Structural characterization and IgE epitope analysis of arginine kinase from Scylla paramamosain

Characterization and Hydrolysis Mechanism Analysis of a Cold-Adapted Trypsin-Like Protease from Antarctic Krill

Cold-adapted proteases are capable of efficient protein hydrolysis at reduced temperatures, which offer significant potential applications in the area of low temperature food processing. In this paper, we attempted to characterize cold-adapted proteases from Antarctic krill. Antarctic krill possesses an extremely active autolytic enzyme system in their bodies, and the production of peptides and free amino acids accompanies the rapid breakdown of muscle proteins following the death. The crucial role of trypsin in this process is recognized. A cold-adapted trypsin named OUC-Pp-20 from Antarctic krill genome was cloned and expressed in Pichia pastoris. Recombinant trypsin is a monomeric protein of 26.8 ± 1.0 kDa with optimum reaction temperature at 25 °C. In addition, the catalytic specificity of OUC-Pp-20 was assessed by identifying its hydrolysis sites through LC-MS/MS. OUC-Pp-20 appeared to prefer Gln and Asn at the P1 position, which is an amino acid with an amide group in its side chain. Hydrolysis reactions on milk and shrimp meat revealed that it can effectively degrade allergenic components in milk and arginine kinase in shrimp meat. These findings update the current knowledge of cold-adapted trypsin and demonstrate the potential application of OUC-Pp-20 in low temperature food processing.

Mutation of Disulfide Bond Sites Reduces the Immunoreactivity of Cra a 4 by Changing the Structural Characteristics

Sarcoplasmic calcium-binding protein (Cra a 4) from Crassostrea angulata belongs to the EF-hand superfamily, and understanding of its structure-allergenicity relationship is still insufficient. In this study, chemical denaturants were used to destroy the structure of Cra a 4, showing that disruption of the structure reduced its IgG-/IgE-binding activity. To explore which critical amino acid site affects the allergenicity of Cra a 4, the mutants were obtained by site-directed mutations in the disulfide bonds site (C97), conformational epitopes (I105, D114), or Ca2+-binding region (D106, D110) and their IgG-/IgE-binding activity was reduced significantly using serological tests. Notably, C97A had the lowest immunoreactivity. In addition, two conformational epitopes of Cra 4 were verified. Meanwhile, the increase of the α-helical content, surface hydrophobicity, and surface electrostatic potential of C97A affected its allergenicity. Overall, the understanding of the structure-allergenicity relationship of Cra a 4 allowed the development of a hypoallergenic mutant.

IgE Epitope Analysis and Hypo-Immunoreactivity Derivative of Arginine Kinase in Mantis Shrimp (Oratosquilla oratoria)

As the main allergenic food, shrimp can trigger allergic reactions in various degrees. In this study, arginine kinase (AK) was identified as an allergen in Oratosquilla oratoria by LC-MS/MS. The open reading frame of AK was obtained, which included 356 amino acids, and recombinant AK (rAK) was expressed in Escherichia coli. The results of immunological analysis and circular dichroism showed that rAK displayed similar IgG-/IgE-binding activity and structure as native AK. Besides, five IgE linear epitopes of AK were verified by serological analysis, on the basis of which an epitope-deleted derivative was obtained and named as mAK-L. It has been shown that mAK-L displayed hypo-immunoreactivity compared to rAK, and the contents of secondary structures were different. In conclusion, these discoveries enrich the overall understanding of crustacean allergens and epitopes and set the foundations for food allergy diagnosis and immunotherapy.

Identification of allergenic epitopes destroyed by two processing technologies of glycinin A2 from soybean

BACKGROUND

Glycinin is one of the most highly allergenic proteins in soybeans, and G2 is one of the five allergenic subunits of glycinin. Compared with the alkaline chain, the acidic chain A2 of the G2 subunit has strong allergenicity. However, the precise epitopes of A2 and the epitopes destroyed during processing are still unknown.

