.48%. Interestingly, the formed complexes were able to protect curcumin from degradation with improved storage stability and bioaccessibility. Thus, PS plus PG could serve as effective modification strategy for utilization of WPI as a promising delivery vehicle for hydrophobic bioactives.

Controlled ethanol-mediated polyphenol removal from sunflower meal: Impact on physicochemical, structural, flow-behavior, and functional characteristics of isolated proteins

BACKGROUND

Polyphenols present in sunflower meal act on sunflower proteins by reacting directly with their structures and thus influencing their purity, solubility, crystallinity, and functionality. However, the effect on these properties of varying concentrations of ethanol used in dephenolization has yet to be explored. The present study aimed to explore the impact of dephenolization using varying ethanol concentrations (60%, 70%, 80%, and 90%) on the physicochemical, color, thermal, structural, functional, and flow behavior of protein isolates extracted from sunflower meal.

RESULTS

Protein isolates originating from meals that were dephenolized using higher ethanol concentrations exhibited a protein content of 836.10 g kg-1. As the concentration of ethanol increased, a reduction in crystallinity was observed from 24% to 14.15%. Fourier transform infrared (FTIR) spectroscopy revealed marked shifts in major peaks within the 1600 to 1700 cm-1 wavelength range, indicating significant structural and conformational changes. Sodium dodecyl-sulfate polyacrylamide gel electrophoresis (SDS-PAGE) results demonstrated that dephenolization caused decline in molecular weight ranging from 25 kDa to 60 kDa. Dephenolization induced significant changes in surface morphology resulting in more heterogeneous and disordered surfaces as indicated by field emission-scanning electron microscopy (FE-SEM) micrographs. Overall improvement in the functional properties was observed, with an increase in solubility from 15.20% to 22.03%. Improvement in the flow behavior with an increase in porosity from 38% to 60% was also observed, due to dephenolization.

CONCLUSION

Dephenolization using 90% ethanol induced structural changes that enhanced physicochemical and functional characteristics of sunflower protein isolates by improving purity and solubility, reducing crystallinity, and increasing flow behavior. © 2024 Society of Chemical Industry.

Structural and functional impacts of glycosylation-induced modifications in rabbit myofibrillar proteins

Rabbit meat, recognized for its nutritional value, is gaining global attention. However, the inferior functional properties of rabbit myofibrillar proteins lead to quality degradation during the production process. Glycosylation represents an effective method for enhancing protein functionality. This study investigated the glycosylation modification of rabbit myofibrillar proteins. The results demonstrated that solubility of glucose-glycosylated products increased by 34 %, while the reduction capacity improved from 0.15 mg/mL to 1.6 mg/mL. The·OH free radical scavenging ability increased from 63.94 % to 94.21 %. β-Glucan-glycosylated products exhibited the highest thermal stability, and their DPPH free radical scavenging rate increased from 19.68 % to 76.21 %. Glycosylation also induced changes in protein conformation, characterized by a 10-30 °C increase in thermal denaturation peak temperature, gradual attenuation of endogenous fluorescence intensity, gradual enhancement of λmax redshift, and a 30-40 % decrease in surface hydrophobicity. Molecular docking simulations revealed that the primary interactions between glucose, lactose, and β-Glucan with myofibrillar proteins involve hydrogen bonds and van der Waals forces. In conclusion, glycosylation can effectively improve the functional properties of proteins, contributing to the development and production of high-quality, stable, and nutritious rabbit meat products.

Solubilization mechanism of self-assembled walnut protein nanoparticles and curcumin encapsulation

BACKGROUND

Native walnut protein is an alkali-soluble protein that seriously limits the application of walnut protein. The pH-shifting method could improve the solubility of walnut proteins and enable the encapsulation of active ingredients. The present study aimed to prepare water-soluble nanoparticles of curcumin using walnut protein and evaluate the process of walnut protein self-assembly, interaction between walnut protein and curcumin, encapsulation properties, and stability of nanoparticles.

