Virtual mutation and directional evolution of anti-amoxicillin ScFv antibody for immunoassay of penicillins in milk

Colorimetric and chemiluminescent enzyme immunoassays based on the alkaline phosphatase-tagged single-chain variable fragment fusion tracer for detecting zearalenone in agro-products

Zearalenone (ZEN) is a non-steroidal estrogenic mycotoxin and seriously threatens food safety, which requires rapid and sensitive detection methods for monitoring ZEN in agro-products. Herein, an alkaline phosphatase-tagged single-chain variable fragment fusion protein (ALP-scFv) was used as a bifunctional tracer to develop a colorimetric enzyme immunoassay (CEIA) and a chemiluminescent enzyme immunoassay (CLEIA) for ZEN. In addition, the interactions between scFv and ZEN were exploited by computer-assisted simulation, and four key amino acid sites were preliminarily identified. After optimization, the CEIA and CLEIA exhibited a limit of detection of 0.02 and 0.006 ng/mL, respectively. Furthermore, both methods showed favorable accuracy in recovery experiments and good selectivity in cross reactions. Moreover, the detection results of the actual samples from both methods correlated well with those from high-performance liquid chromatography. Overall, the ALP-scFv fusion tracer-based CEIA and CLEIA are demonstrated as reliable tools for ZEN detection in food.

Enantioselectivity regulation of antibody against chiral herbicide metolachlor based on interaction at chiral center

Enantioselective antibodies have emerged as great potential biomaterials in the fields of immunoassays and chiral separation. However, cross-reactivity of antibodies to the distomer may severely restrict the application. Comprehending the interaction mechanism between antibodies and enantiomers could be beneficial to produce superior enantioselective antibodies. In this study, a pair of recombinant antibodies (RAbs) against metolachlor enantiomers at chiral carbon (αSS-MET and αSR-MET) were generated and characterized. The αSS-MET-RAb and αSR-MET-RAb showed comparable sensitivity and specificity to the parental monoclonal antibodies by icELISA, with IC50 values of 3.45 and 223.77 ng/mL, respectively. Moreover, the complex structures of RAbs and corresponding eutomer were constructed and analyzed, and site-specific mutagenesis was utilized to verify the reliability of the enantioselective mechanism elucidated. It demonstrated that the strength of the interaction between the chiral center region of eutomer and the antibody was the key factor for the enantioselectivity of antibody. Increasing this interaction could limit the conformational adjustment of the distomer in a specific chiral recognition cavity, thus decreasing the affinity of the antibody to the distomer. This work provided the in-depth analysis of enantioselective mechanism for two RAbs and paved the way to regulate antibody enantioselective performance for immunoassays of chiral compounds.

A Ratiometric Fluorescent Sensor for Penicillin G Based on Color-Tunable Gold-Silver Nanoclusters

Excessive administration of penicillin G and improper disposal of its residues pose a serious risk to human health; therefore, the development of convenient methods for monitoring penicillin G levels in products is essential. Herein, novel gold-silver nanoclusters (AuAgNCs) were synthesized using chicken egg white and 6-aza-2-thiothymine as dual ligands with strong yellow fluorescence at 509 and 689 nm for the highly selective detection of penicillin G. The AuAgNCs were characterized using transmission electron microscopy, X-ray photoelectron spectroscopy, ultraviolet-visible absorption spectrophotometry, and fluorescence spectrophotometry. Under optimum conditions, the fluorescence intensity decreased linearly with the concentration of penicillin G from 0.2 to 6 μM, with a low detection limit of 18 nM. Real sample analyses indicated that a sensor developed using the AuAgNCs could detect penicillin G in urine and water samples within 10 min, with the recoveries ranging from 99.7 to 104.0%. The particle size of the AuAgNCs increased from 1.80 to 9.06 nm in the presence of penicillin G. We believe the aggregation-induced quenching of the fluorescence of the AuAgNCs was the main mechanism for the detection of penicillin G. These results demonstrate the ability of our sensor for monitoring penicillin G levels in environmental and clinic samples.

