Tailoring protein intrinsic charge by enzymatic deamidation for solubilizing chicken breast myofibrillar protein in water

Engineering of protein glutaminase for highly efficient modification of fish myofibrillar protein through structure-based and computational-aided strategy

Protein glutaminase (PG; EC 3.5.1.44) is a class of food-grade enzyme with the potential to significantly improve protein functionality. However, its low catalytic activity and stability greatly hindered industrial application. In this study, we employed structural-based engineering and computational-aided design strategies to target the engineering of protein glutaminase PG5, which led to the development of a combinatorial mutant, MT8, exhibiting a specific activity of 31.1 U/mg and a half-life of 216.2 min at 55 °C. The results indicated that the flexible region in MT8 shifted from the C-terminus to the N-terminus, with increased N-terminal flexibility positively correlating with its catalytic activity. Additionally, MT8 notably boosted fish myofibrillar proteins (MPs) solubility under the absence of NaCl conditions and enhanced their foaming and emulsifying properties. Key residues like Asp31, Ser72, Asn121, Asp471, and Glu485 were crucial for maintaining PG5-myosin interaction, with Ser72 and Asn121 making significant energy contributions.

Complexation between curcumin and walnut protein isolate modified by pH shifting combined with protein-glutaminase

The poor techno-functional properties of walnut protein isolate (WPI) limit its application as carrier to improve bioavailability of curcumin. In this study, WPI was modified by pH-shifting (PS) and protein-glutaminase (PG). Changes on the physicochemical and structural characteristics of WPI and effects on complexation with curcumin were investigated. Treatment of PS plus PG increased electrostatic repulsion of WPI with altered secondary and tertiary structure. Solubility of WPI was greatly improved from 18.09% to 52.90%. The increased flexibility resulted in reduced particle size and increased exposure of hydrophobic groups. The improved amphiphilicity of WPI provided more binding sites for complexation with curcumin. Encapsulation efficiency of curcumin was increased from 32.50% to 94.48%. Interestingly, the formed complexes were able to protect curcumin from degradation with improved storage stability and bioaccessibility. Thus, PS plus PG could serve as effective modification strategy for utilization of WPI as a promising delivery vehicle for hydrophobic bioactives.

Functional and Mechanistic Dissection of Protein Glutaminase PG3 and Its Rational Engineering for Enhanced Modification of Myofibrillar Proteins

Protein glutaminases (PG; EC = 3.5.1.44) are enzymes known for enhancing protein functionality. In this study, we cloned and expressed the gene chryb3 encoding protein glutaminase PG3, exhibiting 39.4 U/mg specific activity. Mature-PG3 featured a substrate channel surrounded by aromatic and hydrophobic amino acids at positions 38-45 and 78-84, with Val81 playing a pivotal role in substrate affinity. The dynamic opening and closing motions between Gly65, Thr66, and Cys164 at the catalytic cleft greatly influence substrate binding and product release. Redesigning catalytic pocket and cocatalytic region produced combinatorial mutant MT6 showing a 2.69-fold increase in specific activity and a 2.99-fold increase at t65 °C1/2. Furthermore, MT6 boosted fish myofibrillar protein (MP) solubility without NaCl. Key residues such as Thr3, Asn54, Val81, Tyr82, Asn107, and Ser108 were vital for PG3-myosin interaction, particularly Asn54 and Asn107. This study sheds light on the catalytic mechanism of PG3 and guided its rational engineering and utilization in low-salt fish MP product production.

