The mechanism of fibril formation of a non-inhibitory serpin ovalbumin revealed by the identification of amyloidogenic core regions

Binding Interaction of Coumarin Derivative Daphnetin with Ovalbumin: Molecular Insights into the Complexation Process and Effects of Metal Ions and pH in the Binding and Antifibrillation Studies

This study investigates the interaction between daphnetin and ovalbumin (OVA) as well as its potential to inhibit OVA fibrillation using both spectroscopic and computational analysis. A moderate binding affinity of 1 × 104 M-1 was observed between OVA and daphnetin, with a static quenched mechanism identified during the fluorescence quenching processes. Metal ions' (Cu2+ and Zn2+) presence led to an increase in the binding affinities of daphnetin toward OVA, mirroring a similar trend observed with the pH variation. Synchronous and 3D fluorescence studies indicated an increase in the polarity of the microenvironment surrounding the Trp residues during binding. Interestingly, circular dichroism and Fourier transform infrared studies showed a significant change in the secondary structure of OVA upon binding with daphnetin. The efficacy of daphnetin in inhibiting protein fibrillation was confirmed through thioflavin T and Congo Red binding assays along with fluorescence microscopic imaging analysis. The thermodynamic assessment showed positive ΔH° [+(29.34 ± 1.526) kJ mol-1] and ΔS° [+(181.726 ± 5.465) J mol-1] values, indicating the presence of the hydrophobic forces, while negative ΔG° signifies spontaneous binding interactions. These experimental findings were further correlated with computational analysis, revealing daphnetin dynamics within the binding site of OVA.

Salt-Promoted Fibrillation of Legume Proteins Enhanced Interfacial Modulus for Stabilization of HIPEs Encapsulating Carotenoids with Improved Nutritional Performance

The thermal acidic-treatment-induced fibrillation of legume proteins isolated from cowpea and mung bean was demonstrated to be promoted by salt. Worm-like thin prefibrilar intermediates were formed in low salt concentrations (0-75 mM), which twisted to be the thick and mature amyloid-like fibrils with multistrands as the salt content was elevated (150-300 mM). Absorption of the fibrils fabricated in high salt concentrations to the oil/water interface constructed the protein layer with a significantly higher interfacial modulus compared with the one formed by the fibrils fabricated in low salt concentrations. Consequently, they showed the superiority in stabilizing high internal phase emulsions (HIPEs) with oil volume fraction ratios higher than 74%. HIPEs stabilized by the high salt-concentration-induced legume protein fibrils had stronger capabilities not only in encapsulating liposoluble carotenoids but also in protecting their stability against heating, ultraviolet, and iron ion stimulus, compared with the one stabilized by the low-salt-concentration-induced legume protein fibrils. Bioaccessibilities of the carotenoids in simulating gastrointestinal (GI) digestion were significantly improved after encapsulation by the HIPEs, which were interestingly increased with the elevation of salt concentrations utilized for preparing the legume protein fibrils. Furthermore, the carotenoids-loading-HIPEs were injectable and showed in vivo nutritional functions of mitigating colitis.

Food amyloid fibrils are safe nutrition ingredients based on in-vitro and in-vivo assessment

Food protein amyloid fibrils have superior technological, nutritional, sensorial, and physical properties compared to native monomers, but there is as yet insufficient understanding of their digestive fate and safety for wide consumption. By combining SDS-PAGE, ELISA, fluorescence, AFM, MALDI-MS, CD, microfluidics, and SAXS techniques for the characterization of β-lactoglobulin and lysozyme amyloid fibrils subjected to in-vitro gastrointestinal digestion, here we show that either no noticeable conformational differences exist between amyloid aggregates and their monomer counterparts after the gastrointestinal digestion process (as in β-lactoglobulin), or that amyloid fibrils are digested significantly better than monomers (as in lysozyme). Moreover, in-vitro exposure of human cell lines and in-vivo studies with C. elegans and mouse models, indicate that the digested fibrils present no observable cytotoxicity, physiological abnormalities in health-span, nor accumulation of fibril-induced plaques in brain nor other organs. These extensive in-vitro and in-vivo studies together suggest that the digested food amyloids are at least equally as safe as those obtained from the digestion of corresponding native monomers, pointing to food amyloid fibrils as potential ingredients for human nutrition.

