Copper-zinc superoxide dismutase active site

Guanine is the most easily oxidizable natural nucleic acid base [8] and many oxidants can selectively oxidize guanine in DNA [95]. Here, we focus on the site-selective oxidation of guanine by the carbonate radical anion, COs , one of the important emerging free radicals in biological systems [96]. The mechanism of COs generation in vivo can involve one-electron oxidation of HCOs at the active site of copper-zinc superoxide dismutase [97, 98], and homolysis of the nitrosoperoxycarbonate anion (0N00C02 ) formed by the reaction of peroxynitrite with carbon dioxide [99-102]. 

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. 

Fig. 6.2. Schematic structure of the active site of copper-zinc superoxide dismutase [72]. The zinc ion can be substituted, among others, by copper(II), cobalt(II) and nickel(II) ions.

The second protein, copper/zinc superoxide dismutase (SOD, EC 1.15.1.1), catalyzes the disproportionation of O2 (Equation (10)) at a heterodimetallic Cu/Zn active sited 489... 

A relatively large number of theoretical studies on the complete calculation of redox potentials of transition metal active sites in metalloproteins have been published. Metalloproteins studied include manganese superoxide dismutase, iron superoxide dismutase, copper-zinc superoxide dismutase, iron-sulfur proteins, cytochrome f, components of the photosynthetic reaction center, and peroxidases. ... 

Kamnakaran, C., Zhang, H., Crow, J.P., Anthohne, W.E., Kalyanaraman, B., 2004. Direct probing of copper active site and free radical formed during bicarbonate-dependent peroxidase activity of bovine and human copper, zinc-superoxide dismutases. Low-temperature electron paramagnetic resonance and electron nuclear double resonance studies. J. Biol. Chem. 279, 32534—32540. 

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. 

The enzymatic activity of the zinc-free protein is pH-dependent whereas native CUjZnjSOD is active over a wide pH-range (pH 5.0-9.5) Cu(II) in the zinc binding site gives little superoxide dismutase activity. This conclusion arises from the observation that AgjCUjSOD in which copper is in the zinc binding site, has almost no activity and that the Cu Cu enzyme has nearly the same activity as... 

Cu—Zn superoxide dismutases (SODs) [87,88] are abundant in eukaryotic cells and may serve to protect cells against the toxic effects of superoxide or deleterious oxy-products derived from 02 . The active site copper and zinc ions are 6.3 A apart and are bridged by a histidine imidazolate. In the oxidized form Cu(II) is roughly pentacoordinate, with four His N s and a water molecule. A highly conserved Arg residue is thought to stabilize Cu(II)-bound anions (e.g., Cu(II)—02 ) a redox reaction releases 02, generating Cu(I), which can reduce more 02 substrate to give peroxide and Cu(II). 

The structure and enzyme kinetics of bovine erythrocyte superoxide dismutase are reviewed. The protein has a novel imidazolate-bridged copper(II)-zinc(II) catalytic center in each of two identical subunits. Since a C /Cu1 redox couple is responsible for the dismutase activity of the enzyme, the role of zinc is of interest. Both 220-MHz NMR measurements of the exchangeable histidine protons and chemical modifications using diethylpyrocarbonate demonstrate that zinc alone can fold the protein chain in the region of the active site into a conformation resembling that of the native enzyme. Other possible roles for zinc are discussed. Synthetic, magnetic, and structural studies of soluble, imidazolate-bridged copper complexes of relevance to the 4 Cu(II) form of the enzyme have been made. 

Copper is an essential element to most life forms. In humans it is the third most abundant trace element only iron and zinc are present in higher quantity. Utilization of copper usually involves a protein active site which catalyzes a critical oxidation reaction, e.g., cytochrome oxidase, amine oxidases, superoxide dismutase, ferroxidases, dopamine-/ -hydrox-ylase, and tyrosinase. Accordingly, animals exhibit unique homeostatic mechanisms for the absorption, distribution, utilization, and excretion of copper (J). Moreover, at least two potentially lethal inherited diseases of copper metabolism are known Wilson s Disease and Menkes s Kinky Hair Syndrome (I). 

The alkaline phosphatase family contains three metals in the active site while the rest of the cocatalytic zinc sites contain two metals (Figme 10) (Table 3). Some of these sites contain metals such as copper, iron, and magnesium in combination with zinc. Combinations of Cu(II)/Zn are seen in the superoxide dismutase (SOD) family see Copper Proteins with Type 2 Sites), Zn/Mg are seen in alkaline phosphatase family and lens aminopeptidase, and Fe(III)/Zn in the purple acid phosphatase family. 

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