Copper-containing glycoprotein

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. 

A blue, copper-containing glycoprotein present in mammalian blood plasma and containing type 1, type 2, and type 3 copper centers. The type 2 and type 3 copper centers are close together, forming a trinuclear copper cluster. Ceruloplasmin has an important role in the transport and storage of copper ions. Thus, it participates in the metabolism of copper-containing enzymes. 

This is a tetrameric copper-containing glycoprotein, which catalyzes the conversion of dopamine into norepinephrine, a hormone/neurotransmitter, and also the hydroxylation of a number of substituted phenylethylamines.1401 1403 Consideration of this enzyme has been hindered by uncertainty over the stoichiometry of binding of copper, and, until recently, there seemed to be good evidence for four copper ions per molecule.1 4 However, there is now convincing support for a stoichiometry of eight copper ions per molecule, with two per subunit.1405,1406 This conclusion is based upon analysis and titration methods. 

The amino acid L-tyrosine is the starting material of melanin biosynthesis. The first step of melanogenesis is centered on tyrosinase (EC 1.14.18.1), a copper-containing glycoprotein. Until recently, this enzyme was believed to catalyze the hydroxylation of L-tyrosine... 

Noradrenergic and adrenergic neurons also contain the enzyme dopamine (i-hydroxylase. Like tyrosine hydroxylase, this enzyme is a mixed function oxygenase. The electron donor in this case is ascorbic acid rather than tetrahydrobiopterin, and dopamine is the primary substrate. The enzyme has been well characterized and is a tetramer of 75,000-Da subunits, which are copper-containing glycoproteins. No major regulatory systems are known for this enzyme. It appears that there are sufficient enzyme molecules present in cells in which this enzyme is expressed to completely convert all the dopamine that is formed into norepinephrine. 

Tyrosinase is a copper-containing glycoprotein that carries a coupled binuclear copper active site capable of catalyzing two distinct reactions (i) hydroxylation of tyrosine to dihydroxyphenylalanin, i.e. dopa (cresolase activity), and (ii) subsequent two-electron oxidation to dopaquinone (catecholase activity) (227). Both reactions require oxygen and the enzyme, i.e. tyrosinase, in reduced cuprous form. Because of these two catalytic functions, tyrosinase is in modern terminology referred to as a mixed function oxidase (757). A mechanism (Fig. 3) for hydroxylation... 

Ceruloplasmin a blue, copper-containing glycoprotein, which is present in mammalian blo plasma, and which is phylogenetically related to plant lac-cases and to ascorbate oxidase, both of which are blue. It is a transport and/or storage protein for copper, and it is also an oxidase. Substrates include unsaturated compounds, such as indole derivatives, amphetamine, adrenalin and dopamine. C. has a central role in copper metabolism when C. is absent, as in Wilson s disease, copper is deposited in the tissues, causing death. 

Caeruloplasmin (Cp) is an acute phase glycoprotein with a copper transport function. At least 90% of total plasma copper is bound to Cp with the remaining 10% associated with albumin, histidine and small peptides. Lipid peroxidation requires the presence of trace amounts of transition metals and the copper-containing active site of Cp endows it with antioxidant capacity... 

Copper is a cofactor in several enzymes, including lysyl oxidase and superoxide dismutase. Ceruloplasmin, a deep-blue glycoprotein, is the principal copper-containing protein in blood. It is used to transport Cu2+ and maintain appropriate levels of Cu2+ in the body s tissues. Ceruloplasmin also catalyzes the oxidation of Fe2+ to Fe3+, an important reaction in iron metabolism. Because the metal is widely found in foods, copper deficiency is rare in humans. Deficiency symptoms include anemia, leukopenia (reduction in blood levels of white blood cells), bone defects, and weakened arterial walls. The body is partially protected from exposure to excessive copper (and several other metals) by metal-lothionein, a small, metal-binding protein that possesses a large proportion of cysteine residues. Certain metals (most notably zinc and cadmium) induce the synthesis of metallothionein in the intestine and liver. 

