Superoxide anion catalysis

Indeed, when present in concentrations sufficient to overwhelm normal antioxidant defences, ROS may be the principal mediators of lung injury (Said and Foda, 1989). These species, arising from the sequential one-electron reductions of oxygen, include the superoxide anion radical, hydrogen peroxide, hypochlorous ions and the hydroxyl radical. The latter species is thought to be formed either from superoxide in the ptesence of iron ions (Haber-Weiss reaction Junod, 1986) or from hydrogen peroxide, also catalysed by ferric