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In page Homeostasis:
"Many aspects of copper homeostasis are known at the molecular level.[3] Copper's essentiality is due to its ability to act as an electron donor or acceptor as its oxidation state fluxes between Cu1+ (cuprous) and Cu2+ (cupric). As a component of about a dozen cuproenzymes, copper is involved in key redox (i.e., oxidation-reduction) reactions in essential metabolic processes such as mitochondrial respiration, synthesis of melanin, and cross-linking of collagen.[4] Copper is an integral part of the antioxidant enzyme copper-zinc superoxide dismutase, and has a role in iron homeostasis as a cofactor in ceruloplasmin.[citation needed]