O2’’ dismutation

Nitric oxide is not in itself very reactive or particularly toxic. However, its interaction with other oxidants may yield products whose toxicity may result of significance. Such is the case of peroxynitrite (ONOO ) which is formed in a non-enzymatic reaction between NO and O2-. This reaction follows a 1 1 stoichiometry and it has a very fast reaction speed (6.7 X 10 mol/L s [40] which is 3 times higher than the capacity of O2 dismutation by SOD and around 1000 times greater than the reaction of NO with iron-sulfiir clusters [41 ]. 

Following discovery of the superoxide dismutase (SOD) enzymes by Keilin and Mann (i), and their ability to catalyze the dismutation of the superoxide radical anion (O2) in laboratory experiments (2), discussion has taken place over the intervening years as to whether such enzymes do catalyze O2 dismutation in vivo, and whether there is a need for a complex enzyme to accelerate a reaction that is already very fast. This is not the place to address these points, but the current feeling is that the answer to both questions is in the affirmative. 

In photosjmthesis, secondary pathways for electron transfer actually lead to superoxide production (9-12), the thylakoid-bound Cu2 Zn2SOD catalyzing the O2 dismutation. Because the chloroplasts lack catalase, the excess H2O2 is a substrate for ascorbate peroxidases to produce dehydroascorbate, which is further re-reduced by NADPH via the glutathione-glutathione reductase pathway (12). 

Studies on Cu2Zn2SOD performed over the past 25 years have produced an enormous amount of information on the system. Nevertheless, several aspects of the mechanism of action of Cu2Zn2SOD toward O2 dismutation are still unclear. In this section we will summarize the facts that have been ascertained about Cu2Zn2SOD chemistry and with which any mechanistic proposal should be consistent. 

The rate of O2 dismutation is affected by the ionic strength of the medium (105, 118, 327). For instance, between pH 7 and 8 the activity of bovine Cu2Zn2SOD diminishes by about 70% on passing from ionic strength I = 0.02 M to / = 0.25 M (118). These results have been confirmed for the human enzjune. Interestingly, the ionic changes at pH 8.0 have a small effect bn the neutral and acidic human SOD mutants of the Arg-143 residue, which possess the lesser activity (95). Above pH 12.0 the effect of solute activity vanishes. This observation implies the presence of electrostatic control of the access of superoxide to the active site (vide infra). 

SCHEME 2 Schematic illustration of analytical mechanism of (a) first-, (b) second-, and (c) third-generation 02 biosensors. Note that the reactions shown in (b) and (c) are bi-directional since SODs are enzymes specifically catalyzing the O2 dismutation, i.e. oxidation into O2 and reduction into H2O2. 

The superoxide anion radical (O2) dismutates to H2O2, and via a transition metal-catalyzed Haber-Weiss and/or Fenton reactions can give rise to the hydroxyl radical (-OH) (see references). 

Hydrogen peroxide, H2O2 Two-electron reduction product of O2 formed from O2 by dismutation or directly from O2. Reactivity of O2 and H2O2 amplified in the presence of heme protein. 

It has been proposed that the actual oxidant of the A" substrate is the newly formed oxygen, as indicated by the quantitative yields of the nonoxidized substrate when O2 from dismutation is continuously purged from the reaction vessel (311, 313). 

Superoxide dismutation bovine erythrocyte superoxide dismutase O2 removal (dismutation)... 

Some substances, e.g., Mn02 aq, Pt, Ag and Au, catalyze the (redox) dismuta-tion of H2O2 into H2O and O2. It is surprising then that the Co(NH3)5 in [Co02(NH3)s] stabilizes the O2 against dismutation. 

Section 5.2.3 in Chapter 5. Superoxide dismutase enzymes catalyze dismutation of the superoxide anion radical (O2 ) according to the summary reactions in equation 7.1 ... 

This is a dismutation reaction because one molecule of O2 is reduced whilst the other is oxidised ... 

The hydroperoxyl radical formed, "00H, has a pKg of 4.75 and exists in water as a superoxide radical anion 02 Either species in water dismutates to produce O2 and hydrogen peroxide, H2O2, or, possibly, react with dissolved or adsorbed phenol. Dilute H2O2 does... 