RESULTS

The solubility of native walnut protein was poor, but the solubility of walnut protein nanoparticles (WPNP) formed by walnut protein after pH-shifting significantly improved to 91.5 ± 1.2%. This is because, during the process of pH changing from 7 to 12 and back to 7, walnut protein first unfolded under alkaline conditions and then refolded under pH drive, finally forming an internal hydrophobic and external hydrophilic shell-core structures. The quenching type of walnut protein and curcumin was static quenching, and the quenching constant was 2.0 × 10¹⁴  mol^-1  L^-1  s^-1 , indicating that the interaction between walnut protein and curcumin was non-covalent. Adding curcumin resulted in the formation of nanoparticles with small particle size compared with the no-load. The loading capacity of curcumin-loaded walnut protein nanoparticles (WPNP-C) was 222 mg g^-1 walnut protein isolate. Under the same mass, the curcumin equivalent concentration in aqueous solution of WPNP-C was 17 000 times higher than that of the native curcumin.

CONCLUSION

The solubility of the self-assembled WPNP significantly increased after pH-shifting treatment. The walnut protein carrier could improve the stability of the encapsulated curcumin. Therefore, walnut proteins could be used as water-soluble carriers for hydrophobic drugs. © 2023 Society of Chemical Industry.

The effects of biliverdin on pressure-induced unfolding of apomyoglobin: The specific role of Zn2+ ions

Apomyoglobin (apoMb), a model protein in biochemistry, exhibits a strong propensity to bind various ligands, which makes it a good candidate as a carrier of bioactive hydrophobic drugs. The stability of its hydrophobic pocket determines its potential as a carrier of bioactive compounds. High pressure (HP) is a potent tool for studying protein stability, revealing the specific role of hydrophobic cavities in unfolding. We probed the effects of biliverdin (BV) binding and its complex with Zn²⁺ ions on the structure and HP stability of apoMb. CD spectroscopy and SAXS measurements revealed that BV and BV-Zn²⁺ complexes make the apoMb structure more compact with higher α-helical content. We performed in-situ HP measurements of apoMb intrinsic fluorescence to demonstrate the ability of BV to stabilise apoMb structure at HP conditions. Furthermore, the presence of Zn²⁺ within the apoMb-BV complex significantly enhances the BV stabilisation effect. In-situ visible absorption study of BV chromophore confirmed the ability of Zn²⁺ to increase the stability of apoMb-BV complex under HP: the onset of complex dissociation is shifted by ~100 MPa in the presence of Zn²⁺. By combining HP-fluorescence and HP-visible absorption spectroscopy, our strategy highlights the crucial role of tetrapyrrole-metal complexes in stabilising apoMb hydrophobic pocket.

Fabrication and characterization of succinylated and glycosylated soy protein isolate and its self-assembled nanogel

In this study, we used succinic anhydride (SA) acylation and dextran (DX) glycosylation modified soybean isolate protein (SPI) to develop self-assembled SPI-SA-DX adduct-based nanogels. Degree of modification, SDS-PAGE, and FT-IR studies showed that the amino group of the SPI was replaced by hydrophilic dextran and succinic acid carboxyl groups. Dextran chain and anhydride group attachment to the soybean protein surface enhanced hydrophilicity and spatial site blocking. Modification-induced protein structure unfolding, free sulfhydryl groups to be converted to disulfide bonds, and reduced surface hydrophobicity (H₀). H₀ was lowest at 33,750 ± 1008.29 when SA content = 10 % protein content (SPI-SA₃-DX). The nanometer gel based on SPI-SA₃-DX had the maximum turbidity and clear transparent solution without precipitation. Its particle size and polymer dispersibility index (PDI) were also the smallest, with values of (106.87 ± 4.51) nm and 0.21 ± 0.009, respectively. Transmission electron microscopy showed that nanogels had subspherical shell-core structures. Nanogels were stable under different pH, ionic strength, high temperature, and storage conditions.

