Copper, like iron, is a redox metal. Like iron, copper exists in multiple valence states; Cu+ and Cu2+(cuprous and cupric) are the most common. Copper enzymes, while not nearly as numerous as zinc, fill important biological functions, mostly within the cytosol. Many fit the category of oxidor-eductases, or more specifically 'oxidases,' meaning they catalyze reactions in which electrons from the substrate are transferred to O2. Copper enzymes can be simple or complex, depending on the number of Cu atoms in the enzyme. Simple enzymes generally contain one Cu per subunit. The more complex enzymes include the multicopper oxidases, which may have as few as four, e.g., laccase, or as many as eight copper atoms per enzyme, e.g., dopamine-¡-monooxygenase. Copper in these enzymes exists in three different chemical environments referred to as type 1, type2, and type 3 copper sites. Cerulo-plasmin, for example, contains 6-7 Cu atoms in three distinct sites. The type 1 copper site gives a blue color to ceruloplasmin and other blue copper proteins. The copper-binding sites in a multicopper oxidase form a triad consisting of one type two and two type 3 coppers arranged as an isosceles triangle. Oxygen binds to the two type 3 coppers at the base of the triangle. Examples of copper enzymes include cytochrome c oxidase, lysyl oxidase, and ascorbate oxidase (Table 5).
Reactivity Because it is prone to accept electrons, copper is a powerful oxidant in biological systems. The copper sites in ceruloplasmin have the capacity to oxidize Fe2+ to Fe3+, which prepares ferric ions to bind to transferrin and deliver iron to the organs and tissues. This reaction links iron with copper metabolism and could explain how an absence of copper in the diet impairs the transport of iron and causes anemia in humans. In Cu2, Zn2 superoxide dismutase, the Cu2+ at the active site removes the single nonbonding electron from one superoxide anion (O2) and transfers it to another:
Seldom is copper destined to perform only a structural role and many enzymes that possess copper as a cofactor use the metal at the active site. More recent studies have linked copper ions with the formation of blood vessels or angiogenesis. One of the more exciting discoveries yet to be fully understood is that depriving an animal or human of copper delays or even inhibits the growth of cancerous tumors. From a nutritional perspective, this could
Table 5 Important copper enzymes
Enzyme Source Function Cu/protein
Ascorbate oxidase Squash Ascorbate catabolism 8
Ceruloplasmin Plasma Iron oxidation 6-7
Cytochrome c oxidase Mitochondria Electron transport 2
Dopamine-ß-monooxygenase Adrenal Noradrenaline synthesis 8
Lysyl oxidase Aorta Collagen, elastin synthesis 1
Superoxide dismutase Erythrocyte Superoxide radical destruction 2
mean that copper is essential for the development of the microvascular system.
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