To estimate the reduction potentials of the electron transfer steps, we used a method that was shown to be the most reliable for calculating reduction potentials of iron-sulfur clusters in proteins. No attempt to modify the method was done, i.e., the best performing functional, basis set, and QM region size was adopted directly from ref. 52. Thus, we performed QM-cluster single-point calculations using a continuum solvent model with a dielectric constant of 80. The default COSMO optimized radii were employed for all atoms except Cu, for which a radius of 2.0 was used. All calculations were based on optimized QM/MM structures, the QM regions were extended by including all functional groups within a radius of 3.5 Å. The total system consisted of approximately 650 atoms. All calculations were performed with TPSS/def2-SV(P) as described above. The reduction potentials were calculated according to: 56