The residues of NeuBAc (a sialic acid) situated at the ends of the oligosaccharide chains of many plasma glycoproteins (Fig. 7-31) protect those proteins from uptake and degradation in the Uver. For example, ceruloplasmin, a copper-containing serum glycoprotein, has several oligosaccharide chains ending in NeuBAc. Re-... 

The laccase molecule is a dimeric or tetrameric glycoprotein, which contains four copper atoms per monomer, distributed in three redox sites. More than 100 types of laccase have been characterized. These enzymes are glycoproteins with molecular weights of 50-130 kDa. Approximately 45% of the molecular weight of this enzyme in plants are carbohydrate portions, whereas fungal laccases contain less of a carbohydrate portion (10-30%). Some studies have suggested that the carbohydrate portion of the molecule ensures the conformational stability of the globule and protects it from proteolysis and inactivation by radicals (Morozova and others 2007). 

Because Cu2+ is the most tightly bound metal ion in most chelating centers (Table 6-9), almost all of the copper present in living cells is complexed with proteins. Copper is transported in the blood by a 132-kDa, 1046-residue sky-blue glycoprotein called ceruloplasmin.471475-477 This one protein contains 3% of the total body copper. 

Ceruloplasmin is an intensely blue glycoprotein of the a2-globulin fraction of mammalian blood, which acts as a copper transfer protein and probably has a role in iron storage. The structure is known it contains three Type 1 (Tl) sites (one of which seems to be inactive) and a Type 2/Type 3 (T2/T3) trinuclear cluster. It is believed to be part of the process of oxidizing Fe(II) to Fe(III) in the transfer of iron from ferritin to transferrin. Reduction of two Tl sites and the T3 pair is fast, but reduction of the T2 Cu site is slow the pathways of electron transfer between the sites have been investigated, but the complete mechanism is still unknown. 

Ceruloplasmin, the copper protein in plasma, deserves special attention. Human plasma contains approximately 32 mg/100 ml of this protein (see Table 6) (C2, C12, R4). According to the recent careful measurements of Kasper and Deutsch (K3), the molecular weight of ceruloplasmin is 160,000, somewhat higher than the 151,000 obtained by Pedersen (P3). Ceruloplasmin is an a2-globulin and contains 8 atoms of copper per molecule (H14). In addition to being a metalloprotein, it is a glycoprotein containing 7% carbohydrate hexose, hexosamine, and neuraminic acid (L4). Ceruloplasmin has been prepared in a pure form from human and pig sera (H14). Several newer methods are now available for its isolation and purification (B29, C20, D4, L4, S4, S7, S39). 

Ceruloplasmin is a blue a-2 glycoprotein of 132 kD. This plasma protein is responsible for the binding of 90 to 95% of the blood plasma copper in vertebrates. In addition to its primary role in copper transport and homeostasis, it possesses a number of additional enzymatic activities (see ref. 179 for a recent review). The complete amino acid sequence of human ceruloplasmin has been determined (42), establishing that this large multicopper oxidase is synthesized as a single chain polypeptide, containing 1046 amino acid residues. 

Ceruloplasmin. This is the most well-known and yet the least understood copper protein. It is an a2-globulin and there is conclusive evidence that this 132000 molecular weight glycoprotein has just one polypeptide chain [9]. It has 7% carbohydrate. It contains six atoms of copper per molecule. Copper in ceruloplasmin exists in both cupric and cuprous forms. Partial removal of copper from ceruloplasmin results in the loss of its characteristic blue color and loss of enzymatic activity. Ceruloplasmin has enzymatic oxidase activity toward several substrates at pH 5.4-S.9. The best substrate is p-phenylenediamine or its dimethyl derivative. Ceruloplasmin can oxidize ferrous ion to ferric ion. The ceruloplasmin level in the newborn is less than 10 mg/100 mL serum. It rises to adult levels (30.4 5 mg/lOO mL) by 2-4 months of age and continues to rise to a peak at 2-3 years and this declines to adult levels by 12 years of age. The level of ceruloplasmin is affected by various pathological states. Usually, a pronounced deficiency of this protein in serum is characteristic of both Wilson s and Menkes s diseases although normal levels have also been reported in the case of Wilson s disease. 

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