According to the Poulos ─ Kraut Mechanism, the Arg and His residues residing in the second sphere contribute to the peroxidase activity by stabilizing the catalytic intermediate, Scheme 1a. We hypothesize that, in our case, the mutations K121H and S112E work in concert to enhance the catalytic activity of Biot2 ─ FePP • Sav S112E K121H.