Reactions were set up in a final volume of 50 µL. The target protein was used at a final concentration of 50 µM (1 eq), the amine donor at a final concentration of 50 µM (1eq) in a buffer containing 50 mM Trizma® base and 4.5 mM DTT at pH 8.0. The bTG enzyme was added last at a final concentration of 75 µM (1.5 eq). Reactions were shaken at 500 rpm and 37°C overnight and analyzed by LCMS experiments. Conversion is defined as the ratio of the product peak area over the sum of the area of the product and starting material multiplied by 100.