Purified protein glutaminase from Chryseobacterium proteolyticum enhances the properties of wheat gluten

Protein glutaminase (PG), originating from Chryseobacterium proteolyticum, can catalyze the deamidation of glutamine residues in plant proteins into glutamic acid, thus enhancing its functional properties. However, the low yield of PG limits its industrial production. In this study, the yield of PG in C. proteolyticum TM1040 increased by 121 %, up to 7.30 U/mL in a 15 L fermenter after medium optimization. Subsequently, purified PG was obtained by cation exchange chromatography (CEX) coupled with hydrophobic interaction chromatography (HIC). The degree of deamidation (DD) of wheat gluten after purified PG deamidation was 87.11 %, which is superior to chemical deamidation in safety and DD. The emulsifying and foaming properties of deamidated wheat gluten were 2.67 and 18.86 times higher, and the water- and oil-holding properties were 4.23 and 18.77 times higher, respectively. The deamidated wheat gluten with enhanced functional properties was used to improve the flavor and texture in baking cakes.

Enhancement of functional properties, digestive properties, and in vitro digestion product physiological activity of extruded corn gluten meal by enzymatic modification

BACKGROUND

Enzymatic modification is an effective means of improving the functional properties, digestive properties, and in vitro digestion product physiological activity of proteins, thus significantly expanding protein uses in various food applications.

RESULTS

In this study, the addition of chymotrypsin (CT) at pH 9.0 and 11.0 was found to significantly improve the functional properties (solubility, foaming properties, water holding capacity, oil holding capacity, etc.) and digestive properties of extruded corn gluten meal (ECGM). Similar changes were observed when treating ECGM with glutaminase, protein glutaminase, and papain. These changes were likely due to the increase in number of carboxyl groups and the multiple effects of change in protein net charge and conformation caused by enzymatic deamidation. Of note, ECGM deamidated by CT showed the highest degree of deamidation, solubility, and gastrointestinal digestibility at pH 11.0, up to 44.92%, 43.75%, and 82.22%, respectively. In addition, CT-ECGM digestion product exhibited strong antioxidant activity and potential to promote alcohol metabolism in both a static digestion model and dynamic digestion model, even comparable to commercial corn peptides (CCP), while being inexpensive and of low bitterness compared to CCP. Meanwhile, the physiological activity enhanced as the molecular weight of digestion product decreased with the digested component having strongest activity.

CONCLUSION

This study may promote the application of ECGM as a food component in the food industry or even as a substitute for CCP. © 2023 Society of Chemical Industry.

Deciphering the thick filaments assembly behavior of myosin as affected by enzymatic deamidation

Enzymatic deamidation is a promising approach in enhancing the solubility of myofibrillar proteins (MPs) in water paving the way of tailor manufacturing muscle protein-based beverages. This work aimed to clarify the solubilization mechanism by deciphering myosin thick filaments assembly as affected by protein-glutaminase deamidation. With the extension of deamidation, filamentous structures in MPs shortened continuously. Dynamic monitoring of quartz crystal microbalance-dissipated showed the adsorption capacity of the deaminated MPs was reduced from 3.66 ng/cm2 to 2.03 ng/cm2, indicating that the ability to assemble myosin thick filaments was significantly weakened. By simulating the surface charge, it was found that deamidation may neutralize the positive charged clusters distanced at 14-29 nm from rod C-terminus. Since this region confers myosin electrostatic property to initiate staggered dimerization, deamidation in this region, which severely affected the electrostatic balance between residues, impaired ordered thick filament growing and elongating, thus promoting the solubilization of MPs in water.

Protein-glutaminase improves water-/oil-holding capacity and beany off-flavor profiles of plant-based meat analogs

