Copper Proteins: Oxidases

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Abstract
Oxidases are enzymes that use dioxygen as the electron acceptor in oxido‐reduction reactions. Many members of this enzyme class (EC 1.) rely on the Cu1+/Cu2+redox cycle of copper prosthetic group(s) in their catalysis of the electron transfer from reducing substrate to O2. Those copper oxidases that catalyze the four‐electron reduction of O2to 2H2O are known as multicopper oxidases (MCO) because each member of this group contains at least four copper atoms of three distinct electronic and spectral types. These three types are: type 1 or ‘blue’ Cu(II), so‐called because of its distinguishing strong absorbance at ∼600 nm; type 2 Cu(II), comparable in electronic structure to that found in square‐planar copper complexes; and type 3 Cu(II), a Cu(II)Cu(II) pair bridged by O(H) and thus diamagnetic. With four 1e−metal redox sites, MCOs are the all‐copper equivalent of the copper‐heme respiratory terminal oxidases. However, MCOs are true oxidases in that all members can use as electron acceptors common (di)phenolic, and aromatic (di)amine substrates, for example, hydroquinone, ascorbic acid, and phenylenediamine. The laccases and ascorbate oxidase are examples of these MCOs (EC 1.10.3). Members of a separate MCO class (EC 1.16.3) also use lower valent metal ions as substrates, for example, Fe2+, Cu1+, and/or Mn2+. These are thereby designated metallo‐oxidases; the most widely studied activity of this class has been towards Fe2+giving rise to the common enzyme name, ferroxidase. The typical MCO structure consists of three ‘cuprodoxin’ domains, a Greek‐key beta‐fold that characterizes a family of mononuclear copper electron‐transfer factors. Among MCOs are both soluble and type 1 membrane proteins. Functionally, MCOs are known to play essential roles in biodegradation and transformation, cell redox balance, and metal metabolism. This article summarizes current knowledge of this class of copper oxidase enzymes.