Mechanisms of tannin-induced trypsin inhibition: a molecular approach

Development of a cell-based quaternary system to unveil the effect of pectic polysaccharides on oral astringency

Phenolic compounds are responsible for food unpleasant taste properties, including astringency, due to their ability to interact with salivary proteins and oral constituents. Astringency is a crucial attribute for consumer's acceptability. To fulfill the demand for both healthy and tasty food, polysaccharides raise as a good alternative to modulate astringency. In this work, a cell-based quaternary system was developed to evaluate the ability of polysaccharides to reduce the interaction between two classes of hydrolysable tannins - gallotannins (tannic acid) and ellagitannins (punicalagin) - and oral constituents (cells, salivary proteins and mucosal pellicle). So, pectic polysaccharide fractions isolated from grape skins, imidazole soluble polysaccharides (ISP) and carbonate soluble polysaccharides (CSP), as well as a commercial pectin, were tested. Results showed that the polysaccharide's effect depends on the structural features of the molecules involved. CSP fraction and pectin were the most effective, reducing the interactions between both tannins and the oral constituents, mainly in the complete oral model. The highest uronic acid content and the presence of methyl esterified groups could explain their high reduction ability. For tannic acid, the reduction effect increased along with the galloylation degree, while the interaction of β-punicalagin with the oral constituents was practically inhibited at 3.0 mg.mL-1.

Monitoring the Interaction between Thermally Induced Whey Protein and Anthocyanin by Fluorescence Quenching Spectroscopy

The color stability of anthocyanins was shown to improve with addition of whey proteins (WP). The goal of this study was to investigate the binding mechanisms of purple corn, grape and black carrot anthocyanin extracts to native and preheated WP (40-80 °C, 3.6 μM) at a pH of 3 using fluorescence quenching spectroscopy. The fluorescence spectra were collected with an excitation wavelength of 280 nm at 25 °C, 35 °C and 45 °C. The quenching data were analyzed by using the Stern-Volmer equation. The fluorescence intensity of WP decreased (up to 73%) and its λmax increased (by ~5 nm) with increasing anthocyanin concentration (0-100 μM). The quenching data showed that the interaction between anthocyanin extracts and WP was a static quenching process. Thermodynamic analysis showed their binding was mainly through hydrophobic interactions. Their binding affinity was higher for preheated WP than native WP and decreased gradually with increasing preheating temperature. Black carrot anthocyanin extract had the lowest binding affinity with WP, likely due to the larger molecular structure. These results help better understand the protection mechanism of native and preheated WP on anthocyanin color stability, expanding the application of anthocyanins as food colorants that better withstand processing and storage.

Polysaccharide selection and mechanism for prevention of protein-polyphenol haze formation in beverages

Polysaccharides have been considered as a group of promising candidate for preventing the protein-polyphenol haze formation in beverages. In order to select effective polysaccharides to prevent the haze formation, four protein-polyphenol haze model systems were successfully established using two proteins (i.e., gelatin and bovine serum albumin) and two polyphenols (i.e., procyanidin [PC] and epigallocatechin gallate [EGCG]). Among seven common polysaccharides, 0.5 mg/mL pectin, 0.05 mg/mL xanthan gum, and 0.01 mg/mL guar gum demonstrated the maximum potential for preventing the formation of four protein-polyphenol hazes. Ultraviolet-visible spectrophotometry confirmed that polysaccharides affected protein-polyphenol interactions. Fluorescence spectrophotometry combined with microscale thermophoresis data indicated the relative affinities of polyphenol to protein and polysaccharide determined the mechanism of polysaccharide for preventing the haze formation. In bovine serum albumin (BSA)/gelatin-EGCG system, polysaccharides (pectin, xanthan gum and guar gum) competed with BSA/gelatin to bind EGCG for prevention the formation of BSA/gelatin-EGCG haze. However, in BSA/gelatin-PC system, polysaccharides (pectin, xanthan gum, and guar gum) formed a ternary complex (protein-tannin-polysaccharide) for increasing the solubility of protein-polyphenol aggregation. From apple juice results, the reduction rates of guar gum in two apple juice systems (gelatin-PC, BSA-PC) were 21% and 56% within 8 weeks, indicating guar gum might be the most effective polysaccharide in preventing the haze formation. PRACTICAL APPLICATION: This experiment data could be used for development of polysaccharide products for prevention of protein-polyphenol haze formation in beverages.