Single-chain fragment variable produced by phage display technology: Construction, selection, mutation, expression, and recent applications in food safety

Immunoassays are reliable, efficient, and accurate methods for the analysis of small-molecule harmful substances (such as pesticides, veterinary drugs, and biological toxins) that may be present in food. However, traditional polyclonal and monoclonal antibodies are limited by animal hosts and hinder further development of immunoassays. With the gradual application of phage display technology as an efficient in vitro selection technology, the single-chain fragment variable (scFv) now provides an exciting alternative to traditional antibodies. Efficiently constructed scFv source libraries and specifically designed biopanning schemes can now yield scFvs possessing specific recognition capabilities. A rational mutation strategy further enhances the affinity of scFv, and allows it to reach a level that cannot be achieved by immunization. Finally, appropriate prokaryotic expression measures ensure stable and efficient production of scFv. Therefore, when developing excellent scFvs, it is necessary to focus on three key aspects of this process that include screening, mutation, and expression. In this review, we analyze in detail the preparation and affinity improvement process for scFv and provide insights into the research progress and development trend of scFv-based immunoassay methods for monitoring small-molecule harmful substances.

Developing of specific monoclonal recombinant antibody fused to alkaline phosphatase (AP) for one-step detection of fig mosaic virus

Present study was performed to develop a fusion recombinant monoclonal antibody for one-step and accurate detection of FMV with a specific single-chain variable fragment (scFv) fused to alkaline phosphatase (AP) named as scFv(FMV-NP)-AP. The gene encoding-specific scFv recombinant antibody binding to nucleocapsid protein of Fig Mosaic Virus (FMV-NP) was fused to upstream of AP gene and integrated in pET26b bacterial expression vector. As vector contain pelB signal peptide, the expressed protein is secreted into periplasmic compartment. Recombinant fusion protein was produced in transformed E. coli following induction by IPTG. Extraction and purification of fusion protein was performed under denatured condition. The results of SDS-PAGE and western blot analysis indicated high integrity and purity with a single band protein with expected size of 72 kDa. The total yield of purified scFv(FMV-NP)-AP fusion protein estimated around 0.5-1 mg/l cultured medium. Subsequent colorimetric analysis confirmed presence of alkaline phosphatase activity in prepared scFv-AP fusion protein. Specificity of generated recombinant fusion antibody against cognate antigen and the native virus presented in infected plant extracts was assessed by ELISA, western blot and dot blot assays. Results revealed that scFv(FMV-NP)-AP is able to detect the presence of FMV in infected fig plants. The novel approach, implementing specific recombinant fusion antibody developed in this research, leads to one-step detection of FMV in plants by avoiding the use of chemical enzyme-labeled secondary antibodies.

Recombinant antibodies and their use for food immunoanalysis

Antibodies are widely employed as biorecognition elements for the detection of a plethora of compounds including food and environmental contaminants, biomarkers, or illicit drugs. They are also applied in therapeutics for the treatment of several disorders. Recent recommendations from the EU on animal protection and the replacement of animal-derived antibodies by non-animal-derived ones have raised a great controversy in the scientific community. The application of recombinant antibodies is expected to achieve a high growth rate in the years to come thanks to their versatility and beneficial characteristics in comparison to monoclonal and polyclonal antibodies, such as stability in harsh conditions, small size, relatively low production costs, and batch-to-batch reproducibility. This review describes the characteristics, advantages, and disadvantages of recombinant antibodies including antigen-binding fragments (Fab), single-chain fragment variable (scFv), and single-domain antibodies (V_HH) and their application in food analysis with especial emphasis on the analysis of biotoxins, antibiotics, pesticides, and foodborne pathogens. Although the wide application of recombinant antibodies has been hampered by a number of challenges, this review demonstrates their potential for the sensitive, selective, and rapid detection of food contaminants.