RESULTS

In the present study, preparation of two specific antibodies damaged by processing and phage display techniques were applied to locate the antigenic epitopes of glycinin A2 polypeptide chains disrupted by two processing techniques (thermal processing and ultra-high pressure combined thermal processing). Bioinformatics methods were used to predict the possible epitopes of the A2 chain. The A2 chain and its overlapping segments were introduced into T7 phages and expressed on phage shell by phage display. An indirect enzyme-linked immunosorbent assay was used to screen for antigenic epitopes that had been disrupted by the two processing technologies. The results showed that the dominant antigenic region disrupted by processing was located mainly in the A2-3-B fragment. The reacting experiment with the serum of allergic patients showed that the A2-3-B fragment protein was not only an antigenic region, but also an allergenic region. The two processing technologies destroyed the allergenic epitopes of A2 chain, thereby reducing the allergenicity of protein. The amino acids where the dominant allergenic region disrupted by processing was located were: ²³³ AIVTVKGGLRVTAPAMRKPQQEEDDDDEEEQPQCVE²⁶⁸ .

CONCLUSION

Precise epitopes of the acidic chain A2 in glycinin were identified and epitopes destroyed in two common processing methods were also obtained. The application products of rapid detection of de-allergenicity effect of processed food can be developed according to the location of processed destruction allergenic region, which is of great significance with respect to preventing the occurrence of soybean allergenic diseases. © 2022 Society of Chemical Industry.

Seafood allergy: Allergen, epitope mapping and immunotherapy strategy

Seafoods are fashionable delicacies with high nutritional values and culinary properties, while seafood belongs to worldwide common food allergens. In recent years, many seafood allergens have been identified, while the diversity of various seafood species give a great challenge in identifying and characterizing seafood allergens, mapping IgE-binding epitopes and allergen immunotherapy development, which are critical for allergy diagnostics and immunotherapy treatments. This paper reviewed the recent progress on seafood (fish, crustacean, and mollusk) allergens, IgE-binding epitopes and allergen immunotherapy for seafood allergy. In recent years, many newly identified seafood allergens were reported, this work concluded the current situation of seafood allergen identification and designation by the World Health Organization (WHO)/International Union of Immunological Societies (IUIS) Allergen Nomenclature Sub-Committee. Moreover, this review represented the recent advances in identifying the IgE-binding epitopes of seafood allergens, which were helpful to the diagnosis, prevention and treatment for seafood allergy. Furthermore, the allergen immunotherapy could alleviate seafood allergy and provide promising approaches for seafood allergy treatment. This review represents the recent advances and future outlook on seafood allergen identification, IgE-binding epitope mapping and allergen immunotherapy strategies for seafood allergy prevention and treatment.

Characterization, Epitope Identification, and Cross-reactivity Analysis of Tropomyosin: An Important Allergen of Crassostrea angulata

Oyster is a common shellfish product in China, which is associated with food allergy. However, there is still lack of research on allergens in oysters. In this study, tropomyosin (TM), an important allergen of Crassostrea angulata, was purified and identified by mass spectrometry. Subsequently, TM was cloned and expressed, with a sequence of size 852 bp, encoding 284 amino acid residues. The results of circular dichroism, digestion assay, inhibition enzyme-linked immunosorbent assay, and basophil activation test showed that recombinant TM had similar physicochemical properties and immunological properties to native TM. Furthermore, two conformational mimotopes were obtained and 10 IgE linear epitopes were verified. Meanwhile, different degrees of cross-reactivity were observed between C. angulata TM and the other 8 shellfish TMs using antibodies and serological analysis, which may relate to the 3 conserved epitope regions. These findings are expected to provide a theoretical basis for the molecular diagnosis of oyster allergy and cross-reactivity among shellfish.

Physicochemical characterization and linear epitopes identification of arginine kinase allergen from Crassostrea gigas

BACKGROUND

Molluscan shellfish, including oysters, often cause allergic reactions in sensitive people throughout the world. It has been demonstrated that arginine kinase (AK) is one of the major allergens of oyster. The present study aimed to evaluate the immunoreactivity and structure of oyster AK as affected by heat treatment, pH change, and in vitro digestion. What is more, the immunoglobulin E-binding epitopes of this allergen were also predicted and validated.

