Structural domains comprising this superfamily share the structure of the bacterial Copper resistance protein K (CopK), a dimeric periplasmic dimeric protein which is synthesised at high levels in the presence of copper, process which leads to its accumulation in the periplasm PMID:21936507. CopK has two distinct copper binding sites and CuICuII-CopK is formed as a results of cooperative binding of CuI and CuII at their corresponding sites under oxidizing conditions. The binding of CuI greatly enhances the specific affinity for CuII and, in turn, CuII binding increases the specific affinity for CuI, although to a lesser extent. This type of cooperative behaviour is unprecedented in copper binding proteins PMID:19236095