Directing the oligomer size distribution of peroxidase-mediated cross-linked bovine alpha-lactalbumin

Preparation of enzymatically cross-linked α-lactalbumin nanoparticles and their application for encapsulating lycopene

High internal phase Pickering emulsions (HIPPEs) stabilized by protein nanoparticles have been widely reported, but the use of enzymatic methods for preparing these nanoparticles remains underexplored. Our hypothesis is that enzymatically crosslinked α-lactalbumin (ALA) nanoparticles (ALATGs) prepared using transglutaminase will demonstrate improved properties as stabilizers for HIPPEs. In this study, we investigated the physicochemical properties and microstructures of ALATGs, finding that enzymatic crosslinking could be enhanced by removing Ca2+ ions from ALA and preheating the proteins (85 °C, 15 min). The electrical charge, secondary structure, and surface hydrophobicity of ALATGs were found to depend on crosslinking conditions. HIPPEs formed with an ALA concentration of 10 mg/mL and an enzyme activity of 120 U/g exhibited the highest apparent viscosity and mechanical strength, as well as significantly improved loading capacity and photostability for the encapsulated lycopene. Overall, our results support the hypothesis that ALATG-nanoparticles show superior performance as emulsifiers compared to ALA-nanoparticles.

Biomimetic Enzymatic Oxidative Coupling of Barley Phenolamides: Hydroxycinnamoylagmatines

Oxidative coupling of hydroxycinnamoylagmatines in barley (Hordeum vulgare) and related Hordeum species is part of the plant defense mechanism. Three linkage types have been reported for hydroxycinnamoylagmatine dimers, but knowledge on oxidative coupling reactions underlying their formation is limited. In this study, the monomers coumaroylagmatine, feruloylagmatine, and sinapoylagmatine were each incubated with horseradish peroxidase. Their coupling reactivity was in line with the order of peak potentials measured: sinapoylagmatine (245 mV) > feruloylagmatine (341 mV) > coumaroylagmatine (506 mV). Structure elucidation of fourteen in vitro coupling products by NMR and MS revealed that the three main linkage types were identical to those naturally present in Hordeum species, namely, 4-O-7'/3-8', 2-7'/8-8', and 8-8'/9-N-7'. Furthermore, we identified two linkage types that were not previously reported for hydroxycinnamoylagmatine dimers, namely, 8-8' and 4-O-8'. We conclude that oxidative coupling by horseradish peroxidase can be used for biomimetic formation of natural antifungal hydroxycinnamoylagmatine dimers from barley.

Mild Enzyme-Induced Gelation Method for Nanoparticle Stabilization: Effect of Transglutaminase and Laccase Cross-Linking

Low-environment-sensitive nanoparticles were prepared by enzymatic cross-linking of electrostatic complexes of dextran-grafted whey protein isolate (WPI-Dextran) and chondroitin sulfate (ChS). The effect of transglutaminase (TG) and laccase cross-linking on nanoparticle stability was investigated. Covalent TG cross-linking and grafted dextran cooperatively contributed to the stability of nanoparticles against dissociation and aggregation under various harsh environmental conditions (sharply varying pH, high ionic strength, high temperature, and their combined effects). However, fragmentation induced by laccase treatment did not promote nanoparticle stability. Structural characterization showed that the compact structure promoted by TG-induced covalent isopeptide bonds repressed dissociation against varying environmental conditions and thermal-induced aggregation. Furthermore, the increasing α-helix and decreasing random coil contents benefited the formation of disulfide bonds, further contributing to the enhanced stability of nanoparticles cross-linked by TG, whereas weak hydrophobic interactions and hydrogen bonding as evidenced by the increase in β-sheet and microenvironmental changes were not able to maintain the stability of nanoparticles treated with laccase. Encapsulated cinnamaldehyde presented sustained release from TG-cross-linked nanoparticles, and the bioaccessibility was considerably enhanced to 50.7%. This research developed a novel mild strategy to enhance nanoparticle stability in harsh environments and digestive conditions, which could be an effective delivery vehicle for hydrophobic nutrients and drug applications in food and pharmaceutical industries.

Enzyme-catalysed polymer cross-linking: Biocatalytic tools for chemical biology, materials science and beyond

Intermolecular cross-linking is one of the most important techniques that can be used to fundamentally alter the material properties of a polymer. The introduction of covalent bonds between individual polymer chains creates 3D macromolecular assemblies with enhanced mechanical properties and greater chemical or thermal tolerances. In contrast to many chemical cross-linking reactions, which are the basis of thermoset plastics, enzyme catalysed processes offer a complimentary paradigm for the assembly of cross-linked polymer networks through their predictability and high levels of control. Additionally, enzyme catalysed reactions offer an inherently 'greener' and more biocompatible approach to covalent bond formation, which could include the use of aqueous solvents, ambient temperatures, and heavy metal-free reagents. Here, we review recent progress in the development of biocatalytic methods for polymer cross-linking, with a specific focus on the most promising candidate enzyme classes and their underlying catalytic mechanisms. We also provide exemplars of the use of enzyme catalysed cross-linking reactions in industrially relevant applications, noting the limitations of these approaches and outlining strategies to mitigate reported deficiencies.

