Fabrication of Fragment Antibody–Enzyme Complex as a Sensing Element for Immunosensing

Dual-functional probe-based multi-signal immunosensor platform for tropane alkaloids: Verification and evaluation

This study aims to realise rapid detecting of tropane alkaloids (TAs) in food. For this purpose, a broad-spectrum single-chain fragment variable was fused with horseradish peroxidase to create an antibody-enzyme complex (AEC) with antigen recognition and catalytic activity. A multi-signal immunosensor platform based on AEC in the direct competitive reaction mode was constructed using 3,3',5,5'-tetramethylbenzidine and 10-acetyl-3,7-dihydroxyphenoxazine as substrates. The sensitivity of TAs in the immunosensor platform ranged from 0.25 μg/kg to 7912.46 μg/kg. Honey was selected as a representative food sample, and the limit of detection of TAs in honey ranged from 0.02 μg/kg to 409.11 μg/kg, with a recovery rate of 65.7 %-117.1 % and a coefficient of variation less than 21.4 %. Results showed that the immunosensor platform possesses satisfactory accuracy and precision, which highlights its potential for practical applications and its suitability as an ideal tool for rapid screening of TAs in food.

Development of tetravalent antibody-enzyme complexes employing a lactate oxidase and the application to electrochemical immunosensors

Antibody-enzyme complexes (AECs) are ideal for immunosensing. Although AECs using antibody fragments can be produced by bacterial hosts, their low affinity limits their sensing applications. We have improved the affinity of AECs by combining two antibodies using Catcher/Tag systems; however, it requires multiple antibodies and an enzyme production process. In this study, to realize the production of AECs harboring multiple antibody fragments in a single production process, we report a versatile development method of unique AECs based on a multimeric enzyme structure. Using the homotetrameric enzyme, lactate oxidase (LOx), as a labeling enzyme, tetravalent AECs were developed as an electrochemical immunosensor. Homogeneous tetravalent AECs were successfully fabricated by fusing the anti-epidermal growth factor receptor (EGFR) variable domain of a heavy chain of heavy chain antibody to the N-terminus of LOx. The prepared AECs bound to EGFR, maintain their enzyme activity, and worked well as sensing elements in electrochemical sandwich enzyme-linked immunosorbent assay. Moreover, tetravalent AECs exhibited higher sensitivity than monovalent AECs because of their avidity. The fabricated AECs were successfully used in a wash-free homogeneous electrochemical detection system combined with magnetic separation. Our findings offer new insights into the applications of the LOx tetrameric enzyme for the fabrication of AECs with tetravalent antibodies, which may serve as scaffolds for immunosensors.

Site-directed immobilization of enzymes on nanoparticles using self-assembly systems

Enzyme immobilization is an effective method for improving the stability and reusability. However, linking at random sites on the enzyme results in low catalytic efficiency due to blockage of the active site or conformational changes. Therefore, controlling the orientation of enzymes on the carrier has been developed. Here, the site-specific mutation and the SpyTag/SpyCatcher systems were used to prepare a site-directed immobilized enzyme. The thermal stability of the immobilized enzyme was better than that of the free enzyme, and ≥80 % of the catalytic activity was retained after 30 days of storage. Furthermore, the Michaelis constant (Km) and the turnover number (kcat) of the immobilized enzyme were 5.23-fold lower and 6.11-fold higher than those of the free enzyme, respectively, which appeared to be related to changes in secondary structure after immobilization. These findings provide a new and effective option for enzyme-directed immobilization.

The Dawn of a New Era: Targeting the "Undruggables" with Antibody-Based Therapeutics

The high selectivity and affinity of antibodies toward their antigens have made them a highly valuable tool in disease therapy, diagnosis, and basic research. A plethora of chemical and genetic approaches have been devised to make antibodies accessible to more "undruggable" targets and equipped with new functions of illustrating or regulating biological processes more precisely. In this Review, in addition to introducing how naked antibodies and various antibody conjugates (such as antibody-drug conjugates, antibody-oligonucleotide conjugates, antibody-enzyme conjugates, etc.) work in therapeutic applications, special attention has been paid to how chemistry tools have helped to optimize the therapeutic outcome (i.e., with enhanced efficacy and reduced side effects) or facilitate the multifunctionalization of antibodies, with a focus on emerging fields such as targeted protein degradation, real-time live-cell imaging, catalytic labeling or decaging with spatiotemporal control as well as the engagement of antibodies inside cells. With advances in modern chemistry and biotechnology, well-designed antibodies and their derivatives via size miniaturization or multifunctionalization together with efficient delivery systems have emerged, which have gradually improved our understanding of important biological processes and paved the way to pursue novel targets for potential treatments of various diseases.

Rapid and sensitive immunoassay for alpha-fetoprotein in serum by fabricating primary antibody-enzyme complexes using protein self-assembly

Primary antibody-enzyme complexes (PAECs) are ideal immunosensing elements that simplify the immunoassay process and improve the uniformity of results due to their ability to both recognize antigens and catalyze substrates. However, the conventional fabrication methods of PAECs, such as direct gene fusion expression, chemical conjugation, enzymatic conjugation, etc., have low efficiency, poor reliability, and other defects, which limit the widespread application of PAECs. Therefore, we developed a convenient method for the fabrication of homogeneous multivalent PAECs using protein self-assembly and validated it using anti-alpha-fetoprotein nanobody (A1) and alkaline phosphatase (ALP) as models. Heptavalent PAECs showed a 4-fold enhancement in enzymatic catalytic activity compared to monovalent PAECs. Further, to verify the application of developed heptavalent PAECs in immunoassay, heptavalent PAECs were used as bifunctional probes to construct a double-antibody sandwich ELISA to detect AFP. The detection limit of the developed heptavalent PAEC-based ELISA is 0.69 ng mL^-1, which is about 3 times higher than that of monovalent PAECs, and the whole detection process can be completed within 3 hours. In short, the proposed protein self-assembling method is a promising technology for developing high-performance heptavalent PACEs, which can simplify the detection process and improve detection sensitivity in various immunoassays.

Strategies to simplify operation procedures for applying labeled antibody-based immunosensors to point-of-care testing

Point-of-care testing (POCT) is an ideal testing format for the rapid and on-site detection of analytes in patients, and facilitates disease diagnosis and monitoring. Molecular recognition elements are required for the specific detection of analytes, and biosensors that use antibodies as the molecular recognition elements are called immunosensors. Traditional immunosensors such as sandwich enzyme-linked immunosorbent assay (ELISA) require complicated procedures to form immunocomplexes consisting of detection antibodies, analytes, and capture antibodies. They also require long incubation times, washing procedures, and large and expensive specialized equipment that must be operated by laboratory technicians. Immunosensors for POCT should be systems that use relatively small pieces of equipment and do not require special training. In this review, to help in the construction of immunosensors for POCT, we have summarized the recently reported strategies for simplifying the operation, incubation, and washing procedures. We focused on the optical and electrochemical detection principles of immunosensors, compared the strategies for operation, sensitivity, and detection devices and discussed the ideal system. Combining detection devices that can be fabricated inexpensively and strategies that enable simplification of operation procedures and enhance sensitivities will contribute to the development of immunosensors for POCT.