An Improved Isolation Method of Soy β-Conglycinin Subunits and Their Characterization

Influence of pH and ionic strength on the physicochemical and structural properties of soybean β-conglycinin subunits in aqueous dispersions

Understanding the impact of pH and ionic strength on the physicochemical and structural properties of soy proteins at subunit level is essential for design and fabrication of many plant-based foods. In this study, soybean β-conglycinin and its subunit fractions αα' and β were dispersed in solutions with different pH values (3.7, 7.6, and 9.0) at low (5 mM NaCl) and high (400 mM NaCl) ionic strengths, respectively. The solubility, rheology, particle size, zeta potential, microstructure, secondary structure, and tertiary structure of the different dispersions were analyzed using a range of analytical methods. The β-conglycinin, αα'- and β-subunits aggregated near the isoelectric point (pH 3.7). Increasing the ionic strength led to the assembly of more homogeneous units. An increase in ionic strength at pH 7.6 and pH 9.0 led to electrostatic screening, which promoted dissociation of the aggregates. The β-subunit showed a greater sensitivity to pH and ionic strength than the αα'-subunits. Based on the evidence from a range of analytical methods, the highly hydrophilic extension region of the αα'-subunits played an important role in determining the stability of the β-conglycinin dispersions under different environmental conditions. Moreover, the N-linked glycans appeared to impact the conformation and aggregation state of the β-conglycinin.

Plant Protein versus Dairy Proteins: A pH-Dependency Investigation on Their Structure and Functional Properties

Plant proteins are constantly gaining attention as potential substitutes for dairy proteins, due to their suitable functionality and nutritional value. This study was designed to compare the structural and functional responses of different plant protein isolates (soy, pea, lentil, and chickpea) with two commonly used dairy protein (whey protein isolates and sodium caseinate) under different pH treatments (pH 3.0, 5.0, 7.0, and 9.0). The results showed that pH had a different alteration on the structural, surface properties and functional properties of plant and dairy proteins. Plant protein generally possessed a darker color, lower solubility, emulsifying properties, and foaming capacity, whereas their foaming stability and water holding capacity were higher than those of dairy proteins. Soy protein isolates were characterized by its comparable proportion of β-turn and random coils, zeta-potential, emulsifying (30.37 m²/g), and water-holding capacity (9.03 g/g) at alkaline conditions and chickpea protein isolates showed good oil-holding capacity (3.33 g/g at pH 9) among plant proteins. Further analysis confirmed that pH had a greater influence on the structural and functional properties of proteins as compared to protein sources, particularly at acidic conditions. Overall, this study might help processors select the appropriate plant protein as dairy alternatives for their target application in plant-based food products.

Heteroprotein Complex Coacervate Based on β-Conglycinin and Lysozyme: Dynamic Protein Exchange, Thermodynamic Mechanism, and Lysozyme Activity

Heteroprotein complex coacervate (HPCC) is a liquid-like protein concentrate produced by liquid-liquid phase separation. We revealed the protein dynamic exchange and thermodynamic mechanism of β-conglycinin/lysozyme coacervate, and clarified the effect of HPCC on protein structure and activity. β-conglycinin and lysozyme assembled into coacervate at pH 5.75-6.5 and assembled into amorphous precipitates at higher pH. As the pH dropped from 8 to 6, the number of binding sites of the complex decreased in half, and the desolvation degree corresponding to the entropy gain was greatly reduced, conducing to the formation of coacervates rather than precipitates. The coacervates achieved the unique dynamic exchange by exchanging proteins with the diluted phase, making the uniform distribution of proteins in coacervates. The lysozyme activity was completely retained in β-conglycinin/lysozyme coacervates. These results proved that β-conglycinin-based heteroprotein complex coacervate is a feasible method to encapsulate and enrich active proteins in a purely aqueous environment.

Nano-architectural assembly of soy proteins: A promising strategy to fabricate nutraceutical nanovehicles

Use of protein-based nanovehicles has been well recognized to be one of the most effective strategies to improve water dispersibility, stability and bioavailability of nutraceuticals or bioactive ingredients. Thanks to their health-benefiting effects and unique assembly behavior, soy proteins seem to be the perfect food proteins for fabricating nanovehicles in this regard. This review presents the state-of-art knowledge about the assembly of soy proteins into nano-architectures, e. g., nanoparticles, nanocomplexes or nanogels, induced by different physicochemical strategies and approaches. The strategies to trigger the assembly of soy proteins into a variety of nano-architectures are highlighted and critically reviewed. Such strategies include heating, enzymatic hydrolysis, pH shift, urea or ethanol treatment, reduction, and static high pressure treatment. The self-assembly behavior of soy proteins (native or denatured) is also reviewed. Besides the assembly of proteins alone, soy proteins can co-assemble with polysaccharides to form versatile nano-architectures, through different processes, e.g., heating or ultrasonication. Finally, recent progress in the development of assembled soy protein nano-architectures as nanovehicles for hydrophobic nutraceuticals is briefly summarized. With the fast increasing health awareness for natural and safe functional foods, this review is of crucial relevance for providing an important strategy to develop a kind of novel soy protein-based functional foods with dual-function health effects from soy proteins and nutraceuticals.

