Cross-linking of β-casein by Trichoderma reesei tyrosinase and Streptoverticillium mobaraense transglutaminase followed by SEC–MALLS

Significant enhancement of the thermal stability and catalytic efficiency of transglutaminase in Streptomyces mobaraensis engineered through the novel S. mobaraensis genomic mutant library construction method GHR/Sml

Herein, we developed a novel Streptomyces mobaraensis genomic mutant library construction method, GHR/Sml, to directly and significantly enhance the thermal stability and catalytic efficiency of TGase in the genome of S. mobaraensis. First, 13 key amino acid residues and their mutations for enhanced thermal stability were identified using error-prone PCR and site-directed mutagenesis. Then, the GHR/Sml method was developed to construct a TGase genomic mutant library with 13 mutations. Positive mutants S23Y/Y24N/S250R, S23Y/Y24N/S303K, S23Y/Y24N/K294L, S23Y/Y24N/S199A/R208L, S23Y/Y24N, and S250R were obtained from 1500 total mutants; their half-life values at 50 °C were increased by 9.3-, 9.5-, 8.7-, 9.0-, 6.9-, and 4.8-fold compared with that of TGLD, respectively. Furthermore, the kcat/Km of mutant S23Y/Y24N/S250R increased by 1.25-fold over that of TGLD. The activity of S23Y/Y24N/S250R reached 65.34 U/mL in a 1000-L fermenter, which was the highest activity reported. This novel GHR/Sml method is of great significance for systematically improving properties of additional enzymes in the genome of S. mobaraensis.

Efficient biosynthesis of transglutaminase in Streptomyces mobaraensis via systematic engineering strategies

Transglutaminases (TGases) have been widely used in food, pharmaceutical, biotechnology, and other industries because of their ability to catalyze deamidation, acyl transfer, and crosslinking reactions between Ƴ-carboxamide groups of peptides or protein-bound glutamine and the Ɛ-amino group of lysine. In this study, we demonstrated an efficient systematic engineering strategy to enhance the synthesis of TGase in a recombinant Streptomyces mobaraensis smL2020 strain in a 1000-L fermentor. Briefly, the enzymatic properties of the TGase TGL2020 from S. mobaraensis smL2020 and TGase TGLD from S. mobaraensis smLD were compared to obtain the TGase TGLD with perfected characteristics for heterologous expression in a recombinant S. mobaraensis smL2020ΔTG without the gene tgL 2020. Through multiple engineering strategies, including promoter engineering, optimizing the signal peptides and recombination sites, and increasing copies of the expression cassettes, the final TGLD activity in the recombinant S. mobaraensis smL2020ΔTG: (PL2020-spL2020-protgLD-tgLD)2 (tgL2020and BT1) reached 56.43 U/mL and 63.18 U/mL in shake flask and 1000-L fermentor, respectively, which was the highest reported to date. With the improvement of expression level, the application scope of TGLD in the food industry will continue to expand. Moreover, the genetic stability of the recombinant strain maintained at more than 20 generations. These findings proved the feasibility of multiple systematic engineering strategies in synthetic biology and provided an emerging solution to improve biosynthesis of industrial enzymes.

Enhanced Thermostability and Catalytic Activity of Streptomyces mobaraenesis Transglutaminase by Rationally Engineering Its Flexible Regions

Streptomyces mobaraenesis transglutaminase can catalyze the cross-linking of proteins, which has been widely used in food processing. In this study, we rationally modified flexible regions to further improve the thermostability of FRAPD-TGm2 (S2P-S23V-Y24N-E28T-S199A-A265P-A287P-K294L), a stable mutant of the transglutaminase constructed in our previous study. First, five flexible regions of FRAPD-TGm2 were identified by molecular dynamics simulations at 330 and 360 K. Second, a script based on Rosetta Cartesian_ddg was developed for virtual saturation mutagenesis within the flexible regions far from the substrate binding pocket, generating the top 18 mutants with remarkable decreases in folding free energy. Third, from the top 18 mutants, we identified two mutants (S116A and S179L) with increased thermostability and activity. Finally, the above favorable mutations were combined to obtain FRAPD-TGm2-S116A-S179L (FRAPD-TGm2A), exhibiting a half-life of 132.38 min at 60 °C (t_1/2(60 °C)) and a specific activity of 79.15 U/mg, 84 and 21% higher than those of FRAPD-TGm2, respectively. Therefore, the current result may benefit the application of S. mobaraenesis transglutaminase at high temperatures.