Amyloid-like aggregation of recombinant β-lactoglobulin at pH 3.5 and 7.0: Is disulfide bond removal the key to fibrillation?

Functional nanofibrils from globular proteins are usually formed by heating for several hours at pH 2.0, which induces acidic hydrolysis and consecutive self-association. The functional properties of these micro-metre-long anisotropic structures are promising for biodegradable biomaterials and food applications, but their stability at pH > 2.0 is low. The results presented here show that modified β-lactoglobulin can also form nanofibrils by heating at neutral pH without prior acidic hydrolysis; the key is removing covalent disulfide bonds. The aggregation behaviour of various recombinant β-lactoglobulin variants was systemically studied at pH 3.5 and 7.0. The suppression of intra- and intermolecular disulfide bonds by eliminating one to three out of the five cysteines makes the non-covalent interactions more prevalent and allow for structural rearrangement. This stimulated the linear growth of worm-like aggregates. Full elimination of all five cysteines led to the transformation of worm-like aggregates into actual fibril structures (several hundreds of nanometres long) at pH 7.0. This understanding of the role of cysteine in protein-protein interactions will help to identify proteins and protein modifications to form functional aggregates at neutral pH.

Morphology, Formation Kinetics and Core Composition of Pea and Soy 7S and 11S Globulin Amyloid Fibrils

Legume seed storage proteins can be induced to form amyloid fibrils upon heating at low pH, which could improve their functionality for use in food and materials. However, the amyloidogenic regions of legume proteins are largely unknown. Here, we used LC-MS/MS to determine the amyloid core regions of fibrils formed by enriched pea and soy 7S and 11S globulins at pH 2, 80 °C, and characterized their hydrolysis, assembly kinetics, and morphology. A lag phase was absent from the fibrillation kinetics of pea and soy 7S globulins, while 11S globulins and crude extracts displayed a similar lag time. Pea and soy protein fibrils differed in morphology, with most pea fibrils being straight and soy fibrils being worm-like. Pea and soy globulins were abundant in amyloid-forming peptides, with over 100 unique fibril-core peptides from pea 7S and around 50 unique fibril-core peptides identified from pea 11S, soy 7S, and soy 11S globulins. Amyloidogenic regions derive predominantly from the homologous core region of 7S globulins and the basic subunit of 11S globulins. Overall, pea and soy 7S and 11S globulins are rich in amyloidogenic regions. This study will help understand their fibrillation mechanism and engineer protein fibrils with specific structures and functionality.

Investigating the Sequence Determinants of the Curling of Amyloid Fibrils Using Ovalbumin as a Case Study

Highly ordered, straight amyloid fibrils readily lend themselves to structure determination techniques and have therefore been extensively characterized. However, the less ordered curly fibrils remain relatively understudied, and the structural organization underlying their specific characteristics remains poorly understood. We found that the exemplary curly fibril-forming protein ovalbumin contains multiple aggregation prone regions (APRs) that form straight fibrils when isolated as peptides or when excised from the full-length protein through hydrolysis. In the context of the intact full-length protein, however, the regions separating the APRs facilitate curly fibril formation. In fact, a meta-analysis of previously reported curly fibril-forming proteins shows that their inter-APRs are significantly longer and more hydrophobic when compared to straight fibril-forming proteins, suggesting that they may cause strain in the amyloid state. Hence, inter-APRs driving curly fibril formation may not only apply to our model protein but rather constitute a more general mechanism.