Comparative study of concatemer efficiency as an isotope-labelled internal standard for allergen quantification

Mass spectrometry-based methods coupled with stable isotope dilution have become effective and widely used methods for the detection and quantification of food allergens. Current methods target signature peptides resulting from proteolytic digestion of proteins of the allergenic ingredient. The choice of appropriate stable isotope-labelled internal standard is crucial, given the diversity of encountered food matrices which can affect sample preparation and analysis. We propose the use of concatemer, an artificial and stable isotope-labelled protein composed of several concatenated signature peptides as internal standard. With a comparative analysis of three matrices contaminated with four allergens (egg, milk, peanut, and hazelnut), the concatemer approach was found to offer advantages associated with the use of labelled proteins, ideal but unaffordable, and circumvent certain limitations of traditionally used synthetic peptides as internal standards. Although used in the proteomic field for more than a decade, concatemer strategy has not yet been applied for food analysis.

Enhancement of Biomimetic Enzymatic Mineralization of Gellan Gum Polysaccharide Hydrogels by Plant-Derived Gallotannins

Mineralization of hydrogel biomaterials with calcium phosphate (CaP) is considered advantageous for bone regeneration. Mineralization can be both induced by the enzyme alkaline phosphatase (ALP) and promoted by calcium-binding biomolecules, such as plant-derived polyphenols. In this study, ALP-loaded gellan gum (GG) hydrogels were enriched with gallotannins, a subclass of polyphenols. Five preparations were compared, namely three tannic acids of differing molecular weight (MW), pentagalloyl glucose (PGG), and a gallotannin-rich extract from mango kernel (Mangifera indica L.). Certain gallotannin preparations promoted mineralization to a greater degree than others. The various gallotannin preparations bound differently to ALP and influenced the size of aggregates of ALP, which may be related to ability to promote mineralization. Human osteoblast-like Saos-2 cells grew in eluate from mineralized hydrogels. Gallotannin incorporation impeded cell growth on hydrogels and did not impart antibacterial activity. In conclusion, gallotannin incorporation aided mineralization but reduced cytocompatibility.

Complex Tannins Isolated from Jelly Fig Achenes Affect Pectin Gelation through Non-Specific Inhibitory Effect on Pectin Methylesterase

Jelly fig (Ficus awkeotsang Makino) is used to prepare drinks and desserts in Asia, owing to the gelling capability of its pectin via endogenous pectin methylesterase (PE) catalyzation. Meanwhile, substances with PE inhibitory activity (SPEI) in jelly fig achenes (JFA) residue were noticed to be able to impede the gelation. In this study, we characterized and isolated SPEI from JFA by a series of PE inhibition-guided isolations. Crude aqueous extract of JFA residue was mixed with acetone, and 90% acetone-soluble matter was further fractionated by Diaion HP-20 chromatography. The retained fraction with dominant PE inhibitory activity was collected from 100% methanol eluate. Results from high-performance liquid chromatography mass spectrometry (HPLC/MS) and hydrolysis-induced chromogenic transition revealed the SPEI as complex tannins. Total tannins content was determined in each isolated fraction, and was closely related to PE inhibitory activity. In addition, SPEI in this study could inhibit activities of digestive enzymes in vitro and may, therefore, be assumed to act as non-specific protein binding agent.