Novel Soy β-Conglycinin Core-Shell Nanoparticles As Outstanding Ecofriendly Nanocarriers for Curcumin

The development of high-performance nanocarriers for nutraceuticals or drugs has become one of the topical research subjects in the functional food fields. In this work, we for the first time propose a novel and ecofriendly process to obtain a kind of nanostructured soy β-conglycinin (β-CG; a major soy storage globulin) as outstanding nanocarriers for poorly soluble bioactives (e.g., curcumin), by a urea-assisted disassembly and reassembly strategy. At urea concentrations > 4 M, the structure of β-CG gradually dissociated into its separate subunits (α, α', and β) and even denatured (depending on the type of subunits); after dialysis to remove urea, the dissociated subunits would reassemble into a kind of core-shell nanostructured particles, in which aggregated β-subunits acted as the core while the shell layer was mainly composed of α- and α'-subunits. The core-shell nanoparticles were favorably formed at protein concentrations of 1.0-2.0 wt %. Curcumin crystals were directly introduced into the β-CG solution at high urea concentrations (e.g., 8 M) and would preferentially interact with the denatured β-subunits. As a consequence, almost all of the curcumin molecules were encapsulated in the core part of the reassembled core-shell nanoparticles. The loading amount of curcumin in these nanoparticles could reach 18 g of curcumin per 100 g of protein, which far exceeds those reported previously. The encapsulated curcumin exhibited a high water solubility, extraordinary thermal stability, and improved bioaccessibility, as well as a sustained release behavior. The findings provide a novel strategy to fabricate a kind of high-encapsulation-performance, organic solvent-free, and biocompatible nanocarrier for hydrophobic nutraceuticals and drugs.

Formation of amyloid fibrils from soy protein hydrolysate: Effects of selective proteolysis on β-conglycinin

The soy protein hydrolysate subjected to selective proteolysis on β-conglycinin (referred to as DβH, contrast group) and a control soy protein isolate sample without addition of protease (referred to as CSPI, blank group) were adopted as experimental samples. By employing the "subtraction" mode of logical thinking, we aimed to compare the differences between CSPI and DβH on fibrillation at pH2.0 with heating at 95°C. The results showed when heated for 60min, CSPI tended to form short worm-like fibrils while DβH long semiflexible fibrils. When heating time was prolonged to 360min, the fibrils formed from them both exhibited cluster. Whereas when heated for 720min, no fibrillar aggregates appeared from them. This study would help explore the effects of β-conglycinin on the fibril formation of soy protein isolate by a new way.

Development of dcELISA Method for Rapid Detection of β-conglycinin in Soybean

In this assay, rabbit anti-β-conglycinin polyclonal antibody (Pab) was used as primary antibody and goat anti-rabbit IgG-horseradish peroxidase (HRP) was used as secondary antibody. The optimum incubating time of blocking, HRP and TMB (3, 3’5, 5’-tetramethylbenzidine) detected by the response surface and orthogonal test was 58, 140, and 20 min, respectively. The practical working range for the determination of β-conglycinin was 20–320 ng ml−1. The recoveries of β-conglycinin in spiked soybean samples were between 95.8 % and 101.2 % with the relative standard deviation less than 6.9 % (intra-assay) and 8.2 % (inter-assay). The current method was used to analyze 80 different soybean obtained from different region of Jilin province and the detected results using the direct ELISA. Compared with the competitive ELISA, The results showed contents had no difference between two methods. The dcELISA assay provides a specific and sensitive method for detecting of soybean β-conglycinin in actual production.

Thermal aggregation behaviour of soy protein: characteristics of different polypeptides and sub-units

BACKGROUND

Due to the differences in structure and composition of glycinin and β-conglycinin, they exhibit different characteristics during heat treatment. In present study, the thermal aggregation behaviour of glycinin, β-conglycinin and their isolated sub-units was investigated at pH 7.0.

RESULTS

Acidic polypeptides, basic polypeptides, αα' and β sub-units of soy protein were denatured during the isolation process. The degree of aggregation of protein fractions after heat treatment was in the order: denatured basic polypeptides > native glycinin > denatured β sub-unit > native β-conglycinin > denatured acidic polypeptides > denatured αα' sub-units. Glycinin, β-conglycinin, acidic polypeptides and αα'/β sub-units exhibited different changing trends of surface hydrophobicity with increasing temperature. The αα' sub-units showed higher ability to suppress thermal aggregation of basic polypeptides than β sub-units during heat treatment. The β sub-units were shown to form soluble aggregates with glycinin after heating.

CONCLUSION

The interaction mechanism of αα' and β sub-units heated with basic polypeptides was proposed. For the β sub-units-basic polypeptides mixed system, more hydrophobic chains were binding together and buried inside during heat treatment, which resulted in lower surface hydrophobicity. The αα' sub-units-basic polypeptides mixed system was considered to be a stable system with higher surface hydrophobicity after being heated.

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.

Growth kinetics of amyloid-like fibrils derived from individual subunits of soy β-conglycinin

The amyloid-like fibrillation of soy β-conglycinin subunits (α, α', and β) upon heating (0-20 h) at 85 °C and pH 2.0 was characterized using dynamic light scattering, circular dichroism (CD), binding to amyloid dyes (Thioflavin T and Congo red), and atomic force microscopy. The fibrillation of all three subunits was accompanied by progressive polypeptide hydrolysis. The hydrolysis behaviors, fibrillation kinetics, and morphologies of amyloid-like fibrils considerably varied among α, α', and β subunits. Faster hydrolysis rates and special fragments were observed for the α and α' subunits compared to the β subunit. However, the order of the fibrillation rate and capacity to form β-sheets was α' > β > α, as evidenced by CD and Thioflavin T data. Moreover, sequential growth of twisted screw-structure fibrils, leading to macroscopic fibrils with distinct morphological characteristics, was observed for β-conglycinin and individual subunits. The different fibrillation kinetics and morphologies of α, α', and β subunits appear to be associated with the differences in the amino acid composition and typical sequence of peptides. Besides, the disruption of ordered structure of fibrils occurred upon further heating (6-20 h) due to extensive hydrolysis. These results would suggest that all subunits are involved in the fibrillation of β-conglycinin, following multiple steps including polypeptide hydrolysis, assembly to amyloid structure, and growth into macroscopic fibrils with a fibril shaving process.