Redox reactions superoxide dismutase

Thus, superoxide can react with almost all redox-active metal centers (Scheme 1). In general, going through similar redox reaction steps metal complexes can interact with superoxide either as catalysts for its dismutation (superoxide dismutase (SOD) mimetics), or in a stoichiometric manner (Scheme 1). 

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. 

Wardman, P. Specificity of superoxide dismutase in catalysing redox reactions A pulse radiolysis study. In Radiation Biology and Chemistry. Research Developments (Edwards, H. E. et al., eds.), Amsterdam-Oxford-New York, Elsevier, 1979, pp. 189-196... 

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). 

Superoxide dismutases are found widely in nature where a variety of redox metals (copper, nickel, iron, and manganese) are used to catalyze the disproportionation reaction 202 + 2H+ > 02 I H202.66 The copper/zinc and manganese-... 

Several metals, for example Cu, Zn, and Mn, are associated with a group of enzymes called superoxide dismutases. These enzymes scavenge the superoxide anion, Oj, which may be a by-product of various redox reactions or the electron transport system (Chapter 17). The superoxide anion gives rise to the very de-... 

Superoxide dismutase (SOD, EC 1.15.1.1) is a scavenger of the superoxide anion, and therefore, provides protection against oxidative stress in biological systems [259]. Most SODs are homodimeric metalloenzymes and contain redox active Fe, Ni, Mn or Cu. The superoxide dismutation by SOD is among the fastest enzyme reactions known. The rate constant for CuZnSOD is = 2x 10 s [260], FeSOD is about one order of... 

Genetic and nutritional studies have illustrated the essential nature of copper for normal brain function. Deficiency of copper during the foetal or neonatal period will have adverse effects both on the formation and the maintenance of myelin (Kuo et al., 2001 Lee et al., 2001 Sun et al., 2007 Takeda and Tamana, 2010). In addition, various brain lesions will occur in many brain regions, including the cerebral cortex, olfactory bulb, and corpus striamm. Vascular changes have also been observed. It is also of paramount importance that excessive amounts of copper do not occur in cells, due to redox mediated reactions such that its level within cells must be carefully controlled by regulated transport mechanisms. Copper serves as an essential cofactor for a variety of proteins involved in neurotransmitter synthesis, e.g. dopamine P-hydroxylase, which transforms dopamine to nor-adrenahne, as well as in neuroprotection via the Cu/Zn superoxide dismutase present in the cytosol. Excess free copper is however deleterious for cell metabolism, and therefore intracellular copper concentration is maintained at very low levels, perhaps as low as 10 M. Brain copper homeostasis is still not well understood. 

The herbicidal effect of paraquat is attributable to the formation of superoxide anion (02 ). Superoxide anion is very toxic compound and is formed by the reaction of oxygen with paraquat radical (paraquat ). Plants, algae, and cyanobacteria have ferredoxin-NADP reductase to form NADPH for the reduction of carbon dioxide (see below). The chemolithoautotrophs also have NAD(P) (NAD and NADP) reductase to form NAD(P)H for the reduction of carbon dioxide. Paraquat [mid-point redox potential at pH 7.0 (Emj 0) = -0.43 V] radical is produced when paraquat is reduced by the catalysis of ferredoxin-NAD(P) reductase or NAD(P) reductase, which catalyzes the reduction of many compounds with of around -0.4 V. Although the aerobic organisms (and even many anaerobic organisms) have superoxide dismutase (SOD) which detoxifies superoxide anion in cooperation with catalase [ascorbate peroxidase in the case of plants (Asada, 1999)], the anion accumulates in the organisms when it is over-produced beyond the capacity of SOD. 

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. 

Solutions of dicyclohexyl-18 crown 6 in DMSO have been used to prepare pale yellow 0.15 mol P solutions of KO2 which contain the O2 anion in approximately the same concentration. The specificity of the superoxide dismutase enzyme was used to show that the solution did indeed contain the superoxide ion in solution. I hc redox potentials of the superoxide and hydroperoxy free radicals, O2 and HO2, have been measured by the fast reaction technique of pulse radiolysis and kinetic absorption spectrophotometry. A d.t.a. study has shown that, during the heating of LiC104-K02 mixtures containing <30% KO2, a eutectic melt occurred at 100—250 C with the loss of superoxide oxygen. At 250—300 °C the mixture melted with loss of peroxide oxygen and at 360— 500 °C the perchlorate decomposed with the loss of all the perchlorate oxygen. 

The mechanisms by which manganese complexes and manganese superoxide dismutase react with superoxide radicals are of interest as knowledge of the kinetic parameters and the reaction pathways may allow the synthesis of model compounds with specific chemical features. These compounds may then have clinical application or may allow the control of specific redox chemistry in catalytic processes. 

In order to monitor SOD activity routinely, assays that require only instrumentation typical of a chemical or biochemical laboratory were set up. The assays consist of a reaction mixture that generates superoxide anion and a coupled redox reaction that scavenges the superoxide ion. The latter reaction is usually followed spectrophoto-metrically. The addition of superoxide dismutase destroys superoxide and inhibits the coupled reaction. The specific activity of the enzyme is determined on the basis of the amount of protein required to slow down to 50% the first-order coupled reaction, i.e., one enzymatic unit is the amount of protein that reduces the rate to 50% and the specific activity corresponds to the number of units per milligram of protein. 

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