Nutritional Composition and Bioactive Content of Legumes: Characterization of Pulses Frequently Consumed in France and Effect of the Cooking Method

Pulses display nutritional benefits and are recommended in sustainable diets. Indeed, they are rich in proteins and fibers, and can contain variable amounts of micronutrients. However, pulses also contain bioactive compounds such as phytates, saponins, or polyphenols/tannins that can exhibit ambivalent nutritional properties depending on their amount in the diet. We characterized the nutritional composition and bioactive compound content of five types of prepared pulses frequently consumed in France (kidney beans, white beans, chickpeas, brown and green lentils, flageolets), and specifically compared the effects of household cooking vs. canning on the composition of pulses that can be consumed one way or the other. The contents in macro-, micronutrients, and bioactive compounds highly varied from one pulse to another (i.e., 6.9 to 9.7 g/100 g of cooked product for proteins, 4.6 to 818.9 µg/100 g for lutein or 15.0 to 284.3 mg/100 g for polyphenols). The preparation method was a key factor governing pulse final nutritional composition in hydrophilic compounds, depending on pulse species. Canning led to a greater decrease in proteins, total dietary fibers, magnesium or phytate contents compared to household cooking (i.e., −30%, −44%, −33% and −38%, p < 0.05, respectively, in kidney beans). As canned pulses are easy to use for consumers, additional research is needed to improve their transformation process to further optimize their nutritional quality.

Inhibition of the Exoglucanase Cel7A by a Douglas-Fir-Condensed Tannin

Douglas-fir forestry residues are a potential feedstock for saccharification-based biofuels, and condensed tannins are expected to make up ∼3% of the dry mass of this feedstock. Condensed tannins are well-known for their ability to interact with proteins and can bind and inhibit cellulase enzymes used in saccharification. In this study, we use molecular docking and classical molecular dynamics simulations to investigate how a characterized condensed tannin from Douglas-fir bark binds to the exoglucanase Cel7A from Trichoderma reesei. Through looking at the "occupancy" and "residency" of specific amino acid residue-tannin interactions, we find that the binding sites are characterized by many simultaneous tannin-enzyme interactions with the strongest occurring on the catalytic module as opposed to the carbohydrate-binding module. The simulations indicate that tannin inhibition can result from binding at or near the catalytic tunnel's entrance and exit. The analyzed tannin further prefers to bind to loops around the catalytic region and has affinity for aromatic and charged amino acid residues. These insights provide direction for the rational design of tannin-resistant cellulases.

Effect of preheat treatment of milk proteins on their interactions with cyanidin-3-O-glucoside

In this study, the binding of cyanidin-3-O-glucoside (C3G) to preheated milk proteins β-lactoglobulin (β-Lg) and β-casein (β-CN) at 55-90 °C under pH 3.6 and pH 6.3 was investigated using multi-spectral techniques. Fluorescence quenching spectroscopy data showed C3G quenched milk proteins' fluorescence strongly. Thermodynamic analysis revealed that C3G bound to β-Lg mainly through hydrogen bonding and hydrophobic interactions, and that their binding affinity increased gradually with increasing preheating temperature at pH 6.3, whereas it decreased at pH 3.6. Hydrogen bonding and van der Waals forces played the major roles in the interaction of β-CN with C3G, their affinity decreasing with increasing preheating temperature at both pH values. The combination of C3G and preheated β-Lg at 85 °C had the strongest binding affinity, with a K_A of 14.10 (±0.33) × 10⁵ M^-1 (pH 6.3, 298 K). Preheating of milk proteins did not change their major forces with C3G. Fourier transform infrared spectra (FT-IR) results showed that C3G binding altered the secondary structures of β-Lg and β-CN by reducing the proportion of α-helix and β-sheet structures and increasing the proportion of random coil and turn structures. The structural changes of preheated β-Lg upon C3G binding were more pronounced than that of native β-Lg, while there was little difference between preheated and native β-CN in their structural changes upon C3G binding. These results will be helpful in better understanding the relevance of native and preheated milk protein-C3G interactions to the stability of C3G, and in promoting its application in the food industry as a natural pigment.