An unresolved challenge for plant-based meat analogs (PBMAs) is their lack of juiciness. Saturated fats significantly contribute to the juiciness of PBMAs, but there are concerns about the undesirable health effects related to saturated fats; thus, demand for their replacement with vegetable unsaturated oils has increased. Although many food additives are used to reduce the leakage of unsaturated oils, this solution cannot meet the clean-label requirements that have been trending in recent years. In this study, we aimed to develop better consumer-acceptable methods using protein-glutaminase (PG) to improve the juiciness of PBMA patties to meet clean-label trends. We found no significant difference between the visual surface of control and PG-treated textured vegetable proteins (TVPs). However, the microstructure of PG-treated TVP had a more rounded shape than that of the control TVP as observed under a scanning electron microscope. After grilling process, the PBMA patties composed of PG-treated TVP showed significantly higher liquid-holding capacities (a juiciness indicator) than the control patties. This suggested that PG treatment could potentially produce PBMA patties with increased juiciness. Interestingly, after the PG-treated TVP underwent the wash process, we found that PG treatment of TVP easily reduced the various beany off-flavor compounds by 58-85%. Moreover, the results of the in vitro protein digestion test showed that the amounts of free amino nitrogen released from PBMA patties composed of PG-treated TVP were 1.5- and 1.7-fold higher than those from control patties in the gastric and intestinal phases, respectively. These findings indicate that PG treatment of TVP could enhance the physical, sensory, and nutritional properties of PBMA patties and meet the clean-label requirements.

Thermo-reversible gelation of myofibrillar protein: Relationship between coiled-coil and thermal reversibility

Thermo-reversible gel of myofibrillar protein (MP) can be made by tactics of elaborate deamidation using protein-glutaminase (PG), and this work aimed to disclose the link between thermally reversible gelation of MP and the coiled-coil (CC). Enzymatic deamidation fragmented myofibril filaments and triggered structural reassembly to create small-sized aggregates. The coiling and dissociation of CC structure in the myosin tails is the fundamental structural basis of the PG deamidated MP (DMP) in the dynamic evolution of reversible gelation. After specific inhibition of CC assembly by trifluoroethanol (TFE), the thermo-reversible gel ability of DMP was impaired, which confirmed that the dynamic assembly of CC with temperature response played a key role in the thermo-reversible gelation of DMP. The findings may broaden the molecular basis of natural CC reversible gelation and foster advances for the development of new muscle protein products.

Insight into Ionic Strength-Induced Solubilization of Myofibrillar Proteins from Silver Carp (Hypophthalmichthys molitrix): Structural Changes and 4D Label-Free Proteomics Analysis

In this study, changes in the physical, structural, and assembly characteristics of silver carp myofibrillar proteins (MPs) at different ionic strength (I) values were investigated. Moreover, the differential proteomic profile of soluble MPs was analyzed using 4D proteomics based on timsTOF Pro mass spectrometry. Solubility of MPs significantly increased at high I (>0.3), and the increase in I enhanced the apparent viscosity, fluorescence intensity, surface hydrophobicity, and α-helix content of MPs solution. Particle size and sodium dodecyl sulfate-polyacrylamide gel electrophoresis patterns also supported the solubility profiles. Transmission electron microscopy and atomic force microscopy observations revealed the morphological assembly and disassembly of MPs under different I conditions. Finally, proteomic analysis revealed the evolution law of salt-induced solubilization of MPs and the critical molecular characteristics in different I environments. The number of differentially abundant proteins (DAPs) decreased with the increase of I, and most DAPs related to the muscle filament sliding, contraction and assembly, actinin binding, and actin filament binding. The soluble abundance of myosin and some structural proteins was dependent on I, and structural proteins in the Z-disk and M-band might contribute to the solubilization of myosin. Our findings provide insightful information about the impact of common I on the solubility pattern of MPs from freshwater fish.

A comparison of the impacts of different polysaccharides on the sono-physico-chemical consequences of ultrasonic-assisted modifications

This study aimed to examine the sono-physico-chemical effects of ultrasound (UND) and its impact on the conjugate rates of morin (MOI) following the addition of polysaccharides in various conditions. In comparison to the control group, the incorporation of quaternary ammonium chitosan decreased the rate of MOI conjugation by 17.38%, but the addition of locust bean gum enhanced the grafting rate by 29.89%. Notably, the highest degree of myofibrillar protein (MRN) unfolding (fluorescence intensity: 114435.50), the most stable state (-44.98 mV), and the greatest specific surface area (393.06 cm2/cm3) were observed in the UMP/LBG group. The outcomes of atomic force microscopy and scanning electron microscopy revealed that the inclusion of locust bean gum led to a different microscopic morphology than the other two polysaccharides, which may be the primary cause of the strongest sono-physico-chemical effects of the system. This work demonstrated that acoustic settings can be tuned based on the characteristics of polysaccharides to maximize the advantages of sono-physico-chemical impacts in UND-assisted MOI processing.

