Copper ions can donate or accept electrons and thus, function as catalysts in a number of metalloenzymes involved in a variety of biochemical processes, such as cellular respiration, iron homeostasis and antioxidant defense. Although copper ions are essential for many cellular processes, excessive free copper ions in the cytoplasm are highly toxic. When the intracellular free copper ion reaches a high level, it will compete with other metal ions for important biological ligands or active sites of enzymes. Furthermore, excessive free copper will generate reactive oxygen species that degrade DNA, proteins and lipids. Consequently, both deficiency and excess of copper ions are disadvantageous, which will result in fatal neurodegenerative diseases, such as Wilson and Menkes disease, Parkinson disease, Alzheimer's diseases and prion diseases.

As for the toxic effects of free copper ions in the cytoplasm, organisms need some strategies to balance the concentration of copper ions. On the other hand, due to the hydrophilic property of metal ions, copper ions cannot permeate through the biomembranes without carriers. To fulfill the task, many copper-chelating proteins came into being within the millions of years' evolution, which can specially bind copper ions, assist it to traverse the membrane and hand it to the copper-dependent proteins, finally complete the compartmentalization of copper ions and maintain the homeostasis of the metal.