DMSO and TMAO-Differences in Interactions in Aqueous Solutions of the K-Peptide

Interactions between a solvent and their co-solute molecules in solutions of peptides are crucial for their stability and structure. The K-peptide is a synthetic fragment of a larger hen egg white lysozyme protein that is believed to be able to aggregate into amyloid structures. In this study, a complex experimental and theoretical approach is applied to study systems comprising the peptide, water, and two co-solutes: trimethylamide N-oxide (TMAO) or dimethyl sulfoxide (DMSO). Information about their interactions in solutions and on the stability of the K-peptide was obtained by FTIR spectroscopy and differential scanning microcalorimetry. The IR spectra of various osmolyte-water-model-peptide complexes were simulated with the DFT method (B3LYP/6-311++G(d,p)). The FTIR results indicate that both solutes are neutral for the K-peptide in solution. Both co-solutes affect the peptide to different degrees, as seen in the shape of its amide I band, and have different influences on its thermal stability. DFT calculations helped simplify the experimental data for easier interpretation.

Processing Induced Changes in Food Proteins: Amyloid Formation during Boiling of Hen Egg White

Amyloid fibrils (AFs) are highly ordered protein nanofibers composed of cross β-structure that occur in nature, but that also accumulate in age-related diseases. Amyloid propensity is a generic property of proteins revealed by conditions that destabilize the native state, suggesting that food processing conditions may promote AF formation. This had only been shown for foie gras, but not in common foodstuffs. We here extracted a dense network of fibrillar proteins from commonly consumed boiled hen egg white (EW) using chemical and/or enzymatic treatments. Conversion of EW proteins into AFs during boiling was demonstrated by thioflavin T fluorescence, Congo red staining, and X-ray fiber diffraction measurements. Our data show that cooking converts approximately 1-3% of the protein in EW into AFs, suggesting that they are a common component of the human diet.

Exploring the potential of infrared spectroscopy in qualitative and quantitative monitoring of ovalbumin amyloid fibrillation

Amyloid fibrils are highly ordered self-assembled (poly)peptide aggregates with cross-β structural pattern. Ovalbumin was used as a model for exploring the potential of infrared spectroscopy in detecting structural transitions and quantitative monitoring of amyloid fibrillation. Low pH (pH 2) and high temperature (90 °C) over the course of 24 h were conditions applied for amyloid formation. Fibrillation of ovalbumin was monitored by ThT and ANS fluorescence, and SDS PAGE. A significant increase in ThT fluorescence with a plateau reached after 4 h of incubation, without the lag phase, was detected. Structural transitions leading to amyloid fibrillation were analysed using all three Amide regions in ATR-FTIR spectra. Significant changes were detected in Amide I and Amide III region (decrease of α-helix and increase of β-sheet peaks). To establish a fast, precise and simple method for quantitative monitoring of amyloid fibrillation, the Amide I/Amide II ratios of aggregation specific β-sheets (1625 and 1695 cm^-1, respectively) with 1540 cm^-1 as internal standard were used, resulting in good correlation (R² = 0.93 and 0.95) with the data observed by monitoring ThT fluorescence. On the other hand, assessing aggregation specific β-sheet contents by self-deconvolution showed lower correlation with ThT fluorescence (R² = 0.75 and 0.64). Here we examined structural transitions during ovalbumin fibrillation in a qualitative and quantitative manner by exploiting the full potential of Amide regions simultaneously. Secondary structure distribution was monitored using second derivative spectra in Amide I region. A novel, simple mathematical calculation for quantitative monitoring of fibrils formation was presented employing that the increase in low and high frequency aggregation specific β-sheet in Amide I region compared to the internal standard in Amide II region is suitable for fibril formation monitoring.

Conditions Governing Food Protein Amyloid Fibril Formation-Part I: Egg and Cereal Proteins

Conditions including heating mode, time, temperature, pH, moisture and protein concentration, shear, and the presence of alcohols, chaotropic/reducing agents, enzymes, and/or salt influence amyloid fibril (AF) formation as they can affect the accessibility of amino acid sequences prone to aggregate. As some conditions applied on model protein resemble conditions in food processing unit operations, we here hypothesize that food processing can lead to formation of protein AFs with a compact cross β-sheet structure. This paper reviews conditions and food constituents that affect amyloid fibrillation of egg and cereal proteins. While egg and cereal proteins often coexist in food products, their impact on each other's fibrillation remains unknown. Hen egg ovalbumin and lysozyme form AFs when subjected to moderate heating at acidic pH separately. AFs can also be formed at higher pH, especially in the presence of alcohols or chaotropic/reducing agents. Tryptic wheat gluten digests can form fibrillar structures at neutral pH and maize and rice proteins do so in aqueous ethanol or at acidic pH, respectively.

