The representative snapshots from cluster analysis were subsequently subjected to the energy minimization by using steepest descend (1,250) and conjugate gradient (1,250) algorithms using Amber20. The water shell within 4Å of the protein or within 20 Å of the QM atoms was retained. The QM region consists of the whole Cpd I molecule, ligands, and the Cysteine residue truncated at Cβ positions (SI scheme 1). A larger QM region consisting of extra three F75, F169, and F395 was also used to perform QM/MM calculations for the hydrogen atom abstraction step (SI scheme 1). The residues which are within 10Å of Cpd I and ligands including water molecules were allowed to move freely and the rest of the system was frozen during the geometry optimization. The hydrogen link atoms were used to saturate the dangling bond at the QM/MM boundary. The reaction coordinate was defined by the distance between the oxygen atom of the Fe(IV)=O in Cpd I and the hydrogen atom to be abstracted of the substrate. The transition state (TS) structure connecting the reactant and the product was obtained by performing a relaxed potential energy surface (PES) scan with an increment of 0.1 Å. All the QM calculations were performed with DFT using UB3LYP with D3 dispersion correction and BJ damping , def2-SVP def2/J basis set on all the atoms. The The RIJCOSX approximation with the TightSCF criteria, slowconv, Grid4 and GridX4 were used in the QM calculations. The energies were also computed at UB3LYP-D3BJ/def2-TZVPP level.