Copper-containing hydroxylases

The copper metalloenzymes are involved in oxygen-using reactions. These enzymes include cytochrome c oxidase (respiratory chain), lysyl oxidase (collagen synthesis), and dopamine [3-hydroxylase (neurotransmitter synthesis). Lysyl oxidase is a small protein with a molecular weight of 32 kDa. This enzyme contains an unusual modification, namely cross-linking between two different parts of its polypeptide chain. The cross-linked region consists of a structure called lysine tyrosylquinone (Klinman, 1996). Two amino acids are involved in this cross-linked structure, and these are Lys 314 and Tyr 349. Lysine tyrosylquinone is used as a cofactor and is necessary for the catalytic activity of the enzyme. Other copper metalloenzymes contain a related cofactor, namely 2,4,5-tiihydrox5q5henylalanine (topaquinone, TPQ). Serum amino oxidase is a copper metalloenzyme that contains TPQ. TPQ consists of a modified residue of phenylalanine. The copper in the active site of the enzyme occurs immediately adjacent to the TPQ cofactor. 

Type 2 copper centers are not uniform in ligand or ligand stereochemistries. One common feature is, however, that in the active enzyme, one coordination site is always free to bind oxygen. The most common ligand in type 2 copper centers is histidine. Tyrosine (often modified), methionine, and cysteine occur as well. There are three histidines and a modified tyrosine in amine oxidase and lysyl oxidase [28]. In diamine oxidase, two of the histidine residues have probably been replaced by cysteines [29]. In galactose oxidase, the copper ion is coordinated by two tyrosines, two histidines and an acetate ion [30]. Dopamine-/J-hydroxylase contains two differently coordinated copper ions per functional unit. One is coordinated by three histidines and a methionine and the other by two histidines and another, yet unknown, ligand [ 31 ]. Last but not least, the type 2 copper ion in Cu,Zn-superoxide dismutase is coordinated by four histidine residues, one of which connects the copper ion to the zinc ion, the second metal ion in the active site of the enzyme [32,33] (Fig. 6). 

Dopamine -hydroxylase has been obtained in essentially pure form from bovine adrenal glands (4). It has a molecular weight of approximately 290,000. Although the protein is colorless, metal analysis showed that it is a copper protein containing between 0.65 and 1.0 /xgram of copper per mg. of protein (4-7 moles of copper/mole protein). Part of the copper is present as Cu and part as Cu. Although the amount of Cu varies from one enzyme preparation to another, the amount of Cu is relatively constant and is equal to about two moles per mole of enzyme. 

Dopamine (5-hydroxylase is a copper-containing enzyme involved in the synthesis of the catecholamines norepinephrine and epinephrine from tyrosine in the adrenal medulla and central nervous system. During hy-droxylation, the Cu+ is oxidized to Cu " reduction back... 

A number of peptide hormones have a carboxyl terminal amide which is derived from a glycine terminal residue. This glycine is hydroxylated on the a-carbon by a copper-containing enzyme, peptidylglycine hydroxylase, which, again, requires ascorbate for reduction of Cu ". ... 

Under conditions of copper deficiency, some methanotrophs can express a cytosolic, soluble form of MMO (sMMO) (20-23), the properties of which form the focus of the present review. The sMMO system comprises three separate protein components which have all been purified to homogeneity (24,25). The hydroxylase component, a 251 kD protein, contains two copies each of three subunits in an a 82y2 configuration. The a subunit of the hydroxylase houses the dinuclear iron center (26) responsible for dioxygen activation and for substrate hydroxylation (27). The 38.6 kD reductase contains flavin adenine dinucleotide (FAD) and Fe2S2 cofactors (28), which enable it to relay electrons from reduced nicotinamide adenine dinucleotide (NADH) to the diiron center in the... 

Dopamine /3-hydroxylase (D/3H) is a copper-containing glycoprotein that hydroxylates dopamine at the benzylic position to norepinephrine.84 During the attempted crystallization of the bis(hydroxide)-bridged dicopper(II) dimer, a side product was subsequently isolated (complex (63)), revealing intramolecular hydroxylation at a formally benzylic position of the tris(imidazo-lyl)phosphine ligand.85 The copper(II) center has an axially compressed TBP structure. 