Morphological Transformation of Peptide Nanoassemblies through Conformational Transition of Core-forming Peptides

Morphological control of nanostructures that are composed of amphiphilic di- or tri-block molecules by external stimuli broadens their applications for molecular containers, nanoreactors, and controlled release materials. In this study, triblock amphiphiles comprising oligo(ethylene glycol), oligo(l-lysine), and tetra(l-phenylalanine) were prepared for the construction of nanostructures that can transform accompanying α-to-β transition of core-forming peptides. Circular dichroic (CD) measurements showed that the triblock amphiphiles adopted different secondary structures depending on the solvent environment: they adopt β-sheet structures in aqueous solution, while α-helix structures in 25% 2,2,2-trifluoroethanol (TFE) solution under basic pH conditions. Transmission electron microscopic (TEM) observation revealed that the triblock amphiphiles formed vesicle structures in 25% TFE aq. Solvent exchange from 25% TFE to water induced morphological transformation from vesicles to arc-shaped nanostructures accompanying α-β conformational transition. The transformable nanostructures may be useful as novel smart nanomaterials for molecular containers and micro reactors.

Rational Design of Amyloid-Like Fibrillary Structures for Tailoring Food Protein Techno-Functionality and Their Potential Health Implications

To control and enhance protein functionality is a major challenge for food scientists. In this context, research on food protein fibril formation, especially amyloid fibril formation, holds much promise. We here first provide a concise overview of conditions, which affect amyloid formation in food proteins. Particular attention is directed towards amyloid core regions because these sequences promote ordered aggregation. Better understanding of this process will be key to tailor the fibril formation process. Especially seeding, that is, adding preformed protein fibrils to protein solutions to accelerate fibril formation holds promise to tailor aggregation and fibril techno-functionality. Some studies have already indicated that food protein fibrillation indeed improves their techno-functionality. However, much more research is necessary to establish whether protein fibrils are useful in complex food systems and whether and to what extent they resist food processing unit operations. In this review the effect of amyloid formation on gelation, interfacial properties, foaming, and emulsification is discussed. Despite their prevalent role as functional structures, amyloids also receive a lot of attention due to their association with protein deposition diseases, prompting us to thoroughly investigate the potential health impact of amyloid-like aggregates in food. A literature review on the effect of the different stages of the human digestive process on amyloid toxicity leads us to conclude that food-derived amyloid fibrils (even those with potential pathogenic properties) very likely have minimal impact on human health. Nevertheless, prior to wide-spread application of the technology, it is highly advisable to further verify the lack of toxicity of food-derived amyloid fibrils.

Ultrasonic Characterization of Amyloid-Like Ovalbumin Aggregation

Thermal processing conditions, pH, and salt content affect the formation of egg white ovalbumin amyloid, which was investigated using high-precision measurements of ultrasonic velocity and attenuation. These were related to fluorescence and particle size measurements. Fluorescence changes indicated the formation of amyloid-like aggregates that was enhanced by increasing time-temperature treatments. The ultrasonic velocity of ovalbumin after heating at neutral pH (60 min at 70 or 80 °C) was lower than that of unheated ovalbumin, whereas the attenuation was higher. The decrease in the velocity represents increased compressibility associated with a change in the compactness of the protein, whereas changes in attenuation are due to protein conformational changes. Heating ramp studies revealed transitions at approximately 58 and 73 °C. During heating at a constant temperature, the ultrasonic velocity decreased slowly with increasing heating time, indicating an increase in ovalbumin compressibility. It is suggested that the obtained amyloid-like ovalbumin aggregates contain a compact core surrounded by loosely packed protein segments.