Development of a monoclonal-based ic-ELISA for the determination of kitasamycin in animal tissues and simulation studying its molecular recognition mechanism

To monitor the residue of kitasamycin (KIT), a monoclonal antibody against KIT was prepared, and a 50% inhibition concentration (IC₅₀) of 5.7 ± 1.4 μg/L was achieved with the most sensitive antibody, KA/2A9, by optimizing ELISA conditions. The LODs for KIT in different animal tissues ranged from 22.47 μg/kg to 29.32 μg/kg, and the recoveries of the fortified tissues were 70% ~ 120% with coefficients of variation below 20%. Then, KIT-specific scFv KA/2A9/3 was prepared for the first time. Homologous modeling and molecular docking results indicated that the key amino acids of KA/2A9/3 scFv are TYR-92 (CDRL3), SER-93 (CDRL3), ASP-155 (CDRH1) and GLY-226 (CDRH3), and the hydrogen bond is the main force. And then, virtual mutation provides a method to evolve KA/2A9/3 scFv antibodies. These results contribute to comprehending the antigen-antibody binding mechanism and provide effective information for in vitro affinity maturation of anti-KIT scFv.

Site-directed mutations of anti-amantadine scFv antibody by molecular dynamics simulation: prediction and validation

A recombinant single-chain variable fragment (scFv) antibody was produced from a hybridoma cell strain secreting the monoclonal antibody for amantadine (AMD), and then its recognition mechanisms for AMD were studied using the molecular docking and molecular dynamics. Complex dockings revealed that three regions are involved in antibody recognition; framework 2 of the VL chain (LFR2) GLU40 and TYR42, complementarity-determining region of the VL chain (LCDR3) TYR116, and framework 2 of the VH chain (HFR2) HIS40 and TRP52 were the key amino acid residues. The results of molecular dynamics show that the most important amino acid residues in the interaction between AMD and scFv are HIS40 and TYR116. On the basis of the results of virtual mutation, the scFv antibody was evolved by directional mutagenesis of amino acid residue GLY107 to PHE. Indirect competitive ELISA (icELISA) results indicated that the scFv mutant had highly increased affinity for AMD with up to 3.9-fold improved sensitivity. Thus, the scFv antibody can be applied for mechanistic studies of intermolecular interactions, and our work offered affinity maturated antibodies by site mutations, which were beneficial for valuable anti-AMD antibody design and preparation in future.

Rapid detection of 21 β-lactams using an immunochromatographic assay based on the mutant BlaR-CTD protein from Bacillus Licheniformis

In this study, a gold immunochromatographic assay (GICA) based on a penicillin receptor protein (PBP) is proposed to simultaneously detect penicillin, cephalosporin, and carbapenem antibiotics in milk and chicken.

Molecular evolution of single chain fragment variable (scFv) for diagnosis of lymphatic filariasis

Endemic countries with lymphatic filariasis are striving towards the Global Program to Eliminate Lymphatic Filariasis (GPELF) by 2020. Efficient and cost-effective diagnostic tools to assess active filarial infection are critical to eradicate lymphatic filariasis. Detection of circulating filarial antigens in sera is one of the precise methods to identify this infection. Monoclonal antibodies and single chain fragment variable (scFv) against Wuchereria bancrofti antigen SXP1 have been developed for antigen detection. Molecular cloning of scFv for recombinant expression has laid a platform for developing novel genetic constructs with enhanced reactivity. In this study, a simple procedure is developed to create diverse libraries of scFv based on a single DNA framework with all the requisites for an in vitro protein synthesis and ribosomal display. Error Prone-PCR was performed to incorporate random mutations and screened by ribosome display technique to isolate evolved scFv. Evolved scFv with six mutations showed tenfold increase in affinity compared to wild-type scFv for rWbSXP1. In silico studies showed that four mutations introduced unique molecular interactions between the evolved scFv and SXP1. Reactivity with asserted clinical samples of endemic normals (EN), microfilariaemic (MF), chronic pathology (CP) and non-endemic normals (NEN) showed significant augment (59.69%, p < 0.0001) in reactivity to MF samples with evolved scFv in comparison to wild-type scFv. Sensitivity of scFv was increased from 15.62 ng to 195 pg by evolved scFv in serum samples. This evolutionary method coupled with ribosome display has facilitated us to improve the reactivity of the ScFv without diminishing the specificity.