Maillard induced aggregation of individual milk proteins and interactions involved

The aggregation of α-lactalbumin, β-lactoglobulin and β-casein after heating in dry state was studied in absence and presence of saccharides. In absence of saccharides, differences were observed in the extent of aggregation. Differences between the proteins were mostly due to differences in covalent aggregation. The presence of glucose during the heat treatment of milk proteins significantly increased the extent of aggregation, and decreased differences between proteins. α-Lactalbumin was selected as a model protein for the study of cross-links formed after heat treatment. In the presence of saccharides, these cross-links were found to consist of 36% of disulphide bridges (compared to >75% in the absence of glucose), followed by other cross-links such as lanthionine. Larger saccharides led to a decrease in Maillard induced aggregation; maltotriose actually even inhibited the formation of α-lactalbumin aggregates.

Mechanistic insights into tyrosinase-mediated crosslinking of soy glycinin derived peptides

Tyrosinase from Bacillus megaterium (TyrBm) was previously used to modulate soy glycinin-based emulsions and gels. To study the crosslinking mechanism, TyrBm oxidation of three tyrosine-containing octapeptides derived from glycinin was analyzed by oxygen consumption measurements, absorbance and mass spectrometry. A significant lag period and lower activity were measured when tyrosine was located in the middle of the peptide chain. Mass spectrometry analysis showed that these peptides are crosslinked via the oxidative quinone ring of the tyrosine residue by aryl-alkylamine addition or aryloxy radical coupling to form di-DOPA (3,4-dihydroxyphenylalanine). In contrast, peptides containing tyrosine in the N- or C-terminus, were rapidly oxidized forming multimer units within thirty minutes. When small amino acids were adjacent to the terminus tyrosine, formation of di-tyrosine was observed. This work confirms that protein crosslinking by TyrBm occurs by several chemical mechanisms and may assist in designing peptide-based inhibitors for the food and cosmetic applications.

Enzymatic cross-linking of α-lactalbumin to produce nanoparticles with increased foam stability

Hard colloidal nanoparticles (e.g. partly hydrophobised silica), are known to make foams with very high foam-stability. Nanoparticles can also be produced from proteins by enzymatic cross-linking. Such protein based particles are more suitable for food applications, but it is not known if they provide Pickering foam stabilisation to the same extent as hard colloidal particles. α-Lactalbumin (α-LA) was cross-linked with either microbial transglutaminase (mTG) or horseradish peroxidase (HRP) to produce α-LA/mTG and α-LA/HRP nanoparticles. With both enzymes a range of nanoparticles were produced with hydrodynamic radii ranging from 20-100 nm. The adsorption of nanoparticles to the air-water interface was probed by increase in surface pressure (Π) with time. In the beginning of the Π versus time curves, there was a lag time of 10-200 s, for nanoparticles with Rh of 30-100 nm, respectively. A faster increase of Π with time was observed by increasing the ionic strength (I = 0-125 mM). The foam-ability of the nanoparticles was also found to increase with increasing ionic strength. At a fixed I, the foam-ability of the nanoparticles decreased with increasing size while their foam-stability increased. Foams produced by low-shear whipping were found to be 2 to 6 times more stable for nanoparticles than for monomeric α-LA (Rh≈ 2 nm). At an ionic strength of 125 mM ionic strength and protein concentration ≥ 10 g L(-1), the foam-stability of α-LA/mTG nanoparticles (Rh = 100 nm, ρapp = 21.6 kg m(-3)) was 2-4 times higher than α-LA/HRP nanoparticles (Rh = 90 nm, ρapp = 10.6 kg m(-3)). This indicated that foam-stablity of nanoparticles is determined not only by size but also by differences in mesoscale structure. So, indeed enzymatic cross-linking of proteins to make nanoparticles is moving a step towards particle like behavior e.g. slower adsorption and higher foam stability. However, the cross-link density should be further increased to obtain hard particle-like rigidity and foam-stability.

Changes in protein conformation and surface hydrophobicity upon peroxidase-catalyzed cross-linking of apo-α-lactalbumin

In this study, we explore the effect of peroxidase-catalyzed cross-linking on the molecular conformation of apo-α-lactalbumin (apo-α-LA) and the resulting changes in protein surface hydrophobicity. In studying conformational changes, we distinguish between early stages of the reaction ("partial cross-linking"), in which only protein oligomers (10(6) Da > Mw ≥ 10(4) Da) are formed, and a later stage ("full cross-linking"), in which larger protein particles (Mw ≥ 10(6) Da) are formed. Partial cross-linking induces a moderate loss of α-helical content. Surprisingly, further cross-linking leads to a partial return of α-helices that are lost upon early cross-linking. At the same time, for partially and fully cross-linked apo-α-LA, almost all tertiary structure is lost. The protein surface hydrophobicity first increases for partial cross-linking, but then decreases again at full cross-linking. Our results highlight the subtle changes in protein conformation and surface hydrophobicity of apo-α-LA upon peroxidase-catalyzed cross-linking.

