Identification of peptides in aggregates formed during hydrolysis of beta-lactoglobulin B with a Glu and Asp specific microbial protease

Physicochemical and Microstructural Characterization of Whey Protein Films Formed with Oxidized Ferulic/Tannic Acids

Protein-based biodegradable packaging films are of environmental significance. The effect of oxidized ferulic acid (OFA)/tannic acid (OTA) on the crosslinking and film-forming properties of whey protein isolate (WPI) was investigated. Both of the oxidized acids induced protein oxidation and promoted WPI crosslinking through the actions of quinone carbonyl and protein sulfhydryl, and amino groups. OTA enhanced the tensile strength (from 4.5 MPa to max 6.7 MPa) and stiffness (from 215 MPa to max 376 MPa) of the WPI film, whereas OFA significantly increased the elongation at break. The water absorption capability and heat resistance of the films were greatly improved by the addition of OTA. Due to the original color of OTA, the incorporation of OTA significantly reduced light transmittance of the WPI film (λ 200–600 nm) as well as the transparency, whereas no significant changes were induced by the OFA treatment. Higher concentrations of OTA reduced the in vitro digestibility of the WPI film, while the addition of OFA had no significant effect. Overall, these two oxidized polyphenols promoted the crosslinking of WPI and modified the film properties, with OTA showing an overall stronger efficacy than OFA due to more functional groups available.

Modulation of amyloid fibrillation of bovine β-lactoglobulin by selective methionine oxidation

Deposition of oxidation-modified proteins during normal aging and oxidative stress are directly associated with systemic amyloidoses. Methionine (Met) is believed to be one of the most readily oxidisable amino acid residues of protein. Bovine beta-lactoglobulin (β-lg), a model globular whey protein, has been presented as a subsequent paradigm for studies on protein aggregation and amyloid formation. Herein, we investigated the effect of t-butyl hydroperoxide (tBHP)-induced oxidation on structure, compactness and fibrillation propensity of β-lg at physiological pH. Notably, whey protein modification, specifically Met residues, plays an important role in the dairy industry during milk processing and lowering nutritional value and ultimately affecting their technological properties. Several bio-physical studies revealed enhanced structural flexibility and aggregation propensity of oxidised β-lg in a temperature dependent manner. A molecular docking study is used to predict possible interactions with tBHP and infers selective oxidation of methionine residues at 7, 24 and 107 positions. From our studies, it can be corroborated that specific orientations of Met residues directs the formation of a partially unfolded state susceptible to fibrillation with possible different cytotoxic effects. Our studies have greater implications in deciphering the underlying mechanism of different whey proteins encountering oxidative stress. Our findings are also important to elucidate the understanding of oxidation induced amyloid fibrillation of protein which may constitute a new route to pave the way for a modulatory role of oxidatively stressed proteins in neurological disorders.

Occurrence of Peptide-Peptide Interactions during the Purification of Self-Assembling Peptide f1-8 from a β-Lactoglobulin Tryptic Hydrolysate

Self-assembling peptides have gained attention because of their nanotechnological applications. Previous work demonstrated that the self-assembling peptide f1-8 (Pf1-8) that is generated from the tryptic hydrolysis of β-lactoglobulin can form a hydrogel after several purification steps, including membrane filtration and consecutive washes. This study evaluates the impact of each processing step on peptide profile, purity, and gelation capacity of each fraction to understand the purification process of Pf1-8 and the peptide-peptide interactions involved. We showed that peptide-peptide interactions mainly occurred through electrostatic and hydrophobic interactions, influencing the fraction compositions. Indeed, the purity of Pf1-8 did not correlate with the number of wash steps. In addition to Pf1-8, two other hydrophobic peptides were identified, peptide f15-20, and peptide f41-60. The gelation observed could be induced either through peptide-peptide interactions or through self-assembling, both being driven by non-covalent bond and more specifically hydrophobic interactions.

