Copper Proteins: Oxidases

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 Cu 1+ /Cu 2+ redox cycle of copper prosthetic group(s) in their catalysis of the electron transfer from reducing substrate to O 2 . Those copper oxidases that catalyze the four‐electron reduction of O 2 to 2H 2 O 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, Fe 2+ , Cu 1+ , and/or Mn 2+ . These are thereby designated metallo‐oxidases; the most widely studied activity of this class has been towards Fe 2+ 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.

Structure-Function Analysis of the Cuprous Oxidase Activity in Fet3p from Saccharomyces cerevisiae

“…This was the first report of cuprous oxidase activity attributable to a eukaryotic ferroxidase; this activity has been reported also in the copper resistance protein in Escherichia coli, CueO (22,23). We have suggested that MCO proteins that exhibit reactivity toward lower valent metal ions be designated metallooxidases (4,20).…”

“…The specificity an MCO has toward the one-electron donor (reducing) substrate is determined by the relative rate of electron transfer from a given substrate to the T1 Cu(II), which is the acceptor site in this outer sphere ET process (2,4,44). An outer sphere electron transfer reaction rate is analytically given by the Marcus equation, Equation 1 (45).…”

“…that exhibit an activity toward Fe(II) not shared by the other members of this family of copper proteins (1)(2)(3)(4). MCO proteins are distinguished by having three distinct copper sites designated copper types 1, 2, and 3 (2,4).…”

“…MCO proteins are distinguished by having three distinct copper sites designated copper types 1, 2, and 3 (2,4). All MCO proteins couple the oxidation of four molecules of a one-electron donor substrate with the reduction of one molecule of dioxygen to 2H 2 O.…”

The copper-iron connection in biology: Structure of the metallo-oxidase Fet3p

“…Cp is a member of a large family of proteins (Ͼ500 homologs found in all three super kingdoms) known as multicopper oxidases (MCOs). MCOs that contain Ϸ500 amino acid residues are composed of three Greek key ␤-barrel cupredoxin (plastocyanin-like) domains (5) that come together to form three spectroscopically distinct copper binding sites termed type 1 (T1), type 2 (T2), and type 3 (T3) (6,7). With Ͼ1,100 residues, Cp differs from Fet3p through duplication of this trimeric cupredoxin assembly (8).…”