Molecular Interaction Between Salivary Proteins and Food Tannins

Polyphenols interaction with salivary proteins (SP) has been related with organoleptic features such as astringency. The aim of this work was to study the interaction between some human SP and tannins through two spectroscopic techniques, fluorescence quenching, and saturation transfer difference-nuclear magnetic resonance (STD-NMR). Generally, the results showed a significant interaction between SP and both condensed tannins and ellagitannins. Herein, STD-NMR proved to be a useful tool to map tannins' epitopes of binding, while fluorescence quenching allowed one to discriminate binding affinities. Ellagitannins showed the greatest binding constants values (K_SV from 20.1 to 94.1 mM^-1; K_A from 0.7 to 8.3 mM^-1) in comparison with procyanidins (K_SV from 5.4 to 40.0 mM^-1; K_A from 1.1 to 2.7 mM^-1). In fact, punicalagin was the tannin that demonstrated the highest affinity for all three SP. Regarding SP, P-B peptide was the one with higher affinity for ellagitannins. On the other hand, cystatins showed in general the lower K_SV and K_A values. In the case of condensed tannins, statherin was the SP with the highest affinity, contrasting with the other two SP. Altogether, these results are evidence that the distinct SP present in the oral cavity have different abilities to interact with food tannins class.

Interaction between Wine Phenolic Acids and Salivary Proteins by Saturation-Transfer Difference Nuclear Magnetic Resonance Spectroscopy (STD-NMR) and Molecular Dynamics Simulations

The interaction between phenolic compounds and salivary proteins is highly related to the astringency perception. Recently, it has been proven the existence of synergisms on the perceived astringency when phenolic acids were tested as mixtures in comparison to individual compounds, maintaining constant the total amount of the stimulus. The interactions between wine phenolic acids and the peptide fragment IB7₁₂ have been studied by saturation-transfer difference (STD) NMR spectroscopy. This technique provided the dissociation constants and the percentage of interaction between both individual and mixtures of hydroxybenzoic and hydroxycinnamic acids and the model peptide. It is noteworthy that hydroxybenzoic acids showed higher affinity for the peptide than hydroxycinnamic acids. To obtain further insights into the mechanisms of interaction, molecular dynamics simulations have been performed. Results obtained not only showed the ability of these compounds to interact with salivary proteins but also may justify the synergistic effect observed in previous sensory studies.

Polyphenolic Compounds and Digestive Enzymes: In Vitro Non-Covalent Interactions

The digestive enzymes–polyphenolic compounds (PCs) interactions behind the inhibition of these enzymes have not been completely studied. The existing studies have mainly analyzed polyphenolic extracts and reported inhibition percentages of catalytic activities determined by UV-Vis spectroscopy techniques. Recently, pure PCs and new methods such as isothermal titration calorimetry and circular dichroism have been applied to describe these interactions. The present review focuses on PCs structural characteristics behind the inhibition of digestive enzymes, and progress of the used methods. Some characteristics such as molecular weight, number and position of substitution, and glycosylation of flavonoids seem to be related to the inhibitory effect of PCs; also, this effect seems to be different for carbohydrate-hydrolyzing enzymes and proteases. The digestive enzyme–PCs molecular interactions have shown that non-covalent binding, mostly by van der Waals forces, hydrogen binding, hydrophobic binding, and other electrostatic forces regulate them. These interactions were mainly associated to non-competitive type inhibitions of the enzymatic activities. The present review emphasizes on the digestive enzymes such as α-glycosidase (AG), α-amylase (PA), lipase (PL), pepsin (PE), trypsin (TP), and chymotrypsin (CT). Existing studies conducted in vitro allow one to elucidate the characteristics of the structure–function relationships, where differences between the structures of PCs might be the reason for different in vivo effects.

Small-Angle X-ray Scattering Study of Protein Complexes with Tea Polyphenols

Exploration of the structure of protein complexes, especially the change in conformation and aggregation behavior of proteins upon ligand binding, is crucial to clarify their bioactivities at the molecular level. We applied solution small-angle X-ray scattering (SAXS) to study the complex structure of bovine serum albumin (BSA) and trypsin binding with tea polyphenols, that is, catechin and epigallocatechin gallate (EGCG). We found that tea polyphenols can steadily promote the aggregation of proteins and protein complexes through their bridging effect. The numbers of proteins in the complexes and in the aggregates of complexes are extracted from SAXS intensity profiles, and their dependences as a function of the molar ratio of polyphenol to protein are discussed. EGCG has stronger capability than catechin to promote complex formation and further aggregation, and the aggregates of complexes have a denser core with a relatively smooth surface. The aggregates induced by catechin are loosely packed with a rough surface. BSA shows higher stability than trypsin in the formation of complex with a well-folded conformation. The synergistic unfolding of trypsin results in larger aggregates in the mixtures with more tea polyphenols. The binding affinity and number of tea polyphenols bound to each protein are further determined using fluorescence spectroscopy. The structure of protein complexes explored in this work is referable in the preparation of protein complex-based particles and the understanding of polyphenol-induced formation and further aggregation of protein complexes.

