Introduction of Reactive Thiol Handles into Tyrosine-Tagged Proteins through Enzymatic Oxidative Coupling

Site-specific protein bioconjugation methods have enabled the development of new therapeutics and materials, and further development of existing techniques has expanded the compatible library of protein substrates for bioconjugation. Among these techniques, the enzyme tyrosinase has demonstrated a promising ability to form protein-protein conjugates between exposed tyrosine and cysteine residues. In this work, we observed that the tyrosinase variant from <i>Bacillus megaterium</i>, termed megaTYR, has an increased tolerance for small-molecule thiol substrates, which can inhibit the activity of other tyrosinases. Among the breadth of thiol substrates that could be reliably coupled to tyrosine-tagged proteins was dithiothreitol (DTT), which effectively introduces a free thiol handle and provides a convenient method to bypass the genetic incorporation of cysteine residues for bioconjugation. Accordingly, these thiolated proteins could undergo additional coupling to commercially available maleimide probes as well as other tyrosine-tagged proteins. This was demonstrated by the conjugation of targeting proteins to drugs, fluorescent probes, and therapeutic enzymes. Of particular note and building on a previous report of a tyrosinase-sensitive tyrosine residue on the Fc region of antibodies, commercially available monoclonal antibodies (mAbs) treated with PNGase F were conjugated to DTT to produce THIOMAB equivalents. These intermediates were subsequently used to make functional antibody-drug and antibody-toxin protein conjugates. This facile method to convert accessible tyrosine residues on proteins to thiol tags extends the use of tyrosinase-mediated oxidative coupling to a broader range of protein substrates.