Conformation changes and emulsifying properties of myofibrillar proteins in water: Effects of electrostatic interaction with chitosan

Great interests have been attracted toward muscle protein in a water-soluble state with improved functionality for further designing meat protein fortified low-salt functional foods. In the present study, electrostatic interaction of chitosan (CH) with myofibrillar proteins (MP) in water aqueous solution was investigated, and the linked structure changes and emulsion stabilization of MP were studied. Results showed that the electrostatic interaction inhibited MP aggregation, and smaller particle size complexes were formed at pH 6.0, leading to the loss of β-sheet contents and recovery of α-helix contents with decreasing MP/CH mixing ratio (5:1 and 1:1). The tertiary structure confirmed the conformation changes of MP in which more hydrophobic groups and active sulfhydryl groups were exposed (P < 0.05), and the fluorescence was also quenched. With decreasing mixing ratio, the droplet size of emulsion decreased (P < 0.05), while the absorbed protein content increased (P < 0.05). After 7 d of storage, complex at a ratio of 1:1 displayed desirable emulsion stability, which could be due to the improved emulsifying capacity, enhanced electrostatic repulsion and steric effects. These findings provide a better understanding of conformation changes of MP in water aqueous solution induced by electrostatic interactions at mild acidic pH and help to fabricate stable protein/polysaccharide emulsification systems for further developing meat protein-based functional food to deliver health.

Succinylation Modified Ovalbumin: Structural, Interfacial, and Functional Properties

In this study, ovalbumin (OVA) was succinylated with the addition of different levels of succinic anhydride, and the structural and functional properties of succinylated OVA (SOVA) were investigated. SDS−PAGE and FTIR spectrum confirmed the covalent attachment of the succinyl group to OVA. Thermal stability and the absolute value of zeta potential (pH 6.0) of SOVA were enhanced by 14.90% and 76.77% higher than that of the native OVA (NOVA), respectively. Circular dichroism (CD) spectra demonstrated that the succinylation decreased the α−helix and increased β−sheet content to 21.31% and 43.28%, respectively. The content of free sulfhydryl groups increased and intrinsic fluorescence spectra suggested the SOVA became more unfolded and flexible as the degree of succinylation enhanced. Furthermore, succinylation effectively enhanced the solubility and decreased the interface tension (oil−water and air−water interface) of OVA. Compared to NOVA, the emulsifying activity and stability of SOVA were increased by 1.6 times and 1.2 times, respectively, and foaming capacity and stability were enhanced by 2.7 times and 1.5 times, respectively.

Physicochemical, functional and structural properties of the major protein fractions extracted from Cordyceps militaris fruit body

The physicochemical and functional as well as structural properties of major protein fractions (albumin, globulin, glutelin) sequentially extracted in water, salt, alkaline solution respectively from Cordyceps militaris Minfu20 fruit body were investigated. The glutelin (43.11%, w/w) was the predominant protein component of C. militaris fruit body followed by albumin (36.47%) and globulin (17.94%). The three proteins extracted from different solvents showed different characteristics, which were related to the alternation of amino acid composition, surface hydrophobicity, and structural feature. Sodium dodecyl sulphate-polyacrylamide gel electrophoresis (SDS-PAGE) showed that the albumin and globulin mainly consisted of polypeptides with size < 20 kDa. The glutelin showed serious staining on the lane which may have a relatively bigger molecular weight. Intrinsic fluorescence intensity (FI) suggested glutelin contained more unfolding conformations (highest FI) which were probably resulted in a better foaming capacity of 151% and emulsion formation with the smallest size oil droplets (10.410 µm). The protein fractions showed great nutritional quality since they satisfied all recommended essential amino acid allowances for adults of Food & Agriculture Organization (FAO)/World Health Organization (WHO). Therefore, Cordyceps militaris Minfu20 fruit body proteins have potential alternative renewable edible fungi (mushroom) protein and could be used effectively as a food ingredient to improve food nutrition and product diversification compared with plant proteins.

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.

