Mechanisms of structure formation in particulate gels of beta-lactoglobulin formed near the isoelectric point

Heat-induced agglomeration of water-soluble cod proteins toward gelled structures

Cod proteins (CPs) have potential applications in designing desirable gel-based products, and this study aimed to unravel their heat-induced aggregation pattern and further probe the roles in protein gels. SDS-PAGE analysis indicated that high-precipitation-coefficient aggregates (HPCAs) of CPs aggregates were composed of considerable polymers of myosin heavy chains and actin, and their low-precipitation-coefficient aggregates (LPCAs) contained myosin light chains and tropomyosin. Studies from correlation analysis between the structure and aggregation kinetics revealed that the generation of β-sheet and SS bonds were responsible for their spontaneous thermal aggregation induced by heating temperature and protein concentration, respectively. Additionally, as protein denaturation ratio increased, more and larger HPCAs were formed, which was evidenced driving the network formation of protein gels and resulting in higher storage modulus (G') values. These novel findings may be applicable to other animal proteins for better tailoring the manufacturing of muscle gel-based products.

Fabrication of Whey Protein Isolate-Pectin Nanoparticles by Thermal Treatment: Effect of Dynamic High-Pressure Treatment

This study investigated the impact of dynamic high-pressure (DHP) treatment on the ability of whey protein isolate (WPI) to form associative complexes with pectin and to form aggregate particles after their subsequent heat treatment. Light scattering showed that DHP treatments disrupted preexisting WPI aggregates and assembled pectin chains. Complexes formed from WPI/pectin mixtures at pH 4.5 were an order of magnitude smaller when formed after DHP treatment, regardless of the degree of esterification. WPI/pectin complexes formed after DHP treatment were more stable against subsequent pH neutralization than complexes formed without DHP treatment, and WPI/high-methoxyl pectin (HMP) complexes had greater stability than WPI/low-methoxyl pectin (LMP) complexes. WPI/pectin particles prepared by thermal treatment of complexes at pH 4.5 were also smaller when prepared after DHP treatment. WPI/HMP particles were stable to subsequent pH neutralization, while WPI/LMP particles became larger after neutralization.

Thermal gelation and digestion properties of hen egg white: Study on the effect of neutral and alkaline salts addition

In this study, the thermal gelation and digestion properties of hen egg white (hen EW) proteins with different salts were investigated. Results show that the addition of neutral salt - sodium chloride (NaCl) decreased the gel hardness/resilience, increased gel lightness, aggregated particle size and digestibility of hen EW proteins significantly. In contrast, alkaline salts - phosphate and carbonate addition increased the gel resilience and strain tolerance as well as reduced the aggregated particle size and gel lightness of hen EW proteins due to the increase of solution pH and negative charge. Correlation analysis shows that the digestibility of hen EW gels was affected by gel viscoelasticity, molecule forces and texture. In conclusion, thermal gelation properties of hen EW proteins could be modulated by salts with different pH/ionic strength, and thus affected the protein digestion and peptide released.

Venturi-based rapid expansion of supercritical solution (Vent-RESS): synthesis of liposomes for pH-triggered delivery of hydrophilic and lipophilic bioactives

Multivitamin-loaded and surface-modified liposomes tailored for simultaneous intestinal delivery of both lipophilic and hydrophilic bioactives were synthesized from sunflower phosphatidylcholine (SFPC). Liposomes (SL) were generated with the aid of a novel, organic solvent free, and environmentally benign process which utilizes venturi-based rapid expansion of supercritical solution (Vent-RESS). Vitamins E and C were used as model lipophilic and hydrophilic bioactives and demonstrated an average encapsulation efficiency of 92 and 70 %, respectively. Synthesized liposomes were coated with a pH-responsive double-wall of chitosan and β-lactoglobulin (βlg-Cs-SL) to develop a biocompatible vehicle for pH-triggered delivery of bioactive cargo(s). To compare the efficacy of this newly developed dual-coating, SL was also coated with a commercially available pH responsive polymer, Eudragit^® S100 (Eu-SL). No organic solvent was used during the surface coating of SLs with these two different types of enteric coatings. The performance of these two coatings was studied by conducting morphological characterization through diameter and ζ-potential measurements along with confocal laser scanning and freeze-fracture cryogenic scanning electron microscopies. The stability of coated and uncoated SFPC liposomes was determined in simulated gastrointestinal fluids. For βlg-Cs-SL and Eu-SL, after 2 h of incubation in simulated gastric condition, less than 5 % of the encapsulated vitamins C and E were released, whereas for SL, 41 and 28 % of vitamins C and E were released within 2 h of incubation period. In simulated intestinal fluid, coated liposomes released most of their remaining payload when incubated for 4 h. The newly developed dual coating was found to be as effective as its commercially available counterpart, Eudragit^® S100 coating; nevertheless, the biocompatible, non-toxic, and non-synthetic nature of this coating makes it an attractive alternative. Modeling the release kinetics of vitamins from coated liposome showed that the release of payload from surface coated liposomes proceeded through a multistep structural disintegration involving both Fickian and non-Fickian types of diffusion. The ability of these surface-coated liposomes to maintain structural integrity under the gastric condition followed by site-specific, pH-triggered release of encapsulated cargo in the intestine will make them highly suitable for oral administration of bioactive compounds in pharmaceutical and food applications.

