Catalytic dismutation

Valentine el al. demonstrated that Cu2+ ions are in the same site in native Cu, Zn-SOD protein and CuESOD (E = empty, i.e. with removal of Zn ion from the native Cu, Zn-SOD) and the activity of the CuESOD is at least 80 5% that of the native Cu, Zn-SOD [99], However, the EZnSOD (E = empty, i.e. with removal of Cu ion from the native Cu, Zn-SOD) does not show any activity for catalytic dismutation of 02. This suggests Cu2+ present in the Cu, Zn-SOD is essential for 02" dismutation even though Zn2+ is also required for dismutase activity [100-102],... 

Some sulfate-reducing bacteria have been shown to contain SODs and catalases, but other species do not demonstrate the expected activities for these enzymes (Hatchikian et al. 1977 van Niel and Gottschal 1998 Dos Santos et al. 2000). The apparent absence of these enzymes can be rationalized on the basis that their catalytic dismutation reactions (Eqs. 10.1 and 10.2) generate dioxygen, which may be disadvantageous for strict anaerobes but raises the question of how these organisms protect themselves against transient air exposure. 

Catalytic Dismutation vs. Reversible Binding of Superoxide Ivana Ivanovic -Burmazovic... 

The catalytic dismutation of superoxide is actually more complicated in E. coli [42] and B. thermophilus [43] Mn-SODs than that of either Cu or Fe proteins since it may involve an inactive form of the enzyme. The inactive form is believed [44] to contain a Mnm-side-on peroxo unit (of the type shown in Figure 29) formed within the hydrophobic environment of MnSOD, in the absence of H+, by the oxidative addition of the superoxide ion to the Mn11 center. When H+ ions are present, an active, end-on peroxo complex forms, yielding successively a bound hydroperoxide ion and free dihydrogen peroxide (cf. Figure 3). Thus, the key parameter that turns the reaction off or on may be the absence or presence of a H+ ion [44],... 

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

Although the catalytic dismutation of phosgene undoubtedly represents a valid and... 

Clearly, there are some very intriguing results that any theory must be able to explain. For example, the two ligands 17 and 18 afford complexes which are either extremely active or inactive. Further the Mn(II) complex of the pentamethyl ligand 10 has no pH dependence to its catalytic rate, even though it has the highest pH-dependent rate measured in this class of complexes, with k = 3.90 x 10 M" s . Thus, any theory which attempts to rationalize the observed reaction rate constants for catalytic dismutation of superoxide must predict why the Mn(II) complex of ligand 10 has no pH-dependent outer-sphere rate. 

Another interesting example of radical-radical reactions is the combination of G(-H) and 02 radicals [29], In healthy tissues, the concentrations of 02 radicals is small and is regulated by SODs [70, 71]. These enzymes rapidly deactivate 07 radicals by catalytic dismutation to oxygen and hydrogen peroxide ... 

SODs are differentiated mainly by the redox-active metal in the active site copper, manganese, or iron. The iron and manganese SODs are structurally similar (5-11) and are structurally distinct from the Cu,Zn SOD (12). The dramatic features of these enzymes are that they catalytically dismutate superoxide at rates that are not only diffusion controlled but have been shown to be electrostatically facilitated (13). In these systems, modifications of amino acid residues near the active site have been shown to alter the enzymatic activity, indicating that superoxide is electrostatically drawn into the active site channel (14). In addition, in contrast to the spontaneous dismutation rate of 02 and the dismutation rates of 02 by many metal complexes, all of which are pH dependent, the enzymatic dismutation rate is largely pH independent over the pH range (5-10). 

Numerous indirect assays, such as the cytochrome c assay, have been used in attempts to measure the SOD activity of putative SOD mimics,However, these assays, which typically rely on a spectrophotometric change of a redox indicator to measure superoxide levels, cannot kinetically distinguish between a catalytic dismutation of superoxide and a stoichiometric interaction of superoxide with the putative SOD mimic. Moreover, the indirect assays are prone to false positives or false negatives respectively, when the putative SOD mimic oxidizes or reduces the spectrophotometric indicator. 

We recognized the need for methodology to measure SOD activity directly that would be more accessible to the bench-top scientist than is the method of pulse radiolysis, another direct measure. Consequently, we developed methodology to measure the catalytic dismutation of superoxide by stopped-flow kinetic analysis.By this technique, we directly monitor the decay of superoxide spectrophotometrically in the presence or absence of a putative SOD mimic at a given pH. Kinetic analysis of this decay can determine whether the complex is a SOD mimic (decay of superoxide becomes first-order in superoxide and first-order in complex see equations 1 and 2), or is inactive (decay of superoxide remains second-order for its self-dismutation see equation 3). At least a tenfold excess of superoxide over the putative SOD mimic is used in the stopped-flow assay, to eliminate contributions due to a stoichiometric reaction of the complex with superoxide. A catalytic rate constant for the dismutation of superoxide by the complex can be determined from the observed rate constants of superoxide decay as a function of catalyst concentration. ... 

It was intriguing to note significant higher rate constants of the catalytic dismutation of superoxide when native SOD was replaced by simple inorganic copper salts However, this phenomenon is only observed in acidic media and in the absence... 

As described above, the self-assembly of Cys promotes the electron transfer between SOD and the Au electrode. This Cys-promoted rapid and direct electron transfer of SOD and its relevance to the redox reaction of the copper complex moiety in SOD formed a strong basis for the development of a SOD-based third-generation biosensor for 02 because the copper complex moiety has been well documented as the active site for the catalytic dismutation of O2 [128]. Figure 10-39 shows CVs at the bare Au, Cys/Au and SOD/Cys/ Au electrodes in PBS (O2 saturated) containing 0.002 unit of XOD and 50 pM xanthine, i.e. in the presence of O2. Both cathodic and anodic peak currents corresponding to the redox reaction of the SOD confined on the electrode are significantly increased, compared with those in the absence of O2 (Fig. 10-36a) [133—135]. Such a redox response was not... 

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