Chemoenzymatic bioconjugation of antibodies: Linking proteins for biomedical applications

Antibodies are useful biomolecules applied in many biomedical applications. The selectivity and specificity of antibodies against the target antigens have gained wide interest for both diagnostic and therapeutic applications. The antibodies are capable of functioning as target-specific carriers to allow site-specific delivery of payloads. However, the challenge has always revolved around the ability to attach designer proteins, enzymes, or drugs to the antibody molecule. The conventional approach involves the use of chemical-based modifications with the introduction of chemical linkers and alteration of chemical functional groups to initiate a covalent attachment of molecules to the antibodies. However, the use of chemically modified strategies to attach antibodies to various molecules has provided several setbacks throughout the years. The major consideration involves the conjugation efficiency, the yield of conjugated product recovered postconjugation, and more importantly the effects to the antibody-binding sites. Therefore, the introduction of bioconjugation approaches utilizing biologically active enzymes to initiate conjugation processes provided researchers with a much-anticipated alternative that was less toxic to the native proteins. This chapter focuses on the application of biologically inspired enzymes that have been used successfully to conjugate proteins or drugs to antibodies in a "green" manner. The enzymes highlighted in this chapter would include sortase, transglutaminase, and formylglycine-generating enzymes. The chapter also highlights the applications of these methods to generate conjugates that have been applied either for diagnostic or therapeutic application.