Ion-Mediated Morphological Diversity in Protein Amyloid Systems

Salt ions are considered among the major determinants ruling protein folding, stability, and self-assembly in the context of amyloid-related diseases, protein drug development, and functional biomaterials. Here, we report that Hofmeister ions not only determine the rate constants of the aggregation reaction for human insulin and hen egg white lysozyme but also control the generation of a plethora of amyloid-like morphologies ranging from the nanoscale to the microscale. We anticipate that the latter is a result of a balance between colloidal and conformational stability combined with an ion-specific effect and highlight the importance of salt ions in controlling the biological functions of protein aggregates.

β-Barrels and Amyloids: Structural Transitions, Biological Functions, and Pathogenesis

Insoluble protein aggregates with fibrillar morphology called amyloids and β-barrel proteins both share a β-sheet-rich structure. Correctly folded β-barrel proteins can not only function in monomeric (dimeric) form, but also tend to interact with one another-followed, in several cases, by formation of higher order oligomers or even aggregates. In recent years, findings proving that β-barrel proteins can adopt cross-β amyloid folds have emerged. Different β-barrel proteins were shown to form amyloid fibrils in vitro. The formation of functional amyloids in vivo by β-barrel proteins for which the amyloid state is native was also discovered. In particular, several prokaryotic and eukaryotic proteins with β-barrel domains were demonstrated to form amyloids in vivo, where they participate in interspecies interactions and nutrient storage, respectively. According to recent observations, despite the variety of primary structures of amyloid-forming proteins, most of them can adopt a conformational state with the β-barrel topology. This state can be intermediate on the pathway of fibrillogenesis ("on-pathway state"), or can be formed as a result of an alternative assembly of partially unfolded monomers ("off-pathway state"). The β-barrel oligomers formed by amyloid proteins possess toxicity, and are likely to be involved in the development of amyloidoses, thus representing promising targets for potential therapy of these incurable diseases. Considering rapidly growing discoveries of the amyloid-forming β-barrels, we may suggest that their real number and diversity of functions are significantly higher than identified to date, and represent only "the tip of the iceberg". Here, we summarize the data on the amyloid-forming β-barrel proteins, their physicochemical properties, and their biological functions, and discuss probable means and consequences of the amyloidogenesis of these proteins, along with structural relationships between these two widespread types of β-folds.

Crocin inhibits urea-induced amyloid formation by bovine β-lactoglobulin

Crocin stabilizes the native structure of β-lactoglobulin and attenuates urea-induced unfolding and loss of β-sheet structure during amyloidogenesis.

Investigating the effect of sugar-terminated nanoparticles on amyloid fibrillogenesis of β-lactoglobulin

In vivo tissue deposition of fibrillar protein aggregates is the cause of several degenerative diseases. Evidence suggests that interfering with the pathology-associated amyloid fibrillogenesis by inhibitory molecules is envisaged as the primary therapeutic strategy. Amyloid fibril formation of proteins has been demonstrated to be influenced by nanoparticles/nanomaterials. As compared with their molecular form counterpart, this work examined the effect of sucrose-terminated nanoparticles on the in vitro amyloid fibrillogenesis and structural properties of β-lactoglobulin at pH 2.0 and 80 °C. ThT binding and electron microscopy results demonstrated that sucrose-terminated nanoparticles were able to suppress β-lactoglobulin fibrillogenesis in a concentration-dependent fashion. Importantly, sucrose-terminated nanoparticles showed better β-lactoglobulin fibril-inhibiting ability than sucrose molecules. ANS fluorescence and right-angle light scattering results showed reduced solvent exposure and decreased aggregation, respectively, in the β-lactoglobulin samples upon treatment with sucrose-terminated nanoparticles. Moreover, fluorescence quenching analyses revealed that the static quenching mechanism and formation of a non-fluorescent fluorophore-nanoparticle complex are involved in the nanoparticle-β-lactoglobulin interaction. We believe that the results from this study may suggest that the nanoparticle form of biocompatible sugar-related osmolytes may serve as effective inhibiting/suppressing agents toward protein fibrillogenesis.

