SyrB2 or SyrB2-F121Y/DOPA (100 µM) was added to RXN buffer containing NaCl (10 mM), 2OG (1 mM). For the WT system, ferrous ammonium sulfate (Sigma: BioUltra, in 2.5 mM H2SO4) was added to the reaction, and the solution was pipette mixed. SyrB1-PPT-Thr was added to the rxn to a final concentration of 100 µM, and the rxn was pipette mixed again. The total reaction volume was 200 µL, and all steps so far were performed on ice. One volume equivalent of O2 saturated buffer was added to the reaction, and each solution was pipette mixed.