Copper is part of several essential enzymes including tyrosinase (melanin production), dopamine beta-hydroxylase (catecholamine production), copper-zinc superoxide dismutase (free radical detoxification), and cytochrome oxidase and ceruloplasmin (iron conversion) (Aaseth and Norseth 1986). All terrestrial animals contain copper as a constituent of cytochrome c oxidase, monophenol oxidase, plasma monoamine oxidase, and copper protein complexes (Schroeder et al. 1966). Excess copper causes a variety of toxic effects, including altered permeability of cellular membranes. The primary target for free cupric ions in the cellular membranes are thiol groups that reduce cupric (Cu+2) to cuprous (Cu+1) upon simultaneous oxidation to disulfides in the membrane. Cuprous ions are reoxidized to Cu+2 in the presence of molecular oxygen molecular oxygen is thereby converted to the toxic superoxide radical O2, which induces lipoperoxidation (Aaseth and Norseth 1986). 

These systems are also described as normal copper proteins due to their conventional ESR features. In the oxidized state, their color is light blue (almost undetectable) due to weak d-d transitions of the single Cu ion. The coordination sphere around Cu, which has either square planar or distorted tetrahedral geometry, contains four ligands with N and/or 0 donor atoms [ 12, 22]. Representative examples of proteins with this active site structure (see Fig. 1) and their respective catalytic function include galactose oxidase (1) (oxidation of primary alcohols) [23,24], phenylalanine hydroxylase (hydroxy-lation of aromatic substrates) [25,26], dopamine- 6-hydroxylase (C-Hbond activation of benzylic substrates) [27] and CuZn superoxide dismutase (disproportionation of 02 superoxide anion) [28,29]. 

Another interesting effect of pyrazole in vivo is to reduce brain noradrenaline levels in treated animals. This activity is shared by 4-methylpyrazole (70) but not by 4-hydroxypyrazole (71), suggesting that this effect is not related to the inhibition of brain catalase activity (81MI10503). Studies have been performed which suggest that pyrazole inhibits dopamine (3- hydroxylase (a copper-containing enzyme implicated in noradrenaline metabolism) in vivo but not in vitro, but methodological problems make these results equivocal (80MI10506). The mechanism by which pyrazole reduces brain noradrenaline levels is therefore as yet unknown. 

Copper has an essential role in a number of enzymes, notably those involved in the catalysis of electron transfer and in the transport of dioxygen and the catalysis of its reactions. The latter topic is discussed in Section 62.1.12. Hemocyanin, the copper-containing dioxygen carrier, is considered in Section 62.1.12.3.8, while the important role of copper in oxidases is exemplified in cytochrome oxidase, the terminal member of the mitochondrial electron-transfer chain (62.1.12.4), the multicopper blue oxidases such as laccase, ascorbate oxidase and ceruloplasmin (62.1.12.6) and the non-blue oxidases (62.12.7). Copper is also involved in the Cu/Zn-superoxide dismutases (62.1.12.8.1) and a number of hydroxylases, such as tyrosinase (62.1.12.11.2) and dopamine-jS-hydroxylase (62.1.12.11.3). Tyrosinase and hemocyanin have similar binuclear copper centres. 

In vivo tolerance to copper is quite high, however, deficiency and excess are serious problems. Infants are particularly vulnerable as they take time to assimilate the correct levels and it is known that trace copper from cooking utensils or water pipes can cause childhood cirrhosis. Copper deficiency leads to arterial weakness and heart enlargement. This is probably caused by a decrease in catecholamine neurotransmitters derived from the biosynthesis of adrenaline which requires the copper-containing enzymes phenylalanine hydroxylase, dopamine P-monooxygenase and tyrosinase. 

Ascorbic acid has specific and weU-deflned roles in two classes of enzymes the copper-containing hydroxylases (such as dopamine /3-hydroxylase and peptidyl glycine hydroxylase) and the 2-oxoglutarate-linked iron-containing hydroxylases, of which the best studied are the proline and lysine hydroxylases involved in maturation of connective tissue (and other) proteins. 

---