Copper enzymes superoxide dismutase

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). 

Although zinc itself is not redox-active, some class I enzymes containing zinc in their active sites are known. The most prominent are probably alcohol dehydrogenase and copper-zinc superoxide dismutase (Cu,Zn-SOD). AU have in common that the redox-active agent is another transition-metal ion (copper in Cu,Zn-SOD) or a cofactor such as nicotinamide adenine dinucleotide (NAD+/NADH). The Zn(II) ion affects the redox reaction only in an indirect manner, but is nevCTtheless essential and cannot be regarded simply as a structural factor. 

The enzyme copper, zinc superoxide dismutase (Cu,Zn-SOD, EC 1.15.1.1) catalyzes the disproportionation of superoxide anion to dioxygen and hydrogen peroxide (equations 1 and 2). Crystallographic data can be found in References 41-46. This antioxidant enzyme is present in the cytosol and mitochondrial intermembrane space of eukaryotic cells and in the periplasmic space of bacterial cells as a homodimer of 32 kDa. Each monomer binds one copper and one zinc ion. The reaction mechanism involves the... 

Starting at the far left, we see a water molecule, two common amino acids, alanine and tryptophan, a segment of a DNA double helix, a segment of a protein single helix, and the folded polypeptide chain of the enzyme copper, zinc superoxide dismutase or SOD. 

Figure 10-12. The copper centre in the enzyme superoxide dismutase from spinach. Why are zinc and copper needed together What is the importance of the bridging histidine ...

C2. Ceballos-Picot, I., Trivier, J.-M., Nicole, A., Sinet, P.-M., and Thevenin, M., Age-correlated modifications of copper-zinc superoxide dismutase and glutathione-related enzyme activities in human erythrocytes. Clin. Chem. 38 66-70 (1992). 

Fig. 1. Schematic overview of copper trafficking and homeostasis inside the yeast cell. The actions of Mad and Ace 1, copper-dependent metalloregulatory transcription factors, control the production of copper import [copper transporter (Ctr) and reductase (Fre)] and detoxification/sequestration [metallothionein (MT)] machineries, respectively. Three chaperone-mediated delivery pathways are shown. Atxl shuttles Cu(I) to the secretory pathway P-type ATPase Ccc2 (right). CCS delivers Cu(I) to the cytoplasmic enzyme copper-zinc superoxide dismutase (SOD) (left). Coxl7 shuttles Cu(I) to cytochrome c oxidase (CCO) in the mitochondria (bottom). Mitochondrial proteins Scol and Sco2 may also play a role in copper delivery to the CuA and CuB sites of CCO. Copper metabolism and iron metabolism are linked through the actions of Fet3, a copper-containing ferroxidase required to bring iron into the cell (lower right) (see text).

Delivery of Copper to the Cytosol The Copper Requiring Superoxide Dismutase Enzyme... 

CD spectroscopy has also provided valuable insight into the chemical stability and chemical denaturation of proteins. A recent study by Rumfeldt etal. examines the guanidinium-chloride induced denaturation of mutant copper-zinc superoxide dismutases (SODs). These mutant forms of the Cu, Zn-SOD enzyme are associated with toxic protein aggregation responsible for the pathology of amyotrophic lateral sclerosis. In this study, CD spectroscopy was used in conjunction with tryptophan fluorescence, enzyme activity, and sedimentation experiments to study the mechanism by which the mutated enzyme undergoes chemical denaturation. The authors found that the mutations in the enzyme structure increased the susceptibihty of the enzyme to form partially unfolded destabilized monomers, rather than the stable metaUated monomer intermediate or native metallated dimer. 

Advances include radioprotective drugs applied during radiotherapy of neoplastic diseases. Effects similar to the enzyme superoxide dismutase have been found in copper complexes of Schiffs bases (derived from different amino acids and salicylaldehyde). Activity of complexes is dependent on their structures. The structural changes of their chelate rings are responsible for their effects on free radicals produced in organisms during radiation. Complexes have square pyramidal pentacoordination, which is similar to the coordination polyhedron in the active center of the Cu-dependent superoxide dismutase. The complexes used as radio-protective drugs play an important role in radiotherapy. They protect healthy tissues and cells from injurious radiation. ° ... 

Oka, S., Ogino, K., Matsuura, S., Yoshimura, S., Yamamoto, K., Okazaki, Y., Takemoto, T., Kato, N., and Uda, T Human serum immuno-reactive copper, zinc-superoxide dismutase assayed with an enzyme monoclonal immunosorbent in patients with digestive cancer. Clin. Chim. Acta 182, 209-220 (1989). 

Copper occurs in almost all life forms and it plays a role at the active site of a large number of enzymes. Copper is the third most abundant transition metal in the human body after iron and zinc. Enzymes of copper include superoxide dismutase, tyrosinase, nitrite reductase and cytochrome c oxidase. Most copper proteins and enzymes have roles as electron transfer agents and in redox reactions, as Cu(II) and Cu(I) are accessible. 

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