RESULTS

Thermal and pH assays revealed that AK was unstable at temperature >40 °C or pH ≤5.0 by sodium dodecyl sulfate polyacrylamide gel electrophoresis and circular dichroism, and the digestibility assays suggested that AK was more easily digested by pepsin than by trypsin and chymotrypsin. The potential epitopes were predicted through immunoinformatics tools, and seven linear epitopes were identified by indirect competition enzyme-linked immunosorbent assay with pooled sera and individual serum from oyster-allergic patients. The critical amino acids in each epitope were also confirmed using mutant peptides. These linear epitopes and critical amino acids were apt to distribute on the outer surface of homology-based AK model. Moreover, the three denaturants (sodium dodecyl sulfate, β-mercaptoethanol, and urea) can destroy the spatial structure of AK and increase or reduce its allergenicity by denaturation treatments.

CONCLUSION

Processing conditions lay the foundation for the variation of allergenicity. Seven linear epitopes and their critical amino acids were identified by indirect competitive enzyme-linked immunosorbent assay. These findings will be helpful in allergy diagnosis and development of hypoallergenic products in the near future. © 2021 Society of Chemical Industry.

Crystal Structure Analysis and IgE Epitope Mapping of Allergic Predominant Region in Scylla paramamosain Filamin C, Scy p 9

Filamin C (FLN c) is a novel allergen in shellfish. In this study, FLN c from Scylla paramamosain was divided into three regions for recombinant expression based on the number of domains and amino acids. Using dot blot and basophil activation tests, the allergic predominant region of FLN c was determined to be 336-531 amino acid positions (named FLN c-M). It was confirmed that by X-ray diffraction, the crystal structure of FLN c-M with immunoglobulin-like folding at a resolution of 1.7 Å was obtained. The monomer was a barrel structure composed of 16 β-strands and 2 α-helices. Three conformational epitopes were predicted, six linear epitopes were verified by serological test, and they were positioned on the crystal structure of FLN c-M. For the first time, the crystal structure of the allergic predominant region of FLN c was determined, and it provided an accurate template for the localization of IgE epitopes.

Identification and characterization of Crassostrea angulata arginine kinase, a novel allergen that causes cross-reactivity among shellfish

Oyster is a common food that causes allergy. However, little information is available about its allergens and cross-reactivity. In this study, arginine kinase (AK) was identified as a novel allergen in Crassostrea angulata. The primary sequence of AK was cloned which encoded 350 amino acids, and recombinant AK (rAK) was obtained. The immunodot results, secondary structure and digestive stability showed that native AK and rAK had similar IgG/IgE-binding activity and physicochemical properties. Serological analysis of 14 oyster-sensitive individuals demonstrated that AK exhibited cross-reactivity among oysters, shrimps, and crabs. Furthermore, nine epitopes in oyster AK were verified using inhibition dot blots and inhibition enzyme linked immunosorbent assay, six of which were similar to the epitopes of shrimp/crab AK. The most conserved epitopes were P5 (121-133) and P6 (133-146), which may be responsible for the cross-reactivity caused by AK. These findings will provide a deeper understanding of oyster allergens and cross-reactivity among shellfish.

Identification and Cross-reactivity Analysis of Sarcoplasmic-Calcium-Binding Protein: A Novel Allergen in Crassostrea angulata

Oysters are an important shellfish group known to cause food allergy; however, knowledge of their sensitization components and cross-reactivity is limited. This study aimed to identify a novel allergen in Crassostrea angulata and investigate its cross-reactivity. To this end, a 20 kDa protein was purified from oyster and confirmed to be a sarcoplasmic-calcium-binding protein (SCP) by LC-MS/MS. A 537 bp open reading frame was obtained from oyster SCP total RNA, which encoded 179 amino acids, and was expressed in Escherichia coli. According to the circular dichroism results, digestion assay, and inhibition ELISA, the recombinant SCP (rSCP) exhibited similar physicochemical properties and IgG-binding activity to native SCP. rSCP displayed stronger IgE-binding activity by immunological method. Moreover, a different intensity of cross-reactivity and sequence homology were demonstrated between shellfish species. Collectively, these findings provide novel insight into shellfish allergens, which can be used to aid in the in vitro diagnosis of oyster-sensitized patients.

