For the calculations of reaction III, we observed for both LsAA9 and TaAA9 that the QM/MM energies for the barrier showed high MM contributions (>33 kJ/mol). We analyzed the energy contributions from individual residues and observed that this was caused by a residue close to the tyrosine (Phe43 and Pro30 in LsAA9 and TaAA9, respectively). Hence, we chose to include these residues in the QM region for this reaction, reducing the MM contributions to ∼13 kJ/mol for both enzymes. Since the reaction for TaAA9 was more favorable for the triplet state and no open-shell singlet could be located for LsAA9, we base the discussion on the triplet state.