Evaluation of protease resistance and toxicity of amyloid-like food fibrils from whey, soy, kidney bean, and egg white

The structural properties of amyloid fibrils combined with their highly functional surface chemistry make them an attractive new food ingredient, for example as highly effective gelling agents. However, the toxic role of amyloid fibrils in disease may cause some concern about their food safety because it has not been established unequivocally if consumption of food fibrils poses a health risk to consumers. Here we present a study of amyloid-like fibrils from whey, kidney bean, soy bean, and egg white to partially address this concern. Fibrils showed varied resistance to proteolytic digestion in vitro by either Proteinase K, pepsin or pancreatin. The toxicity of mature fibrils was measured in vitro and compared to native protein, early-stage-fibrillar protein, and sonicated fibrils in two immortalised human cancer cell lines, Caco-2 and Hec-1a. There was no reduction in the viability of either Caco-2 or Hec-1a cells after treatment with a fibril concentration of up to 0.25 mg/mL.

A kinetic study of ovalbumin fibril formation: the importance of fragmentation and end-joining

The ability to control the morphologies of biomolecular aggregates is a central objective in the study of self-assembly processes. The development of predictive models offers the surest route for gaining such control. Under the right conditions, proteins will self-assemble into fibers that may rearrange themselves even further to form diverse structures, including the formation of closed loops. In this study, chicken egg white ovalbumin is used as a model for the study of fibril loops. By monitoring the kinetics of self-assembly, we demonstrate that loop formation is a consequence of end-to-end association between protein fibrils. A model of fibril formation kinetics, including end-joining, is developed and solved, showing that end-joining has a distinct effect on the growth of fibrillar mass density (which can be measured experimentally), establishing a link between self-assembly kinetics and the underlying growth mechanism. These results will enable experimentalists to infer fibrillar morphologies from an appropriate analysis of self-assembly kinetic data.

Amyloid form of ovalbumin evokes native antigen-specific immune response in the host: prospective immuno-prophylactic potential

Amyloids are highly organized protein aggregates that arise from inappropriately folded versions of proteins or polypeptides under both physiological as well as simulated ambiences. Once thought to be irreversible assemblies, amyloids have begun to expose their more dynamic and reversible attributes depending upon the intrinsic properties of the precursor protein/peptide and experimental conditions such as temperature, pressure, structural modifications in proteins, or presence of chemicals in the reaction mixture. It has been repeatedly proposed that amyloids undergo transformation to the bioactive peptide/protein forms under specific conditions. In the present study, amyloids assembled from the model protein ovalbumin (OVA) were found to release the precursor protein in a slow and steady manner over an extended time period. Interestingly, the released OVA from amyloid depot was found to exhibit biophysical characteristics of native protein and reacted with native-OVA specific monoclonal as well as polyclonal antibodies. Moreover, antibodies generated upon immunization of OVA amyloidal aggregates or fibrils were found to recognize the native form of OVA. The study suggests that amyloids may act as depots for the native form of the protein and therefore can be exploited as vaccine candidates, where slow antigen release over extended time periods is a pre-requisite for the development of desired immune response.

Oxidation of an exposed methionine instigates the aggregation of glyceraldehyde-3-phosphate dehydrogenase