Milk Processing Affects Structure, Bioavailability and Immunogenicity of β-lactoglobulin

Bovine milk is subjected to various processing steps to warrant constant quality and consumer safety. One of these steps is pasteurization, which involves the exposure of liquid milk to a high temperature for a limited amount of time. While such heating effectively ameliorates consumer safety concerns mediated by pathogenic bacteria, these conditions also have an impact on one of the main nutritional whey constituents of milk, the protein β-lactoglobulin. As a function of heating, β-lactoglobulin was shown to become increasingly prone to denaturation, aggregation, and lactose conjugation. This review discusses the implications of such heat-induced modifications on digestion and adsorption in the gastro-intestinal tract, and the responses these conformations elicit from the gastro-intestinal immune system.

Gelation of food protein-protein mixtures

Gelation of proteins is one of the principal means to give desirable texture to food products. Gelation of individual proteins in aqueous solution has been investigated intensively in the past, but in most food products the system contains mixtures of different types of proteins. Therefore one needs to consider interaction between different proteins both before and during gelation. Most food proteins can be classified as globular proteins, but casein and gelatin are also important food proteins. In this review the focus is on gelation induced by heating or cooling, which is the most commonly used method. After briefly discussing general features of protein aggregation and gelation, the literature on gelation of mixtures of different types of globular proteins is reviewed as well as that of mixtures of globular proteins with gelatin or with casein. The effect on the gel stiffness and the microstructure of the gelled mixtures will be discussed in terms of different scenarios that can be envisaged: independent aggregation and gelation, co-aggregation and phase separation.

Inhibition of amyloid fibril formation of β-lactoglobulin by natural and synthetic curcuminoids

The aggregation of proteins has been associated with several aspects of daily life, including food processing, blood coagulation and many neurodegenerative infections.

Self-assembling peptide and protein amyloids: from structure to tailored function in nanotechnology

Self-assembled peptide and protein amyloid nanostructures have traditionally been considered only as pathological aggregates implicated in human neurodegenerative diseases. In more recent times, these nanostructures have found interesting applications as advanced materials in biomedicine, tissue engineering, renewable energy, environmental science, nanotechnology and material science, to name only a few fields. In all these applications, the final function depends on: (i) the specific mechanisms of protein aggregation, (ii) the hierarchical structure of the protein and peptide amyloids from the atomistic to mesoscopic length scales and (iii) the physical properties of the amyloids in the context of their surrounding environment (biological or artificial). In this review, we will discuss recent progress made in the field of functional and artificial amyloids and highlight connections between protein/peptide folding, unfolding and aggregation mechanisms, with the resulting amyloid structure and functionality. We also highlight current advances in the design and synthesis of amyloid-based biological and functional materials and identify new potential fields in which amyloid-based structures promise new breakthroughs.

Formation and functionality of self-assembled whey protein microgels

Whey proteins spontaneously form spherical particles when heated in aqueous solutions at conditions where their net charge density is below a critical value. The particles are microgels consisting of a hydrated crosslinked network of proteins with a diameter between 100nm and 1μm. Stable suspensions of these microgels can be formed in a narrow range of conditions when the protein charge density is low enough to induce their formation, but high enough to inhibit further association into larger clusters or macroscopic gels. The formation of microgels and their application to stabilize emulsions and foams; form core-shell particles; form gels; or modify the texture of polysaccharide solutions and gels are reviewed.

Carboxylate Ion Pairing with Alkali-Metal Ions for β-Lactoglobulin and Its Role on Aggregation and Interfacial Adsorption

We report a combined experimental and computational study of the whey protein β-lactoglobulin (BLG) in different electrolyte solutions. Vibrational sum-frequency generation (SFG) and ellipsometry were used to investigate the molecular structure of BLG modified air-water interfaces as a function of LiCl, NaCl, and KCl concentrations. Molecular dynamics (MD) simulations and thermodynamic integration provided details of the ion pairing of protein surface residues with alkali-metal cations. Our results at pH 6.2 indicate that BLG at the air-water interface forms mono- and bilayers preferably at low and high ionic strength, respectively. Results from SFG spectroscopy and ellipsometry are consistent with intimate ion pairing of alkali-metal cations with aspartate and glutamate carboxylates, which is shown to be more effective for smaller cations (Li(+) and Na(+)). MD simulations show not only carboxylate-alkali-metal ion pairs but also ion multiplets with the alkali-metal ion in a bridging position between two or more carboxylates. Consequently, alkali-metal cations can bridge carboxylates not only within a monomer but also between monomers, thus providing an important dimerization mechanism between hydrophilic surface patches.