Cloning, Expression, and Epitope Identification of Myosin Light Chain 1: An Allergen in Mud Crab

Mud crab (Scylla paramamosain) is a commonly consumed seafood as a result of its high nutritional value; however, it is associated with food allergy. The current understanding of crab allergens remains insufficient. In the present study, an 18 kDa protein was purified from crab muscle and confirmed to be myosin light chain 1 (MLC1) by matrix-assisted laser desorption/ionization-tandem time-of-flight mass spectrometry. Total RNA was isolated and amplified to obtain a MLC1 open reading frame of 462 bp, encoding 154 amino acids. A structural analysis revealed that recombinant MLC1 (rMLC1) expressed in Escherichia coli contained α-helix and random coil. Moreover, rMLC1 displayed strong immunoactivity by dot blot and a basophil activation test. Furthermore, seven allergenic epitopes of MLC1 were predicted, and five critical epitope regions were identified by an inhibition enzyme-linked immunosorbent assay and human mast cell degranulation assay. This comprehensive research of an allergen helps to conduct component-resolved diagnoses and immunotherapies related to crab allergies.

Reducing Allergenicity to Arginine Kinase from Mud Crab Using Site-Directed Mutagenesis and Peptide Aptamers

The mud crab ( Scylla paramamosain) is widely consumed but can cause a severe food allergic reaction. To reduce allergenicity to arginine kinase (AK), site-directed mutagenesis was used to destroy disulfide bonds or mutate critical amino acids of conformational epitopes. Three hypoallergenic mutant AKs (mAK1, mAK2, and mAK3) were generated, with the immunoreactivity decreasing by 54.2, 40.1, and 71.4%, respectively. In comparison to recombinant AK (rAK), the structure of mAKs was clearly changed. Additionally, antisense peptides were designed on the basis of linear epitopes and pepsin-cutting sites of AK. Five peptide aptamers were screened by molecular docking and then analyzed by the immunoglobulin E inhibition enzyme-linked immunosorbent assay and human Laboratory of Allergic Diseases 2 mast cell degranulation assay. The peptide aptamers could significantly inhibit allergenicity of rAK and mAKs, and the inhibitory effect of peptide aptamer 3 was slightly better than the others. These results provide synergistic methods to reduce allergenicity to AK, which could be applied to other shellfish allergens.

A comprehensive analysis of the allergenicity and IgE epitopes of myosinogen allergens in Scylla paramamosain

BACKGROUND

Scylla paramamosain is one of the most common and serious food allergens in Asia. Therefore, research on its prevalence, accurate diagnosis, and IgE-binding pattern of the allergens is crucial.

OBJECTIVE

To identify the IgE epitopes of the myosinogen allergens in S. paramamosain using phage peptide library.

METHODS

The prevalence of allergy to crabs (AC) and of sensitization was analysed using a questionnaire and a serological assay. BAT was performed by flow cytometry, and its diagnostic performance was evaluated in relation to allergens purified from crab myosinogen. IgE-binding epitopes were identified by phage display using the IgE from patients with AC. Sequence- and structure-based bioinformatics analyses were performed to identify allergenic epitopes.

RESULTS

Crab was the most common cause of food allergies in this study. Subjects with AC (n = 30) with clear clinical symptoms were identified by immunoblotting and BAT. All of the myosinogen allergens triggered basophil activation; surface expression of CD63 and CD203c was higher in patients allergic to AK and FLN c than in patients allergic to SCP and TIM. In addition to six conformational epitopes of SCP, six linear epitopes and eight conformational epitopes of AK were identified. Five linear epitopes and three conformational epitopes of TIM, nine linear and ten conformational epitopes of FLN c were also identified, and the sequence VH(I/T) L was appeared in epitopes of both TIM and FLN c. The number of epitopes showed consistency with the value of BAT.

CONCLUSIONS AND CLINICAL RELEVANCE

BAT can be used for accurate diagnosis of AC. Identification of particular allergenic motifs could be a valuable tool for prevention, diagnosis, and treatment of food allergies.

The Maillard Reaction Reduced the Sensitization of Tropomyosin and Arginine Kinase from Scylla paramamosain, Simultaneously

The Maillard reaction was established to reduce the sensitization of tropomyosin (TM) and arginine kinase (AK) from Scylla paramamosain, and the mechanism of the attenuated sensitization was investigated. In the present study, the Maillard reaction conditions were optimized for heating at 100 °C for 60 min (pH 8.5) with arabinose. A low level of allergenicity in mice was shown by the levels of allergen-specific antibodies, and more Th1 and less Th2 cells cytokines produced and associated transcription factors with the Maillard reacted allergen (mAllergen). The tolerance potency in mice was demonstrated by the increased ratio of Th1/Th2 cytokines. Moreover, mass spectrometry analysis showed that some key amino acids of IgE-binding epitopes (K₁₁₂, R₁₂₅, R₁₃₃ of TM; K₃₃, K₁₁₈, R₂₀₂ of AK) were modified by the Maillard reaction. The Maillard reaction with arabinose reduced the sensitization of TM and AK, which may be due to the masked epitopes.