Effects of ultrasonic treatment on the gel properties of microbial transglutaminase crosslinked soy, whey and soy-whey proteins

This paper studied the influences of diverse ultrasonic power treatments on the physico-chemical properties of soy-whey mixed protein induced by microbial transglutaminase. Two groups of 15% (m/v) of protein solution-sole protein (as control group) and mixed protein were prepared and processed under different ultrasonic powers for 30 min. After ultrasonic power treatments, gel properties were significantly increased: under 300 W, the gel hardness of mixed protein reached a maximum of 998.9 g, with its water binding capacity scoring a maximum of 87%. According to the analysis of fluorescence emission spectrum, the fluorescence intensity and maximum absorption peak had changed, for different ultrasonic power treatments had exposed more groups. The Fourier Transform Infrared Spectroscopy also suggested that ultrasonic power treatments could change the secondary structure of gel samples. The scanning electron microscope demonstrated that the network structure of mixed protein gel displayed more regular and uniform after ultrasonic treatments.

Evaluation of the effects of mild heat in bovine milk by time resolved fluorescence

Heat treatment of milk and dairy products are indispensable for the dairy industry. This thermal processing intends to extend shelf life, improve quality of the milk and minimize the health risks associated with milk and dairy products. The use of time-resolved fluorescence techniques to identify conformation and structure changes ok milk fat and proteins could help understand the temperature effects in bovine milk. This study aimed to use fluorescence lifetimes to evaluate the effects of heating fresh cow milk up to 85 °C. We observed different tendencies for fluorescence lifetimes submitted to different heating temperatures. The longer lifetime values decreased for temperatures higher than room temperature until it reached a minimum value near 40 °C and it slowly increased again for temperatures higher than 40 °C, indicating two distinct processes. These results indicate that time-resolved fluorescence can assist on the analysis of heating effects in fluid milk.

Soy protein isolate as a nanocarrier for enhanced water dispersibility, stability and bioaccessibility of β-carotene

BACKGROUND

The incorporation of β-carotene, one of the most common pigments or bioactives, into food formulations has attracted increasing interest from the food industry, due to its good nutrition and potential health effects. However, it is poorly soluble and unstable in water, which greatly limits its applications in foods. This work presented an effective approach to improve the water dispersibility, stability and even bioaccessibility of β-carotene, using soy protein isolate (SPI) to perform as effective nanocarriers for this molecule.

RESULTS

The complexation with SPI remarkably improved the water dispersibility and stability against heating and freeze-drying of β-carotene. However, the encapsulation efficiency and stability of β-carotene in the nanocomplexes with SPI were closely dependent on the applied β-carotene-to-protein ratio, at which the complexation occurred. The best improvement of stability was observed at appropriate β-carotene-to-protein ratios, e.g. 10-20 g kg^-1 . The complexation with β-carotene mainly occurred on the surface of SPI nanoparticles, through hydrophobic interactions. The complexation resulted in inter-particle aggregation, in a concentration-dependent manner. Almost all of the β-carotene molecules in the nanocomplexes could be progressively released into the aqueous phase.

CONCLUSION

SPI exhibits a good potential to perform as a nanocarrier for enhanced water dispersibility, stability and bioaccessibility of β-carotene. © 2016 Society of Chemical Industry.

Time resolved fluorescence of cow and goat milk powder

Milk powder is an international dairy commodity. Goat and cow milk powders are significant sources of nutrients and the investigation of the authenticity and classification of milk powder is particularly important. The use of time-resolved fluorescence techniques to distinguish chemical composition and structure modifications could assist develop a portable and non-destructive methodology to perform milk powder classification and determine composition. This study goal is to differentiate milk powder samples from cows and goats using fluorescence lifetimes. The samples were excited at 315nm and the fluorescence intensity decay registered at 468nm. We observed fluorescence lifetimes of 1.5±0.3, 6.4±0.4 and 18.7±2.5ns for goat milk powder; and 1.7±0.3, 6.9±0.2 and 29.9±1.6ns for cow's milk powder. We discriminate goat and cow powder milk by analysis of variance using Fisher's method. In addition, we employed quadratic discriminant analysis to differentiate the milk samples with accuracy of 100%. Our results suggest that time-resolved fluorescence can provide a new method to the analysis of powder milk and its composition.

