The Copper Enzymes

A widely distributed group of enzymes known as the tyrosinases or polyphenol oxidases catalyzes the oxidation of phenolic substances by oxygen. Where these have been isolated, they have been shown to be copper proteins. These enzymes are particularly abundant in plant tissues where they may function as terminal oxidases in place of the cytochrome system the relative importance of the phenol oxidases in plant cell respiration, however, has not yet been determined.The oxidation of ascorbic acid in plant tissues is also due to the presence of a copper enzyme. 

200 jj. Theorell and K. G. Paul, Arkiv Kemi Mineral. Geol. 18A, No. 12 (1944). 

Nelson, in McElroy and Glass, Copper Metabolism, The Johns Hopkins Press, Baltimore, 1950, p. 76. 

Glucose-6-phosphate + TPN — 6-Phosphogluconate - - TPNH2 Benzoquinone + TPNH2 — Catechol + TPN 

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Figure 2.10 Secondary and tertiary structure of the copper enzyme azurin visualized using Wavefunction, Inc. Spartan 02 for Windows from PDB data deposited as 1JOI. See text for visualization details. Printed with permission of Wavefunction, Inc., Irvine, CA. (See color plate.)...

Copper, like iron, is frequently encountered in reactions involving dioxygen. The copper enzyme laccase catalyses the oxidation of uroshiol (the same poisonous substance found in poison oak and ivy) in the production of Japanese lacquer. It is the products of uroshiol oxidation, which are responsible for the lacquer s remarkable material properties. 

Copper Metalloenzymes and Metalloprotein The copper enzymes are mostly oxidases, that is enzymes which catalyse oxidations. Examples are ... 

The chemical rational for the adoption of the d haem ring has not been rigorously determined. Nitrite reduction is one of many examples where biology can use copper or iron. It is striking that no organic cofactor is needed for the copper enzyme. 

The copper enzymes covered in this article span a wide range in terms of both their structmal and reactivity properties. Yet the copper centers in each share similar coordination geometries and have been designated Type... 

Mimosine (9) is an agonist in folate metabolism and suggested to be an inhibitor toward the iron-containing ribonucleotide reductase, the activity of the transcription of the cytoplasmic serine hydroxymethyltransferase gene (shmtl ) and the copper enzymes... 

Klinman JP. The copper-enzyme family of dopamine 3-mono-oxygenase and peptidylglycine a-hydroxylating monooxygenase resolving the chemical pathway for substrate hydroxylation. J. Biol. Chem. 2006 281 3013-3016. 

Zinc is found in more than 80 enzymes. Two of these, carboxypeptidase and carbonic anhydrase, will be discussed here. Copper is also a common metal in enzymes and is present in four different forms. Two of the copper enzymes will also be described. 

The roles of the copper enzymes in electron transport, oxygen transport, and oxidation reactions have guaranteed continued interest in their study. In addition to studies of the natural compounds, there have been many attempts to design model structures of these enzymes, particularly of the binuclear species. Many of these include both nitrogen and oxygen donors built into macrocyclic ligands, although sulfur has been used as well. ... 

Copper is necessary, together with iron, for hematopoiesis, probably partly because it is needed for the synthesis of fer-roxidase (ceruloplasmin). Many enzymes require copper for activity. Examples of some of the copper-enzymes and their functions are given in Table 37-5. Mitochondrial iron uptake may be blocked by deficiency of a cuproprotein, perhaps cytochrome oxidase. Several inherited diseases involving abnormalities in copper metabolism (Wilson s disease, Menkes syndrome) or copper enzymes (X-linked cutis laxa, albinism) occur in human and in several animal species. 

Although there are flavin-dependent as well as copper-dependent amine oxidases, this section will only deal with the copper enzymes. Amine oxidase (EC 1.4.3.6) catalyzes the oxidative reaction of amines to aldehydes and ammonia. The two-step process generates two electrons which are utilized to reduce oxygen to hydrogen peroxide (H202) [28,113] ... 

Two phylogenetically related forms of phenylalanine hydroxylase exist. One contains iron in its active center, while the other contains copper. Whereas the iron enzyme has been found in a wide variety of organisms, from bacteria to mammalians, the copper enzyme has only been reported for Chromobaktenum violaceum [182], The following pertains solely to the copper form. 

Although there is another nitrite reductase (EC 1.9.3.2), which contains iron and utilizes the redox-active cytochromes c and dx, the following will only treat the copper enzyme [90]. 

The copper enzyme family of D/3M and PHM has recently been reviewed. It could be stated that the solution studies indicate that D/3M and PHM are mechanistically interchangeable. The large body of functional data obtained from studies in solution has been evaluated in the context of the now available structural information and a mechanism common for D/3M and PHM has been proposed. The mechanism is shown in Figure 20. 

The accumulation of iron is dependent on its transport into the cell. Askwith and Kaplan (Chapter 4) discuss iron transport mechanisms in eukaryotic cells, developing models based on studies carried out in the yeast, Saccharomyces cerevisiae. These cells possess both siderophore-dependent and elemental iron transport systems. The latter system relies on cell surface ferrireductases to convert extracellular ferric chelates to ferrous iron, which can be transported through either a high or low affinity iron transport system. Studies on a high affinity ferrous iron transporter (FET3) revealed that the multicopper oxidase will oxidize ferrous to ferric iron, which is then mobilized across the membrane by a ferric transmembrane permease (Ftrlp). This is a highly specific transport system in yeast it only transports iron. In humans, the copper enzyme, ceruloplasmin, is responsible for the radical-free oxidase activity. This plasma protein oxidizes the ferrous iron that is excreted from cells into the transferrin-usable ferric form. 

Some of the copper enzymes that have not been associated with a specific pathological defect include cytochrome c oxidase, the terminal enzyme in the mitochondrial electron transport system superoxide dismutase, the enzyme that catalyzes the dismutation of toxic superoxide anions dopamine-g-hydroxylase, the enzyme that catalyzes the conversion of dopamine to norepinephrine and uricase, the enzyme that catalyzes the oxidation of uric acid. Careful analysis of the pathological defects that accompany copper deficiency will probably lead to association of some of these enz3ones with specific pathological defects. However, some of these enzymes may bind copper very tenaciously or have a slow turnover... 

Formation of metal-oxo species first became realistic in model studies on heme oxygenases (shunt path), and it has also become popular in nonheme systems. As for the metal-jd-peroxo species, the distinctive example of model study has appeared in the copper enzymes. The type 7 peroxo species has been proposed first based on models for dicopper oxy-hemocyanin [19, 20] before demonstration of the same oxygen bonding in enzyme [13]. The type 7 x-peroxo structure has been applied to diiron species in sMMO [14] in addition to 6 [21]. (see Chapter 8)... 

Recently, some low-molecular-weight catalysts have been found that mimic enzymes and have potential practical applications. Reactions of limited complexity, mediated by metalloenzymes, have been reported. One such example is a mimic of the copper enzyme galactose oxidase, which catalyzes the oxidation of primary alcohols to aldehydes in the presence of air. 

The copper enzyme N2O reductase (N2OR) catalyses the final step in the process of bacterial denitrification (vide supra). The two-electron reduction of N2O to dinitrogen is, as mentioned above, thermodynamically a very downhill reaction (Eq. 20), but N2O is kinetically inert and a catalyst is necessary to effect this transformation. 

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