Glyceraldehyde-3-phosphate dehydrogenase (GAPDH) is a ubiquitous and abundant protein that participates in cellular energy production. GAPDH normally exists in a soluble form; however, following necrosis, GAPDH and numerous other intracellular proteins convert into an insoluble disulfide-cross-linked state via the process of "nucleocytoplasmic coagulation." Here, free radical-induced aggregation of GAPDH was studied as an in vitro model of nucleocytoplasmic coagulation. Despite the fact that disulfide cross-linking is a prominent feature of GAPDH aggregation, our data show that it is not a primary rate-determining step. To identify the true instigating event of GAPDH misfolding, we mapped the post-translational modifications that arise during its aggregation. Solvent accessibility and energy calculations of the mapped modifications within the context of the high resolution native GAPDH structure suggested that oxidation of methionine 46 may instigate aggregation. We confirmed this by mutating methionine 46 to leucine, which rendered GAPDH highly resistant to free radical-induced aggregation. Molecular dynamics simulations suggest that oxidation of methionine 46 triggers a local increase in the conformational plasticity of GAPDH that likely promotes further oxidation and eventual aggregation. Hence, methionine 46 represents a "linchpin" whereby its oxidation is a primary event permissive for the subsequent misfolding, aggregation, and disulfide cross-linking of GAPDH. A critical role for linchpin residues in nucleocytoplasmic coagulation and other forms of free radical-induced protein misfolding should now be investigated. Furthermore, because disulfide-cross-linked aggregates of GAPDH arise in many disorders and because methionine 46 is irrelevant to native GAPDH function, mutation of methionine 46 in models of disease should allow the unequivocal assessment of whether GAPDH aggregation influences disease progression.

Role of the N-terminal amphiphilic region of ovalbumin during heat-induced aggregation and gelation

Ovalbumin (OVA), a major globular protein in egg white, forms semiflexible fibrillar aggregates during heat-induced gelation. The N-terminal amphiphilic region (pN1-22) of OVA is removed after treatment with pepsin at pH 4 to leave a large OVA fragment (pOVA). The conformation and thermal stability of pOVA and OVA are similar, but the rheological strength of the heat-induced gel of pOVA is much lower compared to that of OVA. The aggregation rate of pOVA, which forms spherical aggregates, was lower than that of OVA. These results suggest that the N-terminal amphiphilic region of OVA facilitates the α-to-β conformational transition, which triggers OVA fibril formation. Heat treatment of OVA in the presence of pN1-22 consistently resulted in the formation of straight fibrils. The strength of OVA and collagen gels was increased when prepared in the presence of pN1-22, suggesting that pN1-22 may be used to control the properties of protein gels.

Nanoscopic Amyloid Pores Formed via Stepwise Protein Assembly

Protein aggregation leading to various nanoscale assemblies is under scrutiny due to its implications in a broad range of human diseases. In the present study, we have used ovalbumin, a model non-inhibitory serpin, to elucidate the molecular events involved in amyloid assembly using a diverse array of spectroscopic and imaging tools such as fluorescence, laser Raman, circular dichroism spectroscopy, and atomic force microscopy (AFM). The AFM images revealed a progressive morphological transition from spherical oligomers to nanoscopic annular pores that further served as templates for higher-order supramolecular assembly into larger amyloid pores. Raman spectroscopic investigations illuminated in-depth molecular details into the secondary structural changes of the protein during amyloid assembly and pore formation. Additionally, Raman measurements indicated the presence of antiparallel β-sheets in the amyloid core. Overall, our studies revealed that the protein conformational switch in the context of the oligomers triggers the hierarchical assembly into nanoscopic amyloid pores. Our results will have broad implications in the structural characterization of amyloid pores derived from a variety of disease-related proteins.

Aggregation and fibrillogenesis of proteins not associated with disease: a few case studies

While amyloid structures have been well characterised in a medical context, there is increasing interest in studying amyloid-like aggregates in other areas, such as food science and nanomaterials. Several proteins relevant to food processing, including serum albumen, lactoglobulin, lysozyme, ovalbumin, casein, and soy protein isolate have been shown to form fibrillar structures under both physiological and non-physiological conditions. These structures are likely to contribute to the structural characteristics of the final food product. In a biotechnological context, proteins such as insulin and eye lens crystallins can be induced to form amyloid structures which can subsequently be used in biotechnology. One example of this is the use of amyloid fibrils as a scaffold for the immobilisation of enzymes. Another current interest in amyloid fibrils is as a storage form for peptide hormones, including insulin, glucagon and calcitonin. Here, we give an overview of a selection of well characterised proteins that have been studied outside the context of disease.