Enzymatic synthesis of sodium caseinate-EGCG-carboxymethyl chitosan ternary film: Structure, physical properties, antioxidant and antibacterial properties

Proteins and polysaccharides have been frequently used in recent years to prepare environment-friendly packaging materials. However, films based on proteins or polysaccharides alone often have poor performance as packaging, so they need to be combined to improve properties. In this work, we applied enzyme technology to prepare sodium caseinate (SC)-carboxymethyl chitosan (CMC) films, incorporating epigallocatechin gallate (EGCG) as bridging molecules and antibacterial agents. SC-EGCG-CMC ternary conjugate was firstly synthesized by tyrosinase (Tyr), and the composite films were then prepared with the aid of glycerol. Under tyrosinase catalytic conditions, EGCG could cross-link with SC and CMC covalently. The effects of different concentrations of EGCG and tyrosinase on mechanical properties, water vapor permeability, antibacterial properties and free radical scavenging ability were studied. The crosslinking degree and mechanical properties were improved with the increase of EGCG and tyrosinase content. The film showed good antibacterial activity against Gram-positive bacteria. In addition, the antibacterial activity and free radical scavenging ability increased with the increase of EGCG concentration. This work provides an efficient enzymatic method to prepare films with good strength and antibacterial properties, which can be used to improve the storage quality of foods.

Significantly Improving the Thermostability and Catalytic Efficiency of Streptomyces mobaraenesis Transglutaminase through Combined Rational Design

Streptomyces mobaraenesis transglutaminase has been widely used in food processing. We here significantly improved the catalytic properties of S2P-S23V-Y24N-S199A-K294L (TGm1), a highly stabilized variant of the transglutaminase. First, a virtual proline scan was performed based on folding free energy changes to obtain TGm1 variants with enhanced thermostability. Second, the residues within 15 Å of Cys64 in the enzyme-substrate complex of TGm1 were subjected to virtual saturation mutagenesis to generate the variants with reduced binding free energy and increased activity. After combining the favorable mutations, we obtained the variant FRAPD-TGm1-E28T-A265P-A287P (FRAPD-TGm2), exhibiting 66.9 min of half-life at 60 °C (t_1/2(60 °C)), 67.8 °C of melting temperature (T_m), and 71.8 U/mg of specific activity, which are 2-fold, 2.6 °C, and 43.8% higher than those of FRAPD-TGm1, respectively. At last, to increase the surface negative net charge of FRAPD-TGm2, we introduced the mutations N96E-S144E-N163D-R183E-R208E-K325E, yielding FRAPD-TGm3. The latter's t_1/2(60 °C), T_m, and specific activity were 122.9 min, 68.6 °C, and 83.7 U/mg, which are 83.8%, 0.8 °C, and 16.6% higher than the former, respectively. FRAPD-TGm3 is thus a robust candidate for transglutaminase application.

Cysteine inducing formation and reshuffling of disulfide bonds in cold-extruded whey protein molecules: From structural and functional characteristics to cytotoxicity

