All QM/MM calculations were performed on the representative snapshots taken from the MD simulation of complexes 2 and 3 (cf. Scheme 2). In all cases, a truncated heme-porphyrin ring, and the proximal serine (HO-C2H5) residue were kept in the QM zone along with the reactive ligand of the respective complexes. The representative snapshots were based on the closest available distance of interest of the most populated MD trajectories.