Cloning, Expression, and the Effects of Processing on Sarcoplasmic-Calcium-Binding Protein: An Important Allergen in Mud Crab

Shellfish allergy is a prevalent, long-lasting disorder usually persisting throughout life. However, the allergen information is incomprehensive in crab. This study aimed to identify a novel allergen in crab, show its potential in diagnosis and reduce the allergenicity by food processing. A 21-kDa protein was purified from Scylla paramamosain and confirmed as sarcoplasmic calcium binding protein (SCP) by matrix-assisted laser desorption ionization-time-of-flight/time-of-flight mass spectrometry (MALDI-TOF/TOF-MS). Total RNA was isolated from crab muscle, and a rapid amplification of cDNA was performed to obtain an ORF of 579 bp that coded for 193 amino acid residues. According to the results of circular dichroism analysis and ELISA assay, the recombinant SCP (rSCP) expressed in Escherichia coli showed similar physicochemical and immunoreactive properties to native SCP (nSCP). Additionally, the extensive cross reactivity of SCP among different species and the bidirectional IgE cross-reactivity between nSCP and rSCP were detected by iELISA. The allergenicity of rSCP was reduced via Maillard reaction or enzymatic cross-linking reaction, which was confirmed by the results of scanning electron microscopy, dot blot, and digestion assay. A straightforward and reproducible way was developed to obtain high yields of rSCP that maintains structural integrity and full IgE reactivity, which could compensate the low specific IgE-titers of most patient sera for future diagnosis. Furthermore, the Maillard reaction and enzymatic cross-linking reaction were effective approaches for the production of hypoallergenic seafood.

Allergenicity and Oral Tolerance of Enzymatic Cross-Linked Tropomyosin Evaluated Using Cell and Mouse Models

The enzymatic cross-linking of proteins to form high-molecular-weight compounds may alter their sensitization potential. The IgG-/IgE-binding activity, digestibility, allergenicity, and oral tolerance of cross-linked tropomyosin with tyrosinase (CTC) or horseradish peroxidase (CHP) were investigated. ELISA results demonstrated CTC or CHP reduced its IgE-binding activity by 34.5 ± 1.8 and 63.5 ± 0.6%, respectively. Compared with native tropomyosin or CTC, CHP was more easily digested into small fragments; CHP decreased the degranulation of RBL-2H3 cells and increased endocytosis by dendritic cells. CHP can induce oral tolerance and reduce allergenicity in mice by decreasing IgE and IgG1 levels in serum, the production of T-cell cytokines, and the percentage composition of dendritic cells. These findings demonstrate CHP has more potential of reducing the allergenicity than CTC via influencing the morphology of protein, changing the original method of antigen presentation, modulating the Th1/Th2 immunobalance, and inducing the oral tolerance of the allergen tropomyosin.

Identification of triosephosphate isomerase as a novel allergen in Octopus fangsiao

Octopus is an important mollusk in human dietary for its nutritional value, however it also causes allergic reactions in humans. Major allergens from octopus have been identified, while the knowledge of novel allergens remains poor. In the present study, a novel allergen with molecular weight of 28kDa protein was purified from octopus (Octopus fangsiao) and identified as triosephosphate isomerase (TIM) by mass spectrometry. TIM aggregated beyond 45°C, and its IgE-binding activity was affected under extreme pH conditions due to the altered secondary structure. In simulated gastric fluid digestion, TIM can be degraded into small fragments, while retaining over 80% of the IgE-binding activity. The full-length cDNA of O. fangsiao TIM (1140bp) was cloned, which encodes 247 amino acid residues, and the entire recombinant TIM was successfully expressed in Escherichia coli BL21, which showed similar immunoreactivity to the native TIM. Different intensity of cross-reactivity among TIM from related species revealed the complexity of its epitopes. Eight linear epitopes of TIM were predicted following bioinformatic analysis. Furthermore, a conformational epitope (A₇₁G₇₄S₆₉D₇₅T₇₃F₇₂V₆₇) was confirmed by the phage display technology. The results revealed the physicochemical and immunological characteristics of TIM, which is significant in the development of hyposensitivity food and allergy diagnosis.