Polymer chemistry, rheology and cytotoxicity of cysteine initiated S-S redistribution in cold-extruded whey protein (TWPI) molecules were investigated. The locations of disulfide bonds in whey protein isolate (WPI), WPI dried without being extruded (OWPI) and cold-extruded WPI (TWPI), Cysteine (Cys)-treated WPI (WPI-Cys), OWPI (OWPI-Cys) and TWPI (TWPI-Cys) were precisely analyzed using liquid chromatography electrospray ionization tandem mass spectrometry (LC/MS/MS) combined with pLink software approaches. The numbers of intermolecular disulfide cross-linked peptides identified in Cys-treated samples increased by 4, 6 and 1, respectively, in the order of TWPI-Cys, OWPI-Cys and WPI-Cys. Fourier Transform infrared spectroscopy (FTIR) showed cysteine treatment loosed secondary structure of protein samples. Meanwhile, size exclusion chromatography (SEC) assay demonstrated the extensive polymerization in TWPI-Cys. Furthermore, Cys-treatment decreased the gelling temperature of TWPI to 57 °C sharply. Cys-treated TWPI has 19.11 times storage modulus (G') and 25.86 times loss modulus (G") of Cys-untreated TWPI at 85 °C. Additionally, cell viability with Cys addition indicate modified whey proteins are not toxic to human umbilical vein endothelial cells (HUVECs).

Effect of enzymatic cross-linking of naringenin-loaded β-casein micelles on their release properties and fate in in vitro digestion

The microbial transglutaminase (mTG) was used to improve the stability of the naringenin-loaded β-casein micelles (CNMs). The formation of cross-linked CNMs was confirmed by SDS-PAGE electrophoresis, showing a decrease in monomeric β-CN levels with increasing crosslinking time. Dynamic light scattering (DLS) showed that after crosslinking the particle size distribution did not change upon dilution, suggesting occurrence of intra-crosslinking. Fluorescence spectroscopy and circular dichroism (CD) showed that crosslinking induced only minor changes in the structure. Finally, release of naringenin in buffer at pH 7.4 demonstrated a slower release from the cross-linked micelles compared to the untreated micelles. In addition, the cross-linked micelles exhibited a partial resistance to pepsin enzyme. We conclude that crosslinking with mTG is a suitable method to modulate naringenin release kinetics from β-CN micelles and improves the potential of these micelles as delivery systems targeted to the small intestine.

Characterization of transglutaminase from Bacillus subtilis and its cross-linking function with a bovine serum albumin model

Finding new crosslinking enzymes for enzyme-mediated protein conjugation is a great need in the food industry. In this research, the properties of Bacillus subtilis transglutaminase (BTG) were characterized in detail and its protein crosslinking functions with bovine serum albumin (BSA) as a model were studied. Compared to the commercial transglutaminase from Streptoverticillium mobaraense, BTG was more stable in a broad range of temperatures (30-60 °C) and pH values (pH 5.0-9.0), with its maximum enzymatic activity at 60 °C and pH 8.0. The protein function evaluation results demonstrated that the BTG-modified BSA showed better emulsifying and foaming properties (p < 0.05) compared with the native one. Additionally, significant improvements (p < 0.05) were observed in the rheological properties, water holding capacity, and textural properties of the BTG-treated BSA gels. With good thermal and pH stability and excellent crosslinking effects, BTG would be a potential enzyme for food structure engineering to improve the functional properties of food proteins and expand their applications.

Physico-chemical, microstructural and rheological properties of camel-milk yogurt as enhanced by microbial transglutaminase

Camel milk produces watery texture when it is processed to yogurt. Despite the extensive studies on microbial transglutaminase (MTGase) in dairy research, the effect of this enzyme on the properties of yogurt made from camel milk has not been studied. This study aims to investigate the impact of MTGase with and without bovine skimmed milk powder (SMP), whey protein concentrate (WPC),or β-lactoglobulin (β-lg) on physico-chemical, rheological, microstructural, and sensory properties of camel-milk yogurt during 15 days of storage period. MTGase treatment markedly reduced the fermentation time of unfortified and SMP-fortified camel milk. The fortification of camel milk without MTGase failed to give set-type yogurt. The treatment of unfortified milk with MTGase enormously improved the viscosity and the body of yogurt samples. Fortification of MTGase-treated milk impacted positively on the viscosity, the water holding capacity, and the density of the protein matrix in the gel microstructure, which were influenced by the type of dairy ingredients. All MTGase-treated yogurts differed from each other in hardness and adhesiveness values. Electrophoresis results showed that the susceptibility of the individual milk proteins to MTGase varied, and there were differences among the treatments toward the enzyme. SMP-fortified yogurt was the most accepted product. Generally, the addition of MTGase preparation at a concentration of 0.4%, simultaneously with starter culture, to fortified camel milk was considered an effective tool to solve the challenges of producing set-type yogurt from this milk.