Structural characterization of calcium-binding sunflower seed and peanut peptides and enhanced calcium transport by calcium complexes in Caco-2 cells

BACKGROUND

Peptide-Ca complexes can promote Ca absorption. The present study aimed to determine the transport mechanism and structural characteristics of sunflower seed and peanut peptides with high Ca binding capacity with respect to developing third-generation Ca supplements and functional food ingredients.

RESULTS

High Ca-binding fractions of 1-3 kDa sunflower seed peptide (SSP₄ ) and ≥ 10 kDa peanut peptide (PP₁ ) had higher amount of Ca transported than CaCl₂ and two hydrolyzed proteins in Caco-2 cells. SSP₄ and PP₁ were separated by Ca ion metal chelate affinity chromatography, and high Ca-binding fractions were observed for SSP₄ -P₂ and PP₁ -P₂ . The amino acid sequences of SSP₄ -P₂ and PP₁ -P₂ were characterized by high-performance liquid chromatography-electrospray ionization-time of flight mass spectrometry. Seven and eight peptides were identified from SSP₄ -P₂ and PP₁ -P₂ , respectively. These peptides had molecular weights ranging from 1500 Da to 2500 Da and a large number of characteristic amino acid sequences, such as EEEQQQ, EQ-QQQ-QQ, QQ-QQQQQ, E-EEE, EE-EEQ, RR, Q-QQ-QQQ, EE-EQ-EE-Q, QQ-QQQQ, and Q-QQQQ, where 'E' is glutamic acid and 'Q' is glutamine.

CONCLUSION

SSP₄ and PP₁ can promote Ca transport in Caco-2 cells without affecting cell permeability. The amino acid sequences of SSP₄ -P₂ and PP₁ -P₂ with high Ca-binding abilities contain characteristic sequences, such as continuous glutamic acid and glutamine, and have low molecular weights. © 2020 Society of Chemical Industry.

Soy peptide aggregates formed during hydrolysis reduced protein extraction without decreasing their nutritional value

Upon enzymatic hydrolysis, soy protein isolates showed a strong tendency to aggregate, presenting a significant loss of valuable proteins. This study mainly focused on the large insoluble aggregates formed during proteolysis, and the influence of heating was further explored for a better understanding of the mechanism involved. The results from SDS-PAGE and amino acid analysis clearly showed that the insoluble aggregates formed upon hydrolysis were aggregated peptides, mainly attributed to the hydrophobic interactions between peptides with hydrophobic amino acids (Val, Ala, Leu, Ile, Tyr, Phe, and Pro) and sulfur-containing (Met and Cys) residues. Heating of the hydrolysates further enhanced the peptide-protein interactions through hydrophobic forces and disulfide bonds, accelerating the aggregation, where fractions from the basic subunits of glycinin were particularly involved. Furthermore, taking into consideration the fact that aggregates had a high proportion of essential amino acids, the in vitro digestion properties of the aggregates were also investigated. Interestingly, the relatively pepsin-resistant aggregates showed a high degradability toward pancreatin, releasing low molecular weight peptides possessing a higher proportion of antioxidative amino acids, which therefore had a better antioxidant activity. These results indicated a potential use of the insoluble peptide aggregates as protein supplements or active delivery systems for human consumption.

Enzymatic hydrolysis of whey and its analysis

An attempt was made to hydrolyze proteins and lactose in whey to improve the nutritive value of this byproduct, and extend its application as an ingredient to healthy beverages. Flavourzyme in different concentrations was used at pH 7.0 to hydrolyze protein at 50 °C. pH stat method, SDS-PAGE and RP HPLC-MS were used to evaluate degree of protein hydrolysis, pattern of peptide formation and characterize smaller peptides in hydrolysate, respectively. Higher concentration of enzymes produced more number of small peptides. Protein hydrolysate was again hydrolyzed at 30 °C with β-galactosidase at pH 5.5 to hydrolyze lactose. HPLC analysis indicated the degree of lactose hydrolysis and number of tri/poly saccharides formed due to varied enzyme concentration. Results from the experiment can be utilized to formulate healthy whey beverages for specific purpose.