The Gastrointestinal Tract as a Key Target Organ for the Health-Promoting Effects of Dietary Proanthocyanidins

Proanthocyanidins (PACs) are polymers of flavan-3-ols abundant in many vegetable foods and beverages widely consumed in the human diet. There is increasing evidence supporting the beneficial impact of dietary PACs in the prevention and nutritional management of non-communicable chronic diseases. It is considered that PACs with a degree of polymerization >3 remain unabsorbed in the gastrointestinal (GI) tract and accumulate in the colonic lumen. Accordingly, the GI tract may be considered as a key organ for the healthy-promoting effects of dietary PACs. PACs form non-specific complexes with salivary proteins in mouth, originating the sensation of astringency, and with dietary proteins, pancreatic enzymes, and nutrient transporters in the intestinal lumen, decreasing the digestion and absorption of carbohydrates, proteins, and lipids. They also exert antimicrobial activities, interfering with cariogenic or ulcerogenic pathogens in the mouth (Streptococcus mutans) and stomach (Helicobacter pylori), respectively. Through their antioxidant and antiinflammatory properties, PACs decrease inflammatory processes in animal model of gastric and colonic inflammation. Interestingly, they exert prebiotic activities, stimulating the growth of Lactobacillus spp. and Bifidobacterium spp. as well as some butyrate-producing bacteria in the colon. Finally, PACs are also metabolized by the gut microbiota, producing metabolites, mainly aromatic acids and valerolactones, which accumulate in the colon and/or are absorbed into the bloodstream. Accordingly, these compounds could display biological activities on the colonic epithelium or in extra-intestinal tissues and, therefore, contribute to part of the beneficial effects of dietary PACs.

Chemical and functional properties of different common Brazilian bean (Phaseolus vulgaris L.) cultivars

Abstract Six different common bean cultivars (BRS Embaixador, BRS Pitanga, BRS Estilo, Pérola, BRS Campeiro and BRS Esplendor) were characterized aiming to determine possible uses for them in various food products. The samples were analysed to determine their chemical composition, weight per hundred beans, pH, water and oil absorption capacities (WAC and OAC, respectively), foaming at pH 2.5, 5.6 and 8.0 and emulsifying properties. The relationship between the physicochemical and functional properties was described using the Principal Component Analysis (PCA). The results of the chemical composition, weight per hundred beans, WAC and OAC showed differences even between cultivars of the same commercial group. Foaming also varied between the cultivars and foaming capacity and stability were greatest at pH 5.6 and 8.0. The emulsifying capacity proved quite high for all cultivars, as well as the stability of the emulsion. According to these properties, with the contribution of the PCA, each different bean cultivar can be destined to specific applications according to its physicochemical properties.

New Anthocyanin-Human Salivary Protein Complexes

The interaction between phenolic compounds and salivary proteins is considered the basis of the poorly understood phenomenon of astringency. Furthermore, this interaction is an important factor in relation to their bioavailability. In this work, interactions between anthocyanin and human salivary protein fraction were studied by mass spectrometry (MALDI-TOF-MS and FIA-ESI-MS) and saturation-transfer difference (STD) NMR spectroscopy. Anthocyanins were able to interact with saliva proteins. The dissociation constant (KD) between malvidin 3-glucoside and salivary proline-rich proteins was 1.92 mM for the hemiketal form (pH 3.4) and 1.83 mM for the flavylium cation (pH 1.0). New soluble complexes between these salivary proteins and malvidin 3-glucoside were identified for the first time.

