The dual personality of ionic copper in biology

Biological copper is mainly involved in electron transport to catalyse essential oxido-reduction processes. It is an essential trace element which is extremely toxic because exchangeable intracellular copper is Cu(I) which generates reactive oxygen species. To handle this paradox the evolution has led to a fine homeostasis in which copper ions are never free. Intracellular Cu(I) instead is bound to numerous proteins forming specific cascades towards its targets.