Effect of degree of hydrolysis on the bioavailability of corn gluten meal hydrolysates

BACKGROUND

Under the situation that the shortage of proteins and large quantity of corn gluten meal (CGM), which is a superior protein resource, is under-exploited, the enzymatic hydrolysis of CGM was employed to improve its bioavailability because of its special amino acid composition.

RESULTS

The apparent faecal digestibility and true faecal digestibility of all corn gluten meal hydrolysates (CGMHs) decreased in varied amounts compared with those of CGM. However, the protein efficiency ratio, the net protein ratio, the biological value, and the net protein utilization of the CGMHs with degree of hydrolysis (DH) of 4.94% and with DH of 10.06% increased significantly (P < 0.05). The results of in vitro gastro-intestinal digestion showed that the molecular weight distribution and amino acid composition among different DHs resulted in variances in bioavailability.

CONCLUSION

Partial hydrolysis of CGM can improve its bioavailability, providing a future protein supplement for protein resources.

Protein-peptide interaction: study of heat-induced aggregation and gelation of β-lactoglobulin in the presence of two peptides from its own hydrolysate

Two peptides, [f135-158] and [f135-162]-SH, were used to study the binding of the peptides to native β-lactolobulin, as well as the subsequent effects on aggregation and gelation of β-lactoglobulin. The binding of the peptide [f135-158] to β-lactoglobulin at room temperature was confirmed by SELDI-TOF-MS. It was further illustrated by increased turbidity of mixed solutions of peptide and protein (at pH 7), indicating association of proteins and peptides in larger complexes. At pH below the isoelectric point of the protein, the presence of peptides did not lead to an increased turbidity, showing the absence of complexation. The protein-peptide complexes formed at pH 7 were found to dissociate directly upon heating. After prolonged heating, extensive aggregation was observed, whereas no aggregation was seen for the pure protein or pure peptide solutions. The presence of the free sulfhydryl group in [f135-162]-SH resulted in a 10 times increase in the amount of aggregation of β-lactoglobulin upon heating, illustrating the additional effect of the free sulfhydryl group. Subsequent studies on the gel strength of heat-induced gels also showed a clear difference between these two peptides. The replacement of additional β-lactoglobulin by [f135-158] resulted in a decrease in gel strength, whereas replacement by peptide [f135-162]-SH increased gel strength.

Re-formation of fibrils from hydrolysates of β-lactoglobulin fibrils during in vitro gastric digestion

In this study, in vitro digestion of β-lactoglobulin (β-Lg) fibrils and the re-formation of fibril-like structures after prolonged enzymatic hydrolysis (up to 48 h) were investigated using matrix-assisted laser desorption/ionization mass spectrometry (MALDI-MS), thioflavin T fluorescence photometry, and transmission electron microscopy (TEM). Pure β-Lg fibrils that had been formed by heat treatment at pH 2.0 were rapidly hydrolyzed by pepsin in the simulated gastric fluid (pH 1.2), and some new peptides that were suitable for further fibril formation were produced. TEM showed that the new fibrils were long and straight but thinner than the original fibrils, and both TEM and MALDI-MS indicated that the peptides in the new fibrils were shorter/smaller than the peptides in the original fibrils. The formation of new fibrils was found to be affected more by pH than by enzyme activity or temperature.

