The non-blue copper centres are the most common in copper proteins, they are essentially planar, with the copper ion coordinated to the imidazole N of two or three histidine residues and to O-ligands, weak additional coordination in 5th and 6th positions, with consequent Jahn-Teller distortion characteristic of d9 Cu(II) complexes can also occur. Thus the T2 sites are more ionic than T1 sites. Working in cooperation with organic coenzymes, T2 copper sites are attributed to a multitude of biological oxidation reactions, including alcohol oxidation and amine degradation.
In caeruloplasmin, one T2 copper joins a T3 dimer to form a copper trimer, similar clusters are found in laccase and ascorbate oxidase.