Structuring colloidal oat and faba bean protein particles via enzymatic modification

Oat and faba bean protein isolates were treated with transglutaminase from Streptomyces mobaraensis and tyrosinase from Trichoderma reesei to modify the colloidal properties of protein particles in order to improve their colloidal stability and foaming properties. Transglutaminase crosslinked faba bean protein extensively already with 10nkat/g enzyme dosage. Oat protein was crosslinked to some extent with transglutaminase with higher dosages (100 and 1000nkat/g). Transglutaminase increased the absolute zeta-potential values and reduced the particle size of oat protein particles. As a result, the colloidal stability and foaming properties were improved. Tyrosinase had limited crosslinking ability on both plant protein materials. Tyrosinase greatly reduced the solubility of oat protein despite limited crosslinking. Tyrosinase did not have effect on zeta-potential or colloidal stability of either protein, but it impaired foaming properties of both. Thus, the crosslinking enzymes studied caused significantly different end product functionality, presumably due to the different mechanism of action.

Formation and Degradation of Beta-casomorphins in Dairy Processing

Milk proteins including casein are sources of peptides with bioactivity. One of these peptides is beta-casomorphin (BCM) which belongs to a group of opioid peptides formed from β-casein variants. Beta-casomorphin 7 (BCM7) has been demonstrated to be enzymatically released from the A1 or B β-casein variant. Epidemiological evidence suggests the peptide BCM 7 is a risk factor for development of human diseases, including increased risk of type 1 diabetes and cardiovascular diseases but this has not been thoroughly substantiated by research studies. High performance liquid chromatography coupled to UV-Vis and mass spectrometry detection as well as enzyme-linked immunosorbent assay (ELISA) has been used to analyze BCMs in dairy products. BCMs have been detected in raw cow's milk and human milk and a variety of commercial cheeses, but their presence has yet to be confirmed in commercial yoghurts. The finding that BCMs are present in cheese suggests they could also form in yoghurt, but be degraded during yoghurt processing. Whether BCMs do form in yoghurt and the amount of BCM forming or degrading at different processing steps needs further investigation and possibly will depend on the heat treatment and fermentation process used, but it remains an intriguing unknown.

Effect of the mixtures of squid ink tyrosinase and tannic acid on properties of sardine surimi gel

Effect of the mixture of squid ink tyrosinase (SIT) at 300 and 500 U/g protein and tannic acid (TA) at 0.5 and 1 % (based on protein) with different reaction times (90 and 180 min) on gel properties of sardine surimi was investigated. Surimi gel incorporated with mixture of SIT (500 U/g protein) and 1 % TA with a reaction time of 90 min had the highest breaking force and deformation (p < 0.05), in which the increases by 29.3 % and 11.9 % were observed, in comparison with the control. However, gels added with SIT/TA mixture had the lower whiteness, compared to the control (p < 0.05). Gel added with SIT/TA mixture showed more compact and finer network with higher connectivity of strands, compared to the control. This was coincidental with decreased expressible moisture content. Based on sensory evaluation, the highest overall likeness score was found in gel added with the mixture of SIT (500 U/g protein) and 1 % TA (p < 0.05). Therefore the mixture of tyrosinase from squid ink and tannic acid could be used as additives to improve the properties of surimi gel.

Tyrosinase-catalyzed site-specific immobilization of engineered C-phycocyanin to surface

Enzymatic crosslinking of proteins is often limited by the steric availability of the target residues, as of tyrosyl side chains in the case of tyrosinase. Carrying an N-terminal peptide-tag containing two tyrosine residues, the fluorescent protein C-phycocyanin HisCPC from Synechocystis sp. PCC6803 was crosslinked to fluorescent high-molecular weight forms with tyrosinase. Crosslinking with tyrosinase in the presence of L-tyrosine produced non fluorescent high-molecular weight products. Incubated in the presence of tyrosinase, HisCPC could also be immobilized to amino-modified polystyrene beads thus conferring a blue fluorescence. Crosslinking and immobilization were site-specific as both processes required the presence of the N-terminal peptide in HisCPC.