Ultrasound-assisted phosphorylation of goose myofibrillar proteins: improving protein structure and functional properties

BACKGROUND

Goose meat is rough and embedded with dense connective tissue, impairing protein solubility. Therefore, to improve the functional properties of goose myofibrillar protein (GMP), ultrasound was used to assist the phosphorylation of GMP.

RESULTS

The fact that GMP attached covalently with the phosphate group of sodium tripolyphosphate (GMP-STP) was disclosed directly by Fourier transform infrared spectroscopy. Furthermore, ultrasound significantly improved the hydrophobicity and solubility of GMP-STP, which could be attributed to the conversion of α-helix to β-sheet, β-turns, and random coils by sonication. The spatial stabilization of the protein phosphorylation process was boosted by ultrasound, making the droplets more dispersed, and thus an improvement in the functional properties of GMP-STP was observed. Water-holding capacity, oil-binding capacity, and emulsifying and foaming properties were best at an ultrasound power of 400 W.

CONCLUSION

Ultrasound-assisted phosphorylation has great potential to modulate the structure-function relationship of proteins. © 2023 Society of Chemical Industry.

The red, firm, non-exudative and pale, soft, exudative pork have different in vitro digestive properties of protein

Pale, soft and exudative (PSE) meat has worse edible quality than red, firm and non-exudative (RFN) meat, but their difference in nutritional values is still unclear. In this study, the differences in digestive properties between PSE and RFN pork were explored, and the potential mechanisms were analyzed in terms of protein conformation. The PSE pork showed significantly higher digestibility and smaller particle size compared with RFN pork (P < 0.05) after gastrointestinal digestion. Mechanistically, the lower viscosity was seen in the PSE pork digestion system. The protein structure of PSE pork was disordered with weaker hydrogen bond and ionic bond before and after heating. In addition, the protein (mainly salt-soluble protein) of PSE pork was highly oxidized. The results suggested that higher level of oxidation in PSE pork leads to the destruction of the molecular forces, resulting in the impaired protein conformation and disordered protein structure. The serial changes caused the meat proteins more accessible to digestive enzymes, thus improving the digestibility. The findings provide new insights into the evaluating the quality of PSE meat.

Myofibrillar protein can form a thermo-reversible gel through elaborate deamidation using protein-glutaminase

BACKGROUND

Novel thermo-reversible hydrogels that undergo gelation in feedback to external stimuli have numerous applications in the food, biomedical, and functional materials fields. Muscle myofibrillar protein (MP) has long been known for thermally irreversible gelation. Once the reversible gelation of MP is achieved, its scope for research and application will expand.

RESULTS

The work reported here achieved, for the first time, a thermo-reversible MP gelation by elaborate deamidation using protein glutaminase (PG). The protein concentration and PG reaction time within windows of 1.0-2.5% and 8 h or 12 h were observed to be vital for creating thermo-reversible gels. The gel strength increased with protein concentration. The gel displayed a perforated lamellar microstructure, which resulted in a high water-holding capacity. The rheological results revealed the thermo-reversibility of the gel was robust for up to five cycles of heating and cooling. The thermally reversible gelation is closely related to the reversible assembly between individual α-helix and helical coiled coil. Hydrophobic interactions proved to be predominantly involved in the formation and stabilization of the gel network structure.

CONCLUSION

This work increases the scope of research into the thermo-responsive behavior of MP-based gel. It can foster advances in research into the applications of muscle proteins and into the use of PG as a novel ingredient in the food industry. © 2022 Society of Chemical Industry.