Enzyme-catalyzed protein crosslinking

The process of protein crosslinking comprises the chemical, enzymatic, or chemoenzymatic formation of new covalent bonds between polypeptides. This allows (1) the site-directed coupling of proteins with distinct properties and (2) the de novo assembly of polymeric protein networks. Transferases, hydrolases, and oxidoreductases can be employed as catalysts for the synthesis of crosslinked proteins, thereby complementing chemical crosslinking strategies. Here, we review enzymatic approaches that are used for protein crosslinking at the industrial level or have shown promising potential in investigations on the lab-scale. We illustrate the underlying mechanisms of crosslink formation and point out the roles of the enzymes in their natural environments. Additionally, we discuss advantages and drawbacks of the enzyme-based crosslinking strategies and their potential for different applications.

Mushroom tyrosinase oxidizes tyrosine-rich sequences to allow selective protein functionalization

We show that mushroom tyrosinase catalyzes the formation of reactive o-quinones on unstructured, tyrosine-rich sequences such as hemagglutinin (HA) tags (YPYDVPDYA). In the absence of exogenous nucleophiles and at low protein concentrations, the o-quinone decomposes with fragmentation of the HA tag. At higher protein concentrations (>5 mg mL⁻¹), crosslinking is observed. Besthorn's reagent intercepts the o-quinone to give a characteristic pink complex that can be observed directly on a denaturing SDS-PAGE gel. Similar labeled species can be formed by using other nucleophiles such as Cy5-hydrazide. These reactions are selective for proteins bearing HA and other unstructured poly-tyrosine-containing tags and can be performed in lysates to create specifically tagged proteins.

Cross-linking behavior and foaming properties of bovine α-lactalbumin after glycation with various saccharides

α-Lactalbumin was glycated via the Maillard reaction in the dry state using various mono- and oligosaccharides. The reaction resulted not only in coupling of the saccharides to α-lactalbumin but also in cross-linked proteins. The glycation rate and the extent of cross-link formation were highly dependent on the saccharide used. Glycation by arabinose and xylose led to a very fast protein cross-link formation, whereas glucose showed a relatively low protein cross-linking ability. The stability of foams, created using the various glycated protein samples, depended on the type of saccharide used, the extent of glycation, and possibly the amount of cross-linked protein. Compared to nonmodified α-lactalbumin, glycation with rhamnose and fucose improved foam stability, whereas application of glucose, galacturonic acid, and their oligosaccharides did not exert a clear effect. Mass spectrometric analysis revealed that dehydration of the Amadori products is an indicator of the formation of protein cross-links.

Effect of saccharide structure and size on the degree of substitution and product dispersity of α-lactalbumin glycated via the Maillard reaction

The course of the Maillard reaction between α-lactalbumin and various mono- and oligosaccharides in the solid state was studied using UPLC-ESI-TOF-MS. Individual reaction products were monitored for their degree of substitution per protein molecule (DSP). The Maillard reaction rate depended on the saccharide type and decreased when the saccharide size increased. Conjugation with charged saccharides was hindered when a specific average DSP was reached, probably resulting from electrostatic repulsion. The DSP varied between 0 and 15, and the standard deviation of the average DSP, which is a measure for product dispersity, increased to 1.9. Similar experiments were performed with a dipeptide. Relative reaction rates in these experiments were 1 for glucose, 0.28 for maltose, and 0.16 for maltotriose. Comparison of the results obtained using α-lactalbumin and the dipeptide made clear that the Maillard reaction rate is determined by a number of factors, including saccharide reactivity and lysine accessibility.

Identification of the peroxidase-generated intermolecular dityrosine cross-link in bovine α-lactalbumin

The peroxidase-mediated oxidation of calcium-depleted bovine α-lactalbumin generates a mixture of covalently bound protein oligomers with interesting foaming properties. Here, we isolated the initially formed covalent α-lactalbumin dimer and studied its mode of cross-linking. Liquid chromatography-Fourier transform mass spectrometry (LC-FTMS) of proteolytic digests revealed the unambiguous identification of a peroxidase-catalyzed covalent link between Tyr18 and Tyr50. This shows that, although the radical reaction is often regarded as a random reaction, the initial product formation is specific. Protein structural modeling indicates that the conjugation reaction between these tyrosines is sterically favored and involves initial noncovalent protein complex formation through charge compensation, facilitating intermolecular cross-linking. The identification of the Tyr18-Tyr50 cross-link supports the view that the peroxidase-mediated oxidation of apo α-lactalbumin is a sequential process, involving the formation of linear trimers and higher order oligomers.