Versatile peroxidase as a valuable tool for generating new biomolecules by homogeneous and heterogeneous cross-linking

The modification and generation of new biomolecules intended to give higher molecular-mass species for biotechnological purposes, can be achieved by enzymatic cross-linking. The versatile peroxidase (VP) from Pleurotus eryngii is a high redox-potential enzyme with oxidative activity on a wide variety of substrates. In this study, VP was successfully used to catalyze the polymerization of low molecular mass compounds, such as lignans and peptides, as well as larger macromolecules, such as protein and complex polysaccharides. Different analytical, spectroscopic, and rheological techniques were used to determine structural changes and/or variations of the physicochemical properties of the reaction products. The lignans secoisolariciresinol and hydroxymatairesinol were condensed by VP forming up to 8 unit polymers in the presence of organic co-solvents and Mn(2+). Moreover, 11 unit of the peptides YIGSR and VYV were homogeneously cross-linked. The heterogeneous cross-linking of one unit of the peptide YIGSR and several lignan units was also achieved. VP could also induce gelation of feruloylated arabinoxylan and the polymerization of β-casein. These results demonstrate the efficacy of VP to catalyze homo- and hetero-condensation reactions, and reveal its potential exploitation for polymerizing different types of compounds.

High level production of tyrosinase in recombinant Escherichia coli

BACKGROUND

Tyrosinase is a bifunctional enzyme that catalyzes both the hydroxylation of monophenols to o-diphenols (monophenolase activity) and the subsequent oxidation of the diphenols to o-quinones (diphenolase activity). Due to the potential applications of tyrosinase in biotechnology, in particular in biocatalysis and for biosensors, it is desirable to develop a suitable low-cost process for efficient production of this enzyme. So far, the best production yield reported for tyrosinase was about 1 g L(-1), which was achieved by cultivating the filamentous fungus Trichoderma reesei for 6 days.

RESULTS

In this work, tyrosinase from Verrucomicrobium spinosum was expressed in Escherichia coli and its production was studied in both batch and fed-batch cultivations. Effects of various key cultivation parameters on tyrosinase production were first examined in batch cultures to identify optimal conditions. It was found that a culture temperature of 32 °C and induction at the late growth stage were favorable, leading to a highest tyrosinase activity of 0.76 U mL(-1). The fed-batch process was performed by using an exponential feeding strategy to achieve high cell density. With the fed-batch process, a final biomass concentration of 37 g L(-1) (based on optical density) and a tyrosinase activity of 13 U mL(-1) were obtained in 28 hours, leading to a yield of active tyrosinase of about 3 g L(-1). The highest overall volumetric productivity of 103 mg of active tyrosinase per liter and hour (corresponding to 464 mU L(-1) h(-1)) was determined, which is approximately 15 times higher than that obtained in batch cultures.

CONCLUSIONS

We have successfully expressed and produced gram quantities per liter of active tyrosinase in recombinant E. coli by optimizing the expression conditions and fed-batch cultivation strategy. Exponential feed of substrate helped to prolong the exponential phase of growth, to reduce the fermentation time and thus the cost. A specific tyrosinase production rate of 103 mg L(-1) h(-1) and a maximum volumetric activity of 464 mU L(-1) h(-1) were achieved in this study. These levels have not been reported previously.

Protein analysis by 31p NMR spectroscopy in ionic liquid: quantitative determination of enzymatically created cross-links

Cross-linking of β-casein by Trichoderma reesei tyrosinase (TrTyr) and Streptoverticillium mobaraense transglutaminase (Tgase) was analyzed by (31)P nuclear magnetic resonance (NMR) spectroscopy in ionic liquid (IL). According to (31)P NMR, 91% of the tyrosine side chains were cross-linked by TrTyr at high dosages. When Tgase was used, no changes were observed because a different cross-linking mechanism was operational. However, this verified the success of the phosphitylation of phenolics within the protein matrix in the IL. Atomic force microscopy (AFM) in solid state showed that disk-shaped nanoparticles were formed in the reactions with average diameters of 80 and 20 nm for TrTyr and Tgase, respectively. These data further advance the current understanding of the action of tyrosinases on proteins on molecular and chemical bond levels. Quantitative (31)P NMR in IL was shown to be a simple and efficient method for the study of protein modification.