Antihypertensive peptides: production, bioavailability and incorporation into foods

Bioactive food peptides are encrypted within the sequence of food proteins but can be released during food processing (by enzymatic hydrolysis or fermentation) or during gastrointestinal transit. Among bioactive food peptides, those with antihypertensive activity are receiving special attention due to the high prevalence of hypertension in the Western countries and its role in cardiovascular diseases. This paper reviews the current literature on antihypertensive food peptides, focusing on the main methodologies for their production, such as enzymatic hydrolysis, fermentation and the use of recombinant bacteria. This paper also describes the structure/activity relationship of angiotensin-converting enzyme (ACE)-inhibitory peptides, as well as their bioavailability, physiological effects demonstrated by both in vitro and in vivo assays, and the contribution of mechanisms of action other than ACE inhibition. Finally, current reported strategies for incorporation of antihypertensive peptides into foods and their effects on both availability and activity are revised in this manuscript.

In vitro digestion of beta-lactoglobulin fibrils formed by heat treatment at low pH

Extensive studies have been done on beta-lactoglobulin (beta-Lg) fibrils in the past decade due to their potential as functional food ingredients, gelling agents, and encapsulation devices etc. (van der Goot, A. J.; Peighambardoust, S. H.; Akkermans, C.; van Oosten-Manski, J. M. Creating novel structures in food materials: The role of well-defined shear flow. Food Biophys. 2008, 3(2), 120-125 and Loveday, S. M.; Rao, M. A.; Creamer, L. K.; Singh, H. Factors affecting rheological characteristics of fibril gels: The case of beta-lactoglobulin and alpha-lactalbumin. J. Food Sci. 2009, 74 (3), R47-R55). However, most of the studies focus on the formation and mechanism of the fibrils. Little is known about fibril digestibility to date. In this work, in vitro pepsin digestion of bovine beta-lactoglobulin (beta-Lg) fibrils in simulated gastric fluid was investigated using thioflavin T fluorescence photometry, sodium dodecyl sulfate-polyacrylamide gel electrophoresis, size-exclusion chromatography, matrix-assisted laser desorption/ionization mass spectrometry, and transmission electron microscopy (TEM). The fibrils were formed by heating beta-Lg solutions at 80 degrees C and pH 2.0 for 20 h. The fibrils were found to be digested completely by pepsin within 2 min, when long, straight fibrils were no longer observed by TEM. The peptides in the fibrils (2000-8000 Da) could be digested to smaller peptides (mostly <2000 Da) by pepsin. The peptides in the fibrils were believed to be more susceptible for pepsin to access and attack because of their hydrophobic nature. For comparison purposes, solutions of beta-Lg heated at neutral pH (pH 7.4) were also studied under the same conditions.

Isolation and characterization of an aggregating peptide from a tryptic hydrolysate of whey proteins

Spontaneous precipitation of a peptide mixture has been observed during the concentration by reverse osmosis of a tryptic hydrolysate of whey protein. The precipitated material collected by centrifugation could not be solubilized by urea, mercaptoethanol, or sodium dodecyl sulfate. However, a complete solubilization of the aggregates was observed when the pH of the solution was lowered to 2.0. Analysis of the insoluble fraction has allowed the identification of beta-lactoglobulin (beta-lg) fragment 1-8 as the major peptide involved in the formation of aggregates. Peptide beta-lg f1-8 accounted for >94% of the peptide content in the precipitate washed twice with distilled water. The investigation of the secondary structure using circular dichroism evidenced that the peptide beta-lg f1-8 isolated from the flocculated peptide mixture is under random coil conformation at acidic and neutral pH and tends to adopt a beta-sheet conformation at basic pH. The findings of this study provide evidence that peptide beta-lg f1-8 forms aggregates via an efficient self-assembly process.

