The copper protein plastocyanin is formed using phenol oxidases, which contain copper.
It takes part in electron transfer in photosystem I, in which the reversible conversion from Cu+ to Cu2+ + e- takes place. It is a "blue copper" protein.
The coordination sphere of the copper atom is comprised of two strongly bound histidine ligands and one cysteinate ligand. The methionine residue is, in the reduced form shown here, only loosly bound.
The strongly distorted coordination environment of the metal makes the transfer of electron density from the cysteine sulfur atom to the copper easier.