Interaction of gallic acid with trypsin analyzed by spectroscopy

The interactions between trypsin and gallic acid (GA) were investigated by means of fluorescence spectroscopy, UV-vis absorption spectroscopy, resonance light scattering (RLS) spectroscopy, synchronous fluorescence spectroscopy, and enzymatic inhibition assay. It was found that GA can cause the fluorescence quenching of trypsin during the process of formation of GA-trypsin complex, resulting in inhibition of trypsin activity (IC₅₀ = 3.9 × 10⁻⁶ mol/L). The fluorescence spectroscopic data showed that the quenching efficiency can reach about 80%. The binding constants were 1.9371 × 10⁴ L/mol, 1.8192 × 10⁴ L/mol, and 1.7465 × 10⁴ L/mol at three temperatures, respectively. The thermodynamic parameters revealed that hydrogen bonds, van der Waals, hydrophobic, and electrostatic interactions were involved in the binding process of GA to trypsin. Molecular modeling studies illustrated a specific display of binding information and explained most of the experiment phenomena. The microenvironments of tryptophan and tyrosine residue in trypsin were changed by the GA. Results indicated that GA was a strong quencher and inhibitor of trypsin.

The interaction between tannins and gliadin derived peptides in a celiac disease perspective

The demonstration that food tannins are able to complex with gliadin-derived peptides, in a way that is dependent on both structural features, highlights their potential as modulators of celiac disease.

Roles of proanthocyanidin rich extracts in obesity

Obesity is a multifactorial disorder involving an abnormal or excessive amount of body fat.

Effect of tannic acid–fish scale gelatin hydrolysate hybrid nanoparticles on intestinal barrier function and α-amylase activity

Tannic acid/FSGH nanoparticles can ameliorate Cu²⁺ ion induced intestinal epithelial TJ dysfunction without severely inhibiting the activity of digestive enzymes.

Effects of bisphenol S on the structures and activities of trypsin and pepsin

The effects of bisphenol S on the structures and activities of trypsin and pepsin were investigated by various methods like UV-visible absorbance, fluorescence, circular dichroism, and molecular docking. The secondary and tertiary structures of trypsin and pepsin were altered by bisphenol S binding, which resulted in the loosening of the skeletons of trypsin and pepsin. In addition, bisphenol S induced microenvironmental changes around tyrosine and tryptophan residues of trypsin and pepsin. The activity experimental results showed that the activity of pepsin decreases obviously with the increasing concentration of BPS, while the activity of trypsin does not change remarkably. The binding and thermodynamic parameters obtained by molecular docking and fluorescence spectroscopy showed that the bindings of bisphenol S to trypsin and pepsin were spontaneous processes and hydrogen bonding and hydrophobic interactions played a vital role in stabilizing the bisphenol S-trypsin and bisphenol S-pepsin complexes. The binding constants (K(A)) of bisphenol S with trypsin were 7.42 × 10(4) (298 K) and 5.91 × 10(4) L/mol (310 K), and those of pepsin were 5.78 × 10(4) (298 K) and 4.44 × 10(4) L/mol (310 K). Moreover, there was one main kind of binding site for bisphenol S on trypsin or pepsin.

Understanding the molecular mechanism of anthocyanin binding to pectin

Association between anthocyanins and carbohydrates has drawn attention over the past few years and this interaction is of particularly importance in food chemistry since these compounds are often found together in plants and foodstuffs. This work intended to bring insights on the interaction between ionic carbohydrates (pectin) and two anthocyanins (cyanidin-3-O-glucoside, cy3glc and delphinidin-3-O-glucoside, dp3glc). The interaction between the flavylium cation and hemiketal anthocyanin forms was characterized by saturation transfer difference (STD) NMR spectroscopy and the respective dissociation constant (Kd) was obtained. This binding was also studied by Molecular Dynamics simulation. In the presence of the anthocyanin hemiketal form a weak interaction between anthocyanins and pectin seems to occur. A variation in the extent of this interaction was also noticed for the two anthocyanins with dp3glc bearing three hydroxyl groups, revealing to be a stronger binder to pectin (Kd ≈ 180 μM for dp3glc and Kd ≈ 250 μM for cy3glc). Experiments performed at acidic pH (flavylium cation) revealed a much stronger interaction (Kd ≈ 2 μM). These experimental results were also supported by theoretical studies which also revealed a stronger interaction in the presence of the anthocyanin flavylium cation and also a stronger interaction between pectin and dp3glc than with cy3glc (for the hemiketal form).

