The type-1 Cu centers considered here are critical to electron transfer in biological systems. The Cu center is coordinated by two histidines and one cysteine residue in a trigonal planar structure, and mutations or modifications of this site are known to cause disorders related to copper homeostasis. Two protonated forms of these centers exist in solution, which are depicted for the enzyme fern plastocyanin (PDB code 1KDI) in Fig. . In the HetPCM models, a singular histidine residue is exposed to a higher dielectric, as indicated. Following Ref. 107, 1KDI is used as a reference for relative pK a calculations in other enzymes. The free energy of each protein (Pro), relative to that of plastocyanin (Plc), is estimated using the reaction