Hydrolysis of whey protein isolate with Bacillus licheniformis protease: aggregating capacities of peptide fractions

In a previous study, peptides aggregating at pH 7.0 derived from a whey protein hydrolysate made with Bacillus licheniformis protease were fractionated and identified. The objective of the present work was to investigate the solubility of the fractionated aggregating peptides, as a function of concentration, and their aggregating capacities toward added intact proteins. The amount of aggregated material and the composition of the aggregates obtained were measured by nitrogen concentration and size exclusion chromatography, respectively. The results showed that of the four fractions obtained from the aggregating peptides, two were insoluble, while the other two consisted of 1:1 mixture of low and high solubility peptides. Therefore, insoluble peptides coaggregated, assumedly via hydrophobic interactions, other relatively more soluble peptides. It was also shown that aggregating peptides could aggregate intact protein nonspecifically since the same peptides were involved in the aggregation of whey proteins, beta-casein, and bovine serum albumin. Both insoluble and partly insoluble peptides were required for the aggregation of intact protein. These results are of interest for the applications of protein hydrolysates, as mixtures of intact protein and peptides are often present in these applications.

Enzyme-induced aggregation and gelation of proteins

This paper provides a brief overview of the effects of protein hydrolysis on aggregation and gel forming properties of protein systems. Among the food globular proteins, whey proteins and soy proteins are the most extensively studied for their ability to form different textures upon proteolysis. Recent studies were focused on identifying aggregating peptides and on mechanisms of aggregation and gelation.

Hydrolysis of whey protein isolate with Bacillus licheniformis protease: fractionation and identification of aggregating peptides

The objective of this work was to identify the dominant aggregating peptides from a whey protein hydrolysate (degree of hydrolysis of 6.8%) obtained with Bacillus licheniformis protease. The aggregating peptides were fractionated with preparative reversed-phase chromatography and identified with liquid chromatography-mass spectrometry. The results showed that the dominant aggregating peptide, at pH 7.0, was beta-lg AB [f1-45]. In addition, the peptides beta-lg AB [f90-108]-S-S-alpha-la [f50-113], alpha-la [f12-49]-S-S-alpha-la [f50-113], beta-lg AB [f90-108]-S-S-beta-lg AB [f90-108], beta-lg A [f90-157], and beta-lg AB [f135-157/158] were also identified as main aggregating peptides. The results further showed that aggregation, via hydrophobic interactions, prevented further digestion (at pH 8.0), thereby explaining the large size of the aggregating peptides. It is hypothesized that B. licheniformis protease breaks down hydrophilic segments in the substrate and, therefore, preserves hydrophobic segments that aggregate once exposed to the solvent.

Protein-peptide interactions in mixtures of whey peptides and whey proteins

The effects of several conditions on the amounts and compositions of aggregates formed in mixtures of whey protein hydrolysate, made with Bacillus licheniformis protease, and whey protein isolate were investigated using response surface methodology. Next, the peptides present in the aggregates were separated from the intact protein and identified with liquid chromatography-mass spectrometry. Increasing both temperature and ionic strength increased the amounts of both intact protein and peptides in the aggregates. There was an optimal amount of added intact WPI that could aggregate with peptides, yielding a maximal amount of aggregated material in which the peptide/protein molar ratio was around 6. Under all conditions applied, the same peptides were observed in the protein-peptide aggregates formed. The dominant peptides were beta-lg AB [f1-45], beta-lg AB [f90-108], and alpha-la [f50-113]. It was hypothesized that peptides could form a kind of glue network that can include beta-lactoglobulin via hydrophobic interactions at the hydrophobic binding sites at the surface of the protein.

Aggregation properties of whey protein hydrolysates generated with Bacillus licheniformis proteinase activities

Hydrolysis of whey protein concentrate (WPC) with Alcalase 2.4 L, a Bacillus licheniformis proteinase preparation, induces gelation. The aggregation behavior of WPC hydrolysates generated with Alcalase and Prolyve 1000, a Bacillus licheniformis proteinase that did not induce gelation, were studied by turbidity and particle size analysis. With the use of synthetic peptide substrates, it was shown that Alcalase contains a glutamyl endopeptidase (GE) activity not present in Prolyve. Comparison of the aggregation behavior of WPC hydrolysates generated with Alcalase, Prolyve, and combinations of Prolyve with a GE activity isolated from Alcalase showed that GE was responsible for the observed enzyme-induced peptide aggregation in Alcalase hydrolysates. Hydrolysates generated with Prolyve, having a degree of hydrolysis (DH) of 11.8% and 10.4% of peptide material greater than 10 kDa, could be induced to aggregate by the addition of GE. These results emphasize the contribution of enzyme specificity to the physicochemical and functional characteristics of proteinase hydrolysates of WPC.

