Cu-A is a binuclear copper site acting as electron entry port in terminal heme-copper oxidases. In the oxidized form, Cu-A is a mixed valence pair whose electronic structure can be described using a potential energy surface with two minima, sigma(u)* and pi(u), that are variably populated at room temperature. We report that mutations in the first and second coordination spheres of the binuclear metallocofactor can be combined in an additive manner to tune the energy gap and, thus, the relative populations of the two lowest-lying states. A series of designed mutants span sigma(u)*/pi(u) energy gaps ranging from 900 to 13 cm(-1). The smallest gap corresponds to a variant with an effectively degenerate ground state. All engineered sites preserve the mixed-valence character of this metal center and the electron transfer functionality. An increase of the Cu-Cu distance less than 0.06 angstrom modifies the sigma(u)*/pi(u) energy gap by almost 2 orders of magnitude, with longer distances eliciting a larger population of the pi(u) state. This scenario offers a stark contrast to synthetic systems, as model compounds require a lengthening of 0.5 angstrom in the Cu-Cu distance to stabilize the pi(u) state. These findings show that the tight control of the protein environment allows drastic perturbations in the electronic structure of Cu-A sites with minor geometric changes.