Improved water solubility of myofibrillar proteins by ultrasound combined with glycation: A study of myosin molecular behavior

The poor water solubility of myofibrillar proteins (MPs) limits their application in food industry, and is directly related to the molecular behavior associated with myosin assembly into filaments. This study aims to explore the effect of high-intensity ultrasound (HIU) combined with nonenzymatic glycation on the solubility, structural characteristics, and filament-forming behavior of MPs in low ionic strength media. The results showed that the HIU (200-400 W) application could promote the subsequent glycation reaction between MPs and dextran (DX) and interfere with the electrostatic balance between myosin rods, suppressing the formation of filamentous myosin polymers. Glycated MPs pretreated by 400 W HIU had the highest solubility, which corresponded to the smallest particle size, highest zeta potential, and optimum storage stability (P < 0.05). Structure analysis and microscopic morphology observations suggested that the loss of the MP superhelix and the depolymerization of filamentous polymers were the main mechanisms for MP solubilization. In conclusion, HIU combined with glycation can effectively improve the water solubility of MPs by destroying or suppressing the assembly of myosin molecules.

L-Glutamine-, peptidyl- and protein-glutaminases: structural features and applications in the food industry

L-Glutaminases are enzymes that catalyze the cleavage of the gamma-amido bond of L-glutamine residues, producing ammonia and L-glutamate. These enzymes have several applications in food and pharmaceutical industries. However, the L-glutaminases that hydrolyze free L-glutamine (L-glutamine glutaminases, EC 3.5.1.2) have different structures and properties with respect to the L-glutaminases that hydrolyze the same amino acid covalently bound in peptides (peptidyl glutaminases, EC 3.5.1.43) and proteins (protein-glutamine glutaminase, EC 3.5.1.44). In the food industry, L-glutamine glutaminases are applied to enhance the flavor of foods, whereas protein glutaminases are useful to improve the functional properties of proteins. This review will focus on structural backgrounds and differences between these enzymes, the methodology available to measure the activity as well as strengths and limitations. Production methods, applications, and challenges in the food industry will be also discussed. This review will provide useful information to search and identify the suitable L-glutaminase that best fits to the intended application.

Interactions between unfolding/disassembling behaviors, proteolytic subfragments and reversible aggregation of oxidized skeletal myosin isoforms at different salt contents

Myosin filament plays a critical role in water-trapping and thermodynamic regulation during processing of brined muscle foods. The redox state and availability of proteolytic/antioxidant enzymes affected by salt may change the ion-binding capacity of myosin consequently contributing to swelling and rehydration. Thus, this study investigated the impact of different salt content (0%, 1%, 2%, 3%, 4%, 5% NaCl) and oxidation in vitro (10 mM H₂O₂/ascorbate-based hydroxyl radical (OH)-generating system) on the oxidative stability, solubility/dispersion capacity, chymotrypsin digestibility, aggregation site and the microrheological properties of isolated porcine myosin. The result showed that, brining at 2% salt exposed more sulfhydryl groups and inhibited the formation of disulfide bond, whereby smaller dispersed structure (diameter within 10-50 nm) and higher Ca²⁺-ATPase activity of the denatured myosin were observed. Accordingly, gel electrophoresis showed that myosin S1 and HMM subunits were highly oxidized and susceptible to reversible assembles. Despite enhanced hydrophobic interactions between swelled myosin at 3% salt content, ≥4% salt greatly promoted the exposure/polarization of tryptophan and cross-linking structures, mainly occurring at myosin S2 portion. The results of micro-rheology proved that oxidized myosin formed a tighter heat-set network following rehydration at high ion strength (≥4% salt), suggesting an increased inter-droplet resistance and macroscopic viscosity. This work is expected to give some useful insights into improved texture and functionality of engineered muscle foods.