InterPro domain: IPR045087

Copper is one of the most prevalent transition metals in living organisms and its biological function is intimately related to its redox properties. Since free copper is toxic, even at very low concentrations, its homeostasis in living organisms is tightly controlled by subtle molecular mechanisms. In eukaryotes, before being transported inside the cell via the high-affinity copper transporters of the CTR family, the copper (II) ion is reduced to copper (I). In blue copper proteins such as cupredoxin, the copper (I) ion form is stabilised by a constrained His2Cys coordination environment.

Multicopper oxidases oxidise their substrate by accepting electrons at a mononuclear copper centre and transferring them to a trinuclear copper centre; dioxygen binds to the trinuclear centre and, following the transfer of four electrons, is reduced to two molecules of water [ 1 ]. There are three spectroscopically different copper centres found in multicopper oxidases: type 1 (or blue), type 2 (or normal) and type 3 (or coupled binuclear) [ 2 , 3 ]. Multicopper oxidases consist of 2, 3 or 6 of these homologous domains, which also share homology to the cupredoxins azurin and plastocyanin. Structurally, these domains consist of a cupredoxin-like fold, a beta-sandwich consisting of 7 strands in 2 beta-sheets, arranged in a Greek-key beta-barrel [ 4 ]. Multicopper oxidases include:

• Ceruloplasmin ( 1.16.3.1 ) (ferroxidase), a 6-domain enzyme found in the serum of mammals and birds that oxidizes different inorganic and organic substances; exhibits internal sequence homology that appears to have evolved from the triplication of a Cu-binding domain similar to that of laccase and ascorbate oxidase.
• Laccase ( 1.10.3.2 ) (urishiol oxidase), a 3-domain enzyme found in fungi and plants, which oxidizes different phenols and diamines. CueO is a laccase found in Escherichia coli that is involved in copper-resistance [ 5 ].
• Ascorbate oxidase ( 1.10.3.3 ), a 3-domain enzyme found in higher plants.
• Nitrite reductase ( 1.7.2.1 ), a 2-domain enzyme containing type-1 and type-2 copper centres [ 5 , 6 ].

In addition to the above enzymes there are a number of other proteins that are similar to the multi-copper oxidases in terms of structure and sequence, some of which have lost the ability to bind copper. These include: copper resistance protein A (copA) from a plasmid in Pseudomonas syringae; domain A of (non-copper binding) blood coagulation factors V (Fa V) and VIII (Fa VIII) [ 7 ]; yeast FET3 required for ferrous iron uptake [ 8 ]; yeast hypothetical protein YFL041w; and the fission yeast homologue SpAC1F7.08.

1. Dioxygen reduction by multi-copper oxidases; a structural perspective. null , 3507-13
2. The blue oxidases, ascorbate oxidase, laccase and ceruloplasmin. Modelling and structural relationships. Eur. J. Biochem. 187, 341-52
3. A structure-derived sequence pattern for the detection of type I copper binding domains in distantly related proteins. FEBS Lett. 279, 73-8
4. Crystal structure and electron transfer kinetics of CueO, a multicopper oxidase required for copper homeostasis in Escherichia coli. Proc. Natl. Acad. Sci. U.S.A. 99, 2766-71
5. Novel types of two-domain multi-copper oxidases: possible missing links in the evolution. FEBS Lett. 553, 239-44
6. Metal coordination and mechanism of multicopper nitrite reductase. Acc. Chem. Res. 33, 728-35
7. Cofactor proteins in the assembly and expression of blood clotting enzyme complexes. Annu. Rev. Biochem. 57, 915-56
8. The FET3 gene of S. cerevisiae encodes a multicopper oxidase required for ferrous iron uptake. Cell 76, 403-10