Superoxide dismutase dismutation reaction

Superoxide dismutase will scavenge the Of formed and will therefore inhibit the reduction of the dianisidine radical by Of. Consequently the dianisidine radical will dismute to yield the divalently oxidized dianisidine. In the presence of superoxide dismutase this reaction is augmented (Fig. 6). The possibility that Of could reduce the final product of dianisidine oxidation and reverse the change in absorbance at 460 nm was tested and was excluded. The assay has been used to determine the rate constant for purified swordfish liver copper/zinc superoxide dismutase (Bannister et al., 1979) and could be applied to crude extracts. The assay was also found applicable to polyacryalmide gels (Misra and Fridovich, 1977c). Gels soaked in riboflavin plus dianisidine, and subsequently illuminated, developed stable brown bands. Peroxidases are also stained by this procedure due to the photochemical production of hydrogen peroxide. However, the development of bands due to peroxidase activity is much slower than the development of bands due to dismutase activity. 

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 ... 

ROS generation is generally a cascade of reactions that starts with the production of superoxide. Superoxide rapidly dismutates to hydrogen peroxide either spontaneously, particularly at low pH, or catalyzed by superoxide dismutase (SOD). 

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). 

In this reaction (demonstrated in vitro), one of the two radicals is oxidizing while the other is reducing. In vivo, this reaction is catalyzed by one of several isoforms of an enzyme known as superoxide dismutase (SOD). As shown above, hydrogen peroxide may form as a result of the superoxide anion s dismutation reaction however, it may also be produced from a bivalent reduction of 02. The addition of the second electron leads to the formation of hydrogen peroxide, which is a powerful oxidizing agent. Due to the unpaired electrons in their outer shells, free radicals are favored to pair with other molecules during bimolecular collisions. 

HP here is great interest in the biochemistry and relevant coordination chemistry of copper-containing proteins (1,2, 3, 4, 5). They are widely distributed in both plants and animals and are often involved in oxygen metabolism, transport, and use. One of the most actively studied copper proteins is bovine erythrocyte superoxide dismutase (SOD) (6,7,8). This enzyme catalyzes the dismutation of superoxide ion, Reaction 1. 

The reaction products of superoxide ions are believed to be partly responsible for the removal and destruction of bacteria and damaged cells [1]. In view of its low reactivity it is unlikely that superoxide itself is responsible for killing the invading material, but the hydrogen peroxide formed from dismutation by superoxide dismutase can kill some strains of bacteria. Once the phagocytic... 

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... 

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... 

The overall reaction is initiated by an electron transfer from ascorbate to 4-NQO with the production of A and 4-NQO" (Reaction 46, l). The 4-NQO" radical reacts rapidly [reported values for similar compounds range from 10 to 10 M V (62,63)] with molecular oxygen to yield superoxide radical (Reaction 46, 2), which dismutates to peroxide and oxygen (Reaction 46, 3) or reacts with ascorbate (Reaction 46, 4). Specific tests with superoxide dismutase and catalase suggest that OH radicals are formed in this system by a Haber-Weiss (64) and/or Fenton (65) type reaction ... 

Spontaneous dismutation is thus most rapid at the acidic pH values needed to protonate 02% but the rate at neutral or alkaline pH values is greatly accelerated by the presence in chloroplasts of a superoxide dis-mutase enzyme, which catalyzes Reaction 4. Superoxide dismutase in the form of a copper-zinc enzyme is found both free in the stroma and bound to the outside of the chloroplast thylakoids (24-29). 

Manganese compounds of biologic importance are examined by pulse radiolysis e.g., the rate of dismutation of radiation-generated Of is catalyzed by Escherichia coli, Mn-containing superoxide dismutase involving electron transfer in which enzymes with Mn(IV), Mn(IIl), Mn(II) and Mn(I) oxidation states are involved. A kinetic model for the reaction mechanism of an Mn dismutase from Bacillus stearothermophilus accounts for the variation of the rate of decay on the concentrations of Oj, enzyme, HjOj, NaNj, KCN and H+. 

The catalyzed disproportionation (dismutation) of superoxide has been studied extensively, particularly in regard to its enzymatic catalysis by the superoxide dismutases (SODs). The superoxide dismutases are metalloenzymes and can have Cu-Zn, Fe, Mn, or Ni active sites. Simple inorganic species can also catalyze the reaction.28 For example, catalysis by Fe2+/Fe(III) at pH 7.2 has the rate law... 

Free superoxide is dealt with by superoxide dismutase (SOD) via a dismutation reaction (8.2) in which, for two molecules of the same type, one is oxidized and the other reduced. 

The reactions of TTHA (Mn -triethylenetetraminehexaacetate) complexes with H02-02 radicals were studied (pH 2.5-9.S), and a mechanism was suggested that involves the formation of a transient Mn tthaH( 02 f complex. At low pH, this complex is protonated, with the release of H2O2. At higher pH, the dismutation of 02 from the equilibrium complex (Mn TTHA(02 f Mn TTHA + Os") is competitive with protonation. At low pH, the results indicate that there is a rapid first-order process that may be an isomerization from end-bound to side-bound of the attached superoxide radical. In contrast, the kinetics of the dismutation of superoxide radical by Escherichia coh MnSOD (manganese superoxide dismutase) were measured and shown to fit a mechanism involving the rapid reduction of Mn SOD by superoxide followed by both the direct reoxidation of Mn SOD by superoxide and the formation of a Mn SOD(02 ) complex. The differences in the mechanisms are discussed. 

The dismutation of O2 by iron superoxide dismutase was found to be similar to that for the copper/zinc bovine superoxide dismutase. The results obtained by Lavelle et al. (1977) showed that catalysis of dismutation of O2 by the iron superoxide dismutase from Photobacterium leiognathi is first order with respect to substrate concentration for all ratios of substrate to enzyme concentrations reported. Although the enzyme is stable between pH 6.0 and 10.0, the value of the rate constant decreases as the pH increases. The second-order rate constant for the reaction be-... 

Enzymatic mechanisms can defend against ROS, too. Superoxide dismutase (SOD) is a family of metalloenzymes that catalyze dismutation (reactions in which identical molecules have different fates). The reaction catalyzed is as follows ... 

In aprotic media, (O2) behaves like a strong nucleophile but in protic media, it is only weakly nucleophilic, due to the presence of protons. Thus, reactions of (02) in protic media are determined by kinetic rather than by thermodynamic parameters (concentrations, pH, ionic strength). The most important reaction in terms of biological effects is the dismutation of (02) (Eqs. (4) and (5)), which proceeds rather slowly in physiological conditions (ATdis-S X 10 M s at pH 7.4 Halliwell and Gutteridge, 1989), but can be considerably favoured by superoxide dismutases (SOD) (EC 1.15.1.1) (Chance et al., 1979 1.6 x 10 M-> s" ). 

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. 

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