Cu-A centers perform efficient long-range electron transfer. The electronic structure of native Cu-A sites can be described by a double-potential well with a dominant sigma(u)* ground state in fast equilibrium with a less populated pi(u) ground state. Here, we report a Cu-A mutant in which a lysine was introduced in the axial position. This results in a highly unstable protein with a pH-dependent population of the two ground states. Deep analysis of the high-pH form of this variant shows the stabilization of the pi(u) ground state due to direct binding of the Lys residue to the copper center that we attribute to deprotonation of this residue.