Native and thermally modified protein-polyphenol coassemblies: lactoferrin-based nanoparticles and submicrometer particles as protective vehicles for (-)-epigallocatechin-3-gallate

The interactions between native, thermally modified lactoferrin (LF) and (-)-epigallocatechin-3-gallate (EGCG) at pH 3.5, 5.0, and 6.5 were investigated. Turbidity, particle size, and charge of LF-EGCG complexes were mainly dominated by pH value and secondary structure of protein. At pH 3.5 and 5.0, LF-EGCG complexes were nanoparticles which had high ζ-potential, small size, and soluble state. At pH 6.5, they were submicrometer particles which exhibited low ζ-potential, large size, and insoluble state. The infrared spectra of freeze-dried LF-EGCG complexes showed that they were different from LF and EGCG alone. Far-UV CD results indicated that heat denaturation might irreversibly alter the secondary structure of LF and EGCG induced a progressive increase in the proportion of α-helix structure at the cost of β-sheet and unordered coil structure of LF at pH 3.5, 5.0, and 6.5. EGCG exhibited a strong affinity for native LF but a weak affinity for thermally modified LF at pH 5.0 and 6.5. An inverse result was observed at pH 3.5. These results could have potential for the development of food formulations based on LF as a carrier of bioactive compounds.

Heat-induced aggregation of whey proteins in aqueous solutions below their isoelectric point

Processing beverages containing high concentrations of globular proteins represents a technological challenge due to their instability during heating caused by protein aggregation and gelation. Aggregation of whey protein mixtures was investigated in aqueous model systems at pH 3.5, 4.0, and 4.5 at heating conditions resembling conventional industrial treatment (90 °C for 30 s). The extent of aggregation progressively decreased moving away from the pI. Protein aggregates became smaller and had a more open structure compared to higher pH values. Significant loss of protein dispersibility occurred at pH 4.0 and 4.5 above the denaturation T of whey protein (∼70 °C), at which aggregation was caused by intermolecular hydrophobic interactions. Accessible thiol groups were detected in the heat-treated systems, with a higher intensity at higher pH and increasing extent of aggregation. Intermolecular -S-S- bonding played only a minor role in the aggregation at all conditions studied.

Unlocked concanavalin A forms amyloid-like fibrils from coagulation of long-lived "crinkled" intermediates

Understanding the early events during amyloid aggregation processes is crucial to single out the involved molecular mechanisms and for designing ad hoc strategies to prevent and reverse amyloidogenic disorders. Here, we show that, in conditions in which the protein is positively charged and its conformational flexibility is enhanced, Concanavalin A leads to fibril formation via a non-conventional aggregation pathway. Using a combination of light scattering, circular dichroism, small angle X-ray scattering, intrinsic (Tryptophan) and extrinsic (ANS) fluorescence and confocal and 2-photon fluorescence microscopy we characterize the aggregation process as a function of the temperature. We highlight a multi-step pathway with the formation of an on-pathway long-lived intermediate and a subsequent coagulation of such “crinkled” precursors into amyloid-like fibrils. The process results in a temperature-dependent aggregation-coagulation pathway, with the late phase of coagulation determined by the interplay between hydrophobic and electrostatic forces. Our data provide evidence for the complex aggregation pathway for a protein with a highly flexible native conformation. We demonstrate the possibility to generate a long-lived intermediate whose proportion and occurrence are easily tunable by experimental parameters (i.e. temperature). As a consequence, in the case of aggregation processes developing through well-defined energy barriers, our results can open the way to new strategies to induce more stable in vitro on-pathway intermediate species through a minute change in the initial conformational flexibility of the protein. This will allow isolating and experimentally studying such transient species, often indicated as relevant in neurodegenerative diseases, both in terms of structural and cytotoxic properties.

Effect of fudosteine, a cysteine derivative, on airway hyperresponsiveness, inflammation, and remodeling in a murine model of asthma

AIMS

Fudosteine is a cysteine derivative that is used as an expectorant in chronic bronchial inflammatory disorders. It has been shown to decrease the number of goblet cells in an animal model. This study examined the effects of fudosteine on airway inflammation and remodeling in a murine model of chronic asthma.

