Copper can be found in the active sites of most enzymes that catalyze redox reactions (i.e., oxidoreductases), as it facilitates single electron transfers while reversibly oscillating between the Cu and Cu redox states. Enzymes typically contain between one (mononuclear) and four (tetranuclear) copper centers, which enable enzymes to catalyze different reactions. These copper centers coordinate with different ligands depending on the Cu redox state. Oxidized Cu preferentially coordinates with "hard donor" ligands (e.g., N- or O-containing ligands such as histidine, aspartic acid, glutamic acid or tyrosine), while reduced Cu preferentially coordinates with "soft donor" ligands (e.g., S-containing ligands such as cysteine or methionine). Copper's powerful redox capability makes it critically important for biology, but comes at a cost: Cu is a highly toxic metal to cells because it readily abstracts single electrons from organic compounds and cellular material, leading to production of free radicals. Thus, cells have evolved specific strategies for carefully controlling the activity of Cu while exploiting its redox behavior.