Gel Formation of Peptides Produced by Extensive Enzymatic Hydrolysis of β-Lactoglobulin

The purpose of the present study was to identify which peptides were responsible for enzyme-induced gelation of extensively hydrolyzed beta-lactoglobulin with Alcalase in order to gain insight into the mechanism of gelation. Dynamic rheology, aggregation measurements, isoelectrofocusing as well as chromatography and mass spectrometry were used to understand the gel formation. A transparent gel was formed above a critical concentration of peptides while noncovalently linked aggregates appear with increasing time of hydrolysis. Extensive hydrolysis was needed for gelation to occur as indicated by the small size of the peptides. Isoelectrofocusing was successful at separating the complex mixture, and 19 main peptides were identified with molecular weight ranging from 265 to 1485 Da. Only one fragment came from a beta-sheet rich region of the beta-lactoglobulin molecule, and a high proportion of peptides had proline residues in their sequence.

Enzyme-induced gelation of extensively hydrolyzed whey proteins by Alcalase: peptide identification and determination of enzyme specificity

Extensive hydrolysis of whey protein isolate by Alcalase was shown to induce gelation mainly via hydrophobic interactions. The aim of this work was to characterize the peptides released in order to better understand this phenomenon. The apparent molecular mass distribution indicated that aggregates were formed by small molecular mass peptides (<2000 Da). One hundred and thirty peptides with various lengths were identified by reversed-phase high-performance liquid chromatography coupled with electrospray ionization mass spectrometry. Alcalase was observed to have a high specificity for aromatic (Phe, Trp, and Tyr), acidic (Glu), sulfur-containing (Met), aliphatic (Leu and Ala), hydroxyl (Ser), and basic (Lys) residues. Most peptides had an average hydrophobicity of 1-1.5 kcal/residue and a net charge of 0 at the pH at which gelation occurred (6.0). Therefore, an intermolecular attractive force such as hydrophobic interaction suggests the formation of aggregates that further leads to the formation of a gel.

Effect of physicochemical conditions on peptide-peptide interactions in a tryptic hydrolysate of beta-lactoglobulin and identification of aggregating peptides

The objective of this study was to characterize the changes in peptide solubility resulting from changing some physicochemical conditions in a tryptic hydrolysate of beta-lactoglobulin (beta-LG). The turbidity (500 nm) of a 1% solution of tryptic peptides was measured at pH 3-10, at 5, 25, and 50 degrees C, in the presence of different salt concentrations (0, 0.5, and 1 M NaCl), in the presence of denaturing and reducing agents (6 M urea, 5% SDS, or 5% beta-mercaptoethanol), and under an electric field (isoelectric focusing). The results reveal an increase in turbidity of the peptide solution at pH 4, but a slight increase in turbidity was also observed at pH 8, which is attributable to peptides linked by disulfide bridges. The effect of temperature and ionic strength on the turbidity occurring at pH 4 indicates that mainly hydrophobic interactions are involved in the aggregation process. The material in the precipitate at pH 4 was identified as the peptides beta-LG 1-8, 15-20, and 41-60 and non-hydrolyzed alpha-lactalbumin. These results suggest that a limited number of peptides are involved in the aggregation process observed at pH 4, some of which having bioactive (beta-LG 15-20, ACE inhibitor, and opioid) or emulsifying properties (beta-LG 41-60). Aggregation of these peptides at acidic pH indicates that a simple acidification step could represent an easy process for isolating peptidic fractions enriched in bioactive or functional peptides.