The superoxide anion radical (O2 ) is produced when oxygen accepts one electron. This radical has a short lifetime in aqueous solutions, where it mainly undergoes spontaneous dismutation to hydrogen peroxide and oxygen (Reaction 1). The superoxide radical is in equilibrium with its conjugated acid, the hydroperoxyl radical (HO2), which is a stronger oxidant and generally more reactive than O2... 

SODs. Nevertheless, the D. baarsii Rbo was reported to reduce the steady-state level of intracellular superoxide in the SOD-deficient E. coli strain (Liochev and Fridovich 1997). This Rbo in vitro showed evidence for superoxide reductase (SOR) activity (Lombard et al. 2000), i.e., reduction of O2, presumably to FI2O2, without dismutation (Eq. 10.3). 

A mechanism without reduction of Cu, similar to the reaction between HO and O2, was presented for the catalysis of the dismutation of superoxide by the Cu cation... 

The demonstration that PMNs formed O2- in the respiratory burst necessitated the consideration of all the species which result when dioxygen is reduced one electron at a time (Fig. 1). Superoxide, the result of the reduction of dioxygen by one electron, appears to act mainly as a mild reductant in aqueous solutions. But when it coexists with H2O2, its spontaneous dismutation product, O can initiate a number of potentially injurious events [reviewed by Fridovich The primary means by which cells deal with superoxide anions appears to be through the catalysis of their dismutation by a family of metalloenzymes collectively designated superoxide dismutases. 

The rate constants of O catalytic dismutation, determined for the nitroxides tested, increased with [H+], indicating that OOH, but not O2 oxidizes nitroxide ... 

Nevertheless, with respect to the catalysis of the dismutation of O2, little difference has been found among the three types of SOD.I0) The reaction rate constants between O2 and the enzyme are about 2X109 M- s at neutral pH. However, although the catalytic rate of the CuZn-SOD is constant between pH 5 and 9.5, those of the Fe- and Mn-SODs become progressively lower as the pH is raised above 8.5. 

The dismutation product of O2, H2O2, inactivates CuZn-SOD82 and Fe-SOD, but not Mn-SOD83> (Table 10.1). 

As the two one-electron redox potentials involved are close to 1V, many aromatic compounds can be oxidized to a radical cation. This is a common source of free radical production during intoxication with xenobiotics [31]. The large difference in redox potential between the two couples O2/H2O2 and H2O2/H2O provides a thermodynamic driving force for H2O2 dismutation catalyzed by catalase ... 

The redox-active copper of copper/zinc-superoxide dismutases [24,37] is associated with an E° for [Cun/Cur] of +0.3-0.4 V, approximately mid-way between the redox potential E° aq for O2 reduction to 02 relative to 1M 02, and that for 02 reduction to H2O2, enabling dismutation to take place. 

Salts of 02 can be solubilized as crown-ethers in aprotic solvents, where 02 then behaves as a powerful nucleophile [73], but this property is essentially not observed in water where its spontaneous dismutation to O2 and H2O2 proceeds efficiently around neutral pH. The pKa of H02/02 is only 4.8, but deprotonation of many organic substrates by 02" is driven to the right by the accelerated dismutation of (H02 + 02 ) ... 

As illustrated in Pig. 1, reactions of nonprotein and protein-bound coenzymes and metals with O2 or with H2O2 can serve as routes of production of damaging oxygen radical species. H202 can be formed from the dismutation of O2" (Eqn 4) ... 

Reaction of superoxide with water-soluble Fe porphyrins results in dismutation of superoxide to O2 and H2O2 (equation 19) (see Oxygen Inorganic Chemistry). ... 

Lastly the copper oxidases that appear in mammals are listed. Many of these seem similar to their counterparts in the plant kingdom with the exception of some forms of superoxide dismutase. In mammals it is a zinc-copper enzyme and catalyzes the dismutation of the 02" radical according to the reaction Oj" + O2" + 2H" — H2O2 + O2. This reaction... 

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