Assessment of the sensitizing capacity and allergenicity of enzymatic cross-linked arginine kinase, the crab allergen

SCOPE

The enzymatic cross-linking of an allergen by food processing may alter its sensitization potential. In this study, the IgE-binding activity and allergenicity of cross-linked thermal polymerized arginine kinase (CL-pAK) were investigated.

METHODS AND RESULTS

The IgE-binding activity and stability of CL-pAK were analyzed by immunological and proteomics methods. The sensitization and potency to induce oral tolerance of CL-pAK were tested using in vivo assays and a cell model. According to the results of inhibition of ELISA, the half inhibitory concentration of AK after cross-linking changed from 1.13 to 228.36 μg/mL. The results of in vitro digestion demonstrated that CL-pAK showed more resistance to gastrointestinal digestion than native AK. Low allergenicity and capacity to induce oral tolerance in mice were shown by the sera levels of AK-specific antibodies and T-cell cytokine production. Exposure of RBL-2H3 cells to CL-pAK compared with AK, resulted in lower levels of mast degranulation and histamine.

CONCLUSION

Enzymatic cross-linking with thermal polymerization of AK by tyrosinase and caffeic acid had high potential in mitigating IgE-binding activity and allergenicity, which were influenced by altering the molecular and immunological features of the shellfish protein.

Purification, Characterization, and Analysis of the Allergenic Properties of Myosin Light Chain in Procambarus clarkii

Myosin light chain (MLC) plays a vital role in cell and muscle functions and has been identified as an allergen in shrimp. In this study, MLC with a molecular mass of 18 kDa was purified from crayfish (Procambarus clarkii) muscle. Its physicochemical characterization showed that the purified MLC is a glycoprotein with 4.3% carbohydrate, highly stable to heat, acid-alkali, and digestion, and weakly retains IgE-binding activity when its secondary structure was altered. Serological assays suggested that conformational epitopes predominate over linear epitopes in the purified MLC. Two isoforms of the MLC gene (MLC1 and MLC2) were cloned, and the purified MLC was identified as MLC1. Analysis of the secondary and tertiary structures of the MLCs indicated that MLC1 has four conformational epitopes and three linear epitopes, whereas MLC2 had a major conformational epitope and three linear epitopes. These results are significant for understanding hypersensitization of humans to crayfish.

The Inhibitory Effects of Cu(2+) on Exopalaemon carinicauda Arginine Kinase via Inhibition Kinetics and Molecular Dynamics Simulations

We studied the Cu(2+)-mediated inhibition and aggregation of Exopalaemon carinicauda arginine kinase (ECAK). We found that Cu(2+) significantly inactivated ECAK activity and double-reciprocal kinetics demonstrated that Cu(2+) induced noncompetitive inhibition of arginine and ATP (IC50 = 2.27 ± 0.16 μM; K i for arginine = 13.53 ± 3.76; K i for ATP = 4.02 ± 0.56). Spectrofluorometry results showed that Cu(2+) induced ECAK tertiary structural changes including the exposure of hydrophobic surfaces that directly induced ECAK aggregation. The addition of osmolytes such as glycine and proline successfully blocked ECAK aggregation induced by Cu(2+) and recovered ECAK activity. We built a 3D structure for ECAK using the ECAK ORF gene sequence. Molecular dynamics (MD) and docking simulations between ECAK and Cu(2+) were conducted to elucidate the binding mechanisms. The results showed that Cu(2+) blocked the entrance to the ATP active site; these results are consistent with the experimental result that Cu(2+) induced ECAK inactivation. Since arginine kinase (AK) plays an important role in cellular energy metabolism in invertebrates, our study can provide new information about the effect of Cu(2+) on ECAK enzymatic function and unfolding, including aggregation, and the protective effects of osmolytes on ECAK folding to better understand the role of the invertebrate ECAK metabolic enzyme in marine environments.