Our results provide a mechanistic explanation for that, similar to other oxoreductases, the LPMO active-site tyrosine is critical for initializing the protective hole-hopping mechanism. This tyrosine is widely conserved in most LPMO families, except for the majority of AA10 LPMOs, where it is replaced by phenylalanine. Our results can thus explain that it has recently been discovered that fungal AA9 LPMOs are less prone to oxidative damage than their bacterial (AA10) counterparts. In a previous paper 40 we compared calculated UV-vis spectra with the characteristic Shown are only the most feasible energetics for B3LYP/def2-TZVPP (structures were optimized with TPSS/def2-SV(P). Energies in grey are from ref. Note that the reaction barrier TS III was obtained with a bigger QM region. observed bands around 400-420 nm for 2, Considering that we carried out the TD-DFT calculations as vacuum calculations, the intense peak from the open-shell singlet at 386 nm (3.22 eV) is in reasonable correspondence with the experimental room temperature absorption spectrum for TaAA9 with the most intense peak at 420 nm (2.95 eV). The corresponding experimental value for LsAA9 is 414 nm (2.99 eV) 41other AA9 LPMOs show intense peaks in the same region (see refs. ). For the triplet state of TaAA9, we also tried to calculate the spectrum of 2 including electrostatics the enzyme as point-charges, but the effect of the point-charges on the position of the intense transition due to the tyrosyl is minimal (see Fig. in the SI).