The met axial ligand determines the redox potential in Cu-A sites

The replacement of the axial methionine ligand in a native Cu-A protein rendered a series of stable mutants with spectroscopic features of a mixed valence center. The mutations resulted in minor perturbations of the electronic structure of this site but led to significant changes in the redox potential.

This suggests that the axial ligand plays a role in tuning the redox potential Of Cu-A, resembling the behavior observed in blue copper sites.