Plastocyanin is one of the family of blue copper proteins that are involved in electron transfer reactions. The copper-binding site is described as distorted trigonal pyramidal. The trigonal plane of the pyramidal base is composed of two nitrogen atoms (N and N) from separate histidines and a sulfur (S) from a cysteine. Sulfur (S) from an axial methionine forms the apex. The distortion occurs in the bond lengths between the copper and sulfur ligands. The Cu−S contact is shorter (207 pm) than Cu−S (282 pm).
The elongated Cu−S bonding destabilizes the Cu(II) form and increases the redox potential of the protein. The blue color (597 nm peak absorption) is due to the Cu−S bond where S(pπ) to Cu(d−y</sub>) charge transfer occurs.
In the reduced form of plastocyanin, His-87 will become protonated with a pK of 4.4. Protonation prevents it acting as a ligand and the copper site geometry becomes trigonal planar.