Effect of protein structural integrity on cross-linking by tyrosinase evidenced by multidimensional heteronuclear magnetic resonance spectroscopy

Enzymatic cross-linking of proteins can be catalyzed either by transferase-type enzymes, e.g., transglutaminases, or by oxidoreductases, e.g., tyrosinases or laccases. Three-dimensional structure of protein substrate plays a key role in these reactions, that is, the reactivity and end product are strongly modulated by the accessibility of target amino acid residues to the cross-linking enzyme. Typically structural integrity of protein can be distorted by heat, pH, or mechanical action, as well as by varying ionic concentration of the solution. In this study we used partially unfolded protein (wild-type DrkN SH3) and its structurally stabilized mutant (T22G) to investigate the impact of folded/unfolded conformations on cross-linking by Trichoderma reesei tyrosinase. Our results clearly showed formation of intermolecular cross-links solely between unfolded conformations, making them superior substrates to folded proteins when using tyrosinase as a cross-linking enzyme. Multidimensional heteronuclear magnetic resonance experiments in solution state were employed to investigate cross-linked end-products. The results presented in this study form basis for application development in food, medical, cosmetic, textile, packing and other sectors. In addition, the outcome of this study has a high value for the basic understanding of reaction mechanism of tyrosinases on proteins.

Advances in ingredient and processing systems for meat and meat products

Changes in consumer demand of meat products as well as increased global competition are causing an unprecedented spur in processing and ingredient system developments within the meat manufacturing sector. Consumers demand healthier meat products that are low in salt, fat, cholesterol, nitrites and calories in general and contain in addition health-promoting bioactive components such as for example carotenoids, unsaturated fatty acids, sterols, and fibers. On the other hand, consumers expect these novel meat products with altered formulations to taste, look and smell the same way as their traditionally formulated and processed counterparts. At the same time, competition is forcing the meat processing industry to use the increasingly expensive raw material "meat" more efficiently and produce products at lower costs. With these changes in mind, this article presents a review of novel ingredient systems and processing approaches that are emerging to create high quality, affordable meat products not only in batch mode but also in large-scale continuous processes. Fat replacers, fat profile modification and cholesterol reduction techniques, new texture modifiers and alternative antioxidant and antimicrobial systems are being discussed. Modern processing equipment to establish continuously operating product manufacturing lines and that allow new meat product structures to be created and novel ingredients to be effectively utilized including vacuum fillers, grinders and fine dispersers, and slicers is reviewed in the context of structure creation in meat products. Finally, trends in future developments of ingredient and processing systems for meat products are highlighted.

Novel casein hydrogels: formation, structure and controlled drug release

To develop biocompatible, non-toxic materials for pharmaceutical and biomedical applications, the enzyme-assisted formation and structural characteristics of novel casein hydrogels were investigated by dynamic rheology and fractal analyses. As revealed by oscillatory time sweep and stress relaxation tests, the gelation time was shortened greatly and the hydrogel strength was enhanced obviously when a natural tissue enzyme, microbial transglutaminase (MTGase), was used. For aqueous system containing 10.0 wt% casein and 0.05 wt% MTGase, temperature dependence of the gelation time could be described by an Arrhenius plot with its apparent activation energy of 95.4 kJ/mol. In particular, the resultant casein hydrogel was found to show a "weak-link" behavior with fractal character. The use of the enzyme resulted in the increase of the fractal dimension and the formation of a more "tight" network structure. By means of this enzyme-assisted gelation, Vitamin B12 as the model drug could be incorporated into the casein hydrogel matrix under mild conditions and then show a prolonged release behavior.