Multicopper oxidases

Solomon El, Sundaram UM, Machonkin TE. 1996. Multicopper oxidases and oxygenases. Chem Rev 96 2563-2605. 

Zheng W, Li QE, Su L, Yan YM, Zhang J, Mao LQ. 2006. Direct electrochemistry of multicopper oxidases at carhon nanotubes noncovalently functionalized with cellulose derivatives. Electroanalysis 18 587-594. 

Figure 12.4 Proposed path for the intracellular transfer of Cu(I) by Atxl. Copper destined for incorporation into the vascular multicopper oxidase Fet3 requires both Ctrl and Ccc2. Cytoplasmic Cu(I)-Atxl, but not apo-Atxl, associates with the amino-terminal domain of Ccc2 and Cu(I) is transferred to the latter. (Inset) A proposed mechanism for the exchange of Cu(I) involving two- and three-coordinate Cu-bridged intermediates. The human homologues of Atxl (Hahl), Ccc2 (Menkes and Wilson s proteins) and Fet3 (ceruloplasmin) are likely to employ similar mechanisms. Reprinted with permission from Pufahl et al., 1997. Copyright (1997) American Association for the Advancement of Science.

Copper oxidases Blue oxidases (multicopper oxidases) Laccase Ascorbate oxidase Ceruloplasmin... 

Harris, Z.L., Davis-Kaplan, S.R., Gitlin, J.D. and Kaplan, J. (2004) A fungal multicopper oxidase restores iron homeostasis in aceruloplasminemia, Blood, 103, 4672-4673. 

Figure 3. Molecular mechanism for the 4e reduction of O2 to H2O by the multicopper oxidases. Reprinted with permission from ref 99. Copyright 2001 American Chemical Society.

The hemocyanlns which cooperatively bind dioxygen are found in two invertebrate phyla arthropod and mollusc. The mollusc hemocyanlns additionally exhibit catalase activity. Tyrosinase, which also reversibly binds dioxygen and dlsmutates peroxide, is a monooxygenase, using the dloxygen to hydroxylate monophenols to ortho-diphenols and to further oxidize this product to the quinone. Finally, the multicopper oxidases (laccase, ceruloplasmin and ascorbate oxidase) also contain coupled binuclear copper sites in combination with other copper centers and these catalyze the four electron reduction of dloxygen to water. 

Type 2, Cu11, present in multicopper oxidases, refers to those Cu11 ions which give rise to more usual visible and ESR spectra. This means that their visible absorption is generally too weak to be observed, although their ESR spectra are just observable. The type 2 Cu ions work in conjunction with the type 1 in the overall electron transfer process. 

Type 3, Cu, also present in multicopper oxidases, is the ESR-silent Cu ions, either Cu1 or binuclear coupled Cu11 ions. Both may be present in some cases, e.g. ceruloplasmin.140 The binuclear centres give rise to intense absorptions around 330 nm and may be dioxygen coordination sites. 

Blue Multicopper Oxidases. These include laccases, ascorbate oxidase, and ceruloplasmin [22,61], which along with cytochrome c oxidase (CcO with Fe and Cu) can couple the one-electron oxidation of substrates (e.g., ascorbate, diamines, monophenols Fe2+ for ceruloplasmin cytochrome c, for CcO) to the full reduction of dioxygen to water (i.e., 02 + 4c + H+ —> 2H20). 

Other Copper Oxidases. A number of additional multicopper oxidases have been detected [22], including phenoxazinone synthase (Table 1). This enzyme catalyzes the overall six-electron oxidative coupling of 2-aminophenols to form 1-aminophenoxazinone, the final step in the bacterial (Streptomyces) biosynthesis of the antineoplastic agent actinomycin. 

Some proteins contain more than one copper site, and are therefore among the most complicated and least understood of all. The active site known as type 4 is usually composed of a type 2 and a type 3 active site, together forming a trinuclear cluster. In some cases, such proteins also contain at least one type 1 site and are in this case termed multicopper oxidases, or blue oxidases [3], Representatives of this class are laccase (polyphenol oxidase) [7-9], ascorbate oxidase (Figure 5.Id) [10], and ceruloplasmin [11], which catalyze a range of organic oxidation reactions. 

The multicopper oxidases (laccase, ascorbate oxidase, and ceruloplasmin) catalyze a four-electron reduction of dioxygen to water (285-287). Consistent with the four-electron stoichiometry, the enzymes contain four copper ions. One of the copper ions is type I, causing an intensely blue color of the proteins, thus the enzymes of this family are referred to as blue oxidases. They also contain a monomeric copper site that exhibits normal spectroscopic features, whereas the other two copper... 

Multicopper Oxidases Type 1 (Blue) Type 2 (Normal) Type 3 (Coupled Binuclear) Total Cu... 

The goal of our research on the multicopper oxidases has been to determine the spectral features of the type 3 (and type 2) centers, to use these spectral features to define geometric and electronic structural differences relative to hemocyanin and tyrosinase, and to understand how these structural differences contribute to their variation in biological function. The hemocyanins and tyrosinases reversibly bind and activate dioxygen whereas the multicopper oxidases catalyze its four-electron reduction to water. 

This research was supported by the National Science Foundation (CHE-9217628) for the blue copper studies and by the National Institutes of Health (DK-31450) for the coupled binuclear and multicopper oxidase studies. Edward I. Solomon expresses his sincere appreciation to all his students and collaborators who are listed as co-authors in the references for their commitment and contributions to this science. 

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