Effect of ligand binding on the dynamics of trypsin. Comparison of different approaches

The intramolecular signal transduction induced by the binding of ligands to trypsin was investigated by molecular dynamics simulations. Ligand binding changes the residue-residue interaction energies and suppresses the mobility of loops that are in direct contact with the ligand. The reduced mobility of these loops results in the altered flexibility of the nearby loops and thereby transmits the information from ligand binding site to the remote sites. The analysis of the flexibility of all residues confirmed the coupling between loops L1 (185-188) and L2 (221-224) and the residues in the active center. The significance of S1 pocket residues for the signal transduction from the active center to the substrate-binding site was confirmed by the dynamical network and covariance matrix analyses. Gaussian network model and principal component analysis demonstrated that the active center residues had zero amplitude in the slowest fluctuations acting as hinges or anchors. Overall, our results provide a new insight into protein-ligand interactions and show how the allosteric signaling may occur.

Mechanism of cinnamic acid-induced trypsin inhibition: a multi-technique approach

In order to investigate the association of the protease trypsin with cinnamic acid, the interaction was characterized by using fluorescence, UV-vis absorption spectroscopy, molecular modeling and an enzymatic inhibition assay. The binding process may be outlined as follows: cinnamic acid can interact with trypsin with one binding site to form cinnamic acid-trypsin complex, resulting in inhibition of trypsin activity; the spectroscopic data show that the interaction is a spontaneous process with the estimated enthalpy and entropy changes being -8.95 kJ mol(-1) and 50.70 J mol(-1) K(-1), respectively. Noncovalent interactions make the main contribution to stabilize the trypsin-cinnamic acid complex; cinnamic acid can enter into the primary substrate-binding pocket and alter the environment around Trp and Tyr residues.

Interactions between tea catechins and casein micelles and their impact on renneting functionality

Many studies have shown that tea catechins bind to milk proteins. This research focused on the association of tea polyphenols with casein micelles, and the consequences of the interactions on the renneting behaviour of skim milk. It was hypothesized that epigallocatechin-gallate (EGCG), the main catechin present in green tea, forms complexes with the casein micelles and that the association modifies the processing functionality of casein micelles. The binding of EGCG to casein micelles was quantified using HPLC. The formation of catechin-casein micelles complexes affected the rennet induced gelation of milk, and the effect was concentration dependent. Both the primary as well as the secondary stage of gelation were affected. These experiments clearly identify the need for a better understanding of the effect of tea polyphenols on the processing functionality of casein micelles, before milk products can be used as an appropriate platform for delivery of bioactive compounds.

The colloidal state of tannins impacts the nature of their interaction with proteins: the case of salivary proline-rich protein/procyanidins binding

While the definition of tannins has been historically associated with its propensity to bind proteins in a nonspecific way, it is now admitted that specific interaction also occurs. The case of the astringency perception is a good example to illustrate this phenomenon: astringency is commonly described as a tactile sensation induced by the precipitation of a complex composed of proline-rich proteins present in the human saliva and tannins present in beverages such as tea or red wines. In the present work, the interactions between a human saliva protein segment and three different procyanidins (B1, B3, and C2) were investigated at the atomic level by NMR and molecular dynamics. The data provided evidence for (i) an increase in affinity compared to shortest human saliva peptides, which is accounted for by protein "wraping around" the tannin, (ii) a specificity in the interaction below tannin critical micelle concentration (CMC) of ca. 10 mM, with an affinity scale such that C2 > B1 > B3, and (iii) a nonspecific binding above tannin CMC that conducts irremediably to the precipitation of the tannins/protein complex. Such physicochemical findings describe in accurate terms saliva protein-tannin interactions and provide support for a more subtle description by oenologists of wine astringency perception in the mouth.