Azurin takes part in bacterial photosynthesis. It belongs to the "blue copper" proteins. The coordination sphere of the copper atom is made up of two strongly bound histidine ligands and one cysteinate ligand. The methionine residue and another oxygen atom of a peptide group are only weakly bound.
The strongly distorted 3+1+1 coordination environment of the metal makes it easier to transfer electron density from the cysteine sulfur atom to the copper.