Structural and Functional Properties of Soy Protein Isolates Modified by Soy Soluble Polysaccharides

Aiming to achieve the modification to soy protein isolate (SPI) by soy soluble polysaccharides (SSPS), electrically driven complex systems were first established in the environment of pH 3.0, and then reconstituted SPI particles with different SPI-SSPS ratios were obtained under freeze-drying process. Through this treatment, the structures of SPI particles were partly unfolded and adsorbed SSPS mainly via hydrophobic interactions and hydrogen bonding with larger particle sizes. The adherence of SSPS decreased the surface hydrophobicity of reconstituted SPI particles, but exerted not much influence on the emulsifying and foaming activities and increased the corresponding stabilities due to enhancing the unfolded extent of structure and improving the conformation flexibility. Reconstituted SPI-SSPS particles might rearrange and link each other due to the presence of SSPS on the air-water interface to better stabilize these systems. At SPI-SSPS ratio of 10:1, lower temperature was required to form gels with lower gel intensity and porous structure. The findings provide a further comprehension to the relationship between structures and functional properties of SPI modified by SSPS and the feasibility of applying these reconstituted particles to needed areas.

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

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

Emulsifying and interfacial properties of vicilins: role of conformational flexibility at quaternary and/or tertiary levels

Although the functionality of plant proteins (and soy proteins in particular) has been widely investigated in the last decades, the importance of conformational characteristics to their functionalities is still far away from being understood. The aim of the present work was to unravel the role of conformational flexibility at the quaternary and/or tertiary levels in the emulsifying and interfacial properties of phaseolin, an ideal vicilin (or 7S globulin) from red kidney bean. The conformational flexibility at quaternary and tertiary levels of phaseolin was modulated by urea with increasing concentrations from 0 to 8 M, as characterized by using dynamic light scattering (DLS), intrinsic fluorescence and derivative UV spectroscopy, and differential scanning calorimetry (DSC). The emulsifying and interfacial properties, including emulsifying ability, flocculated state of oil droplets (in fresh emulsions), emulsion stability against creaming, and adsorption dynamics at the oil-water interface, were characterized at a specific protein concentration of 0.5% (w/v). The results indicated that increasing the urea concentration resulted in a progressive dissociation of trimeric phaseolin molecules into monomeric subunits, and even a structural unfolding of dissociated subunits; the urea-induced conformational changes at quaternary and/or tertiary levels were reversible, and the molecules at high urea concentrations shared similar structural features to the "molten globule state". On the other hand, increasing the urea concentration progressively improved the emulsifying ability of the protein, and flocculated extent of oil droplets in the fresh emulsions, but led to a progressive decrease in interfacial protein concentration. The improvement of the emulsifying ability was not related to diffusion (during initial adsorption) and penetration at the interface, but highly dependent on ease of structural rearrangement of the adsorbed proteins. These observations clearly confirmed that the flexibility of phaseolin at quaternary and/or tertiary levels plays a vital role in its emulsifying ability, mainly through the way of affecting the ease of structural rearrangement of adsorbed proteins at the interface. The findings could provide an in-depth understanding of the importance of conformational flexibility for the emulsifying properties of oligomeric storage globulins, and thus are of great help to guide the modifications of the proteins for better emulsifying properties.

Structural and physicochemical property relationships of cruciferin homohexamers

Heteromeric cruciferin from wild type (WT) Arabidopsis thaliana and homomeric cruciferin CRUA, CRUB, and CRUC composed of identical subunits obtained from double-knockout mutant lines were investigated for their structural and physicochemical properties. A three-step chromatographic procedure allowed isolation of intact cruciferin hexamers with high purity (>95%). FT-IR and CD analysis of protein secondary structure composition revealed that all cruciferins were folded into higher order structures consisting of 44-50% β-sheets and 7-9% α-helices. The structural and physicochemical properties of homohexameric CRUC deviated from that of CRUA and CRUB and exhibited a compact, thermostable, and less hydrophobic structure, confirming the predictions made using 3D homology structure models.