All QM/MM calculations employed a subtractive QM/MM scheme with electrostatic embedding with van der Waals linkatom corrections and the Z1 charge-distribution scheme . They were performed in ComQum, an interface that combines the QM software TURBOMOLE (V7.5.0) with the MM software AMBER 22 . Geometry optimizations and linear transit calculations were generally performed with the TPSS functional, and def2-SV(P) basis set keeping the geometry of the MM part fixed. Reaction 2 --→ 3 was additionally optimized with the B3LYP functional and def2-SV(P) basis set. The second-coordination-sphere histidine HIS208 was used as proton donor. Histidines can adopt different protonation states. We label them as HIE if N ε2 is protonated, HID if N δ 1 is protonated and HIP if both nitrogen atoms are protonated. All energies reported are from single-point calculations employing a def2-TZVPP basis set based on the optimized geometries and using TPSS, B3LYP and in some cases (reactions 2 --→ 3, 3 --→ 4 and 3 red --→ 4 red ) CAM-B3LYP . We mainly report B3LYP energies, but use TPSS to compare to the results by Caldararu et al.. All calculations included D3 dispersion correction with Becke-Johnson damping and the TPSS calculations employed the resolution of identity (RI) approximation using the corresponding standard auxiliary basis sets. Reactions were calculated as linear transits with a single restrained reaction coordinate. The point with the highest energy was assigned to be the transition state (TS).