MAIN METHODS

BALB/c mice were sensitized by an intraperitoneal injection of ovalbumin (OVA), and subsequently challenged with nebulized ovalbumin three days a week for four weeks. Seventy-two hours after the fourth challenge, airway hyperresponsiveness (AHR) and the cell composition of bronchoalveolar lavage (BAL) fluid were assessed. Fudosteine was administered orally at 10mg/kg or 100mg/kg body weight from the first to the fourth challenge.

KEY FINDINGS

We investigated the effects of fudosteine on the development of allergic airway inflammation and airway hyperresponsiveness after chronic allergen challenges. The administration of fudosteine during the challenge with ovalbumin prevented the development of airway hyperresponsiveness and accumulation of lymphocytes in the airways. Eotaxin, IL-4, and TGF-β levels and the relative intensity of matrix metalloproteinase-2 and matrix metalloproteinase-9 (MMP-2 and MMP-9) in BAL fluid were reduced by the fudosteine treatment; however, the number of eosinophils in BAL fluid and serum IgE levels did not change. The expression of TGF-β, the development of goblet cell hyperplasia, subepithelial collagenization, and basement membrane thickening were also reduced by the fudosteine treatment.

SIGNIFICANCE

These results indicate that fudosteine is effective in reducing airway hyperresponsiveness, airway inflammation, and airway remodeling in a murine model of chronic asthma.

Relation between gelation conditions and the physical properties of whey protein particles

Whey protein particles have several applications in modulating food structure and for encapsulation, but there is a lack of methods to prepare particles with a very high internal protein content. In this study whey protein particles with high internal protein content were prepared through emulsification and heat gelation of 25% (w/w) whey protein isolate solution at different pH (6.8 or 5.5) and NaCl concentrations (50, 200, or 400 mM). Particles formed at pH 6.8 were spherical, whereas those formed at pH 5.5 were irregular and had a cauliflower-like appearance. Both particles had an average size of few micrometers, and the particles formed at pH 5.5 had higher protein content (∼39% w/v) than the particles formed at pH 6.8 (∼18% w/v). Similarly, particle morphology and protein density were also affected by initial NaCl concentration: particles formed at 50 mM NaCl (pH 6.8) were spherical, whereas particles formed at either 200 mM NaCl (pH 6.7) or 400 mM NaCl (pH 6.6) were irregular and protein density of the particles increased with increasing initial NaCl concentration. Whey protein particles formed at pH 5.5 showed an excellent heat stability: viscosity of the suspensions containing approximately 30% of protein particles formed at pH 5.5 did not show any change after heating at 90 °C for 30 min while the viscosity of suspensions containing protein particles prepared at other conditions increased after heating. In summary, whey protein particles with varying microstructure, shape, internal protein density, and heat stability can be formed by using heat-induced gelation of whey protein isolate at different gelling conditions.

On the crucial importance of the pH for the formation and self-stabilization of protein microgels and strands

Stable suspensions of protein microgels are formed by heating salt-free β-lactoglobulin solutions at concentrations up to about C = 50 g·L(-1) if the pH is set within a narrow range between 5.75 and 6.1. The internal protein concentration of these spherical particles is about 150 g·L(-1) and the average hydrodynamic radius decreases with increasing pH from 200 to 75 nm. The formation of the microgels leads to an increase of the pH, which is a necessary condition to obtain stable suspensions. The spontaneous increase of the pH during microgel formation leads to an increase of their surface charge density and inhibits secondary aggregation. This self-stabilization mechanism is not sufficient if the initial pH is below 5.75 in which case secondary aggregation leads to precipitation. Microgels are no longer formed above a critical initial pH, but instead short, curved protein strands are obtained with a hydrodynamic radius of about 15-20 nm.

Recent progress in biopolymer nanoparticle and microparticle formation by heat-treating electrostatic protein-polysaccharide complexes

Functional biopolymer nanoparticles or microparticles can be formed by heat treatment of globular protein-ionic polysaccharide electrostatic complexes under appropriate solution conditions. These biopolymer particles can be used as encapsulation and delivery systems, fat mimetics, lightening agents, or texture modifiers. This review highlights recent progress in the design and fabrication of biopolymer particles based on heating globular protein-ionic polysaccharide complexes above the thermal denaturation temperature of the proteins. The influence of biopolymer type, protein-polysaccharide ratio, pH, ionic strength, and thermal history on the characteristics of the biopolymer particles formed is reviewed. Our current understanding of the underlying physicochemical mechanisms of particle formation and properties is given. The information provided in this review should facilitate the rational design of biopolymer particles with specific physicochemical and functional attributes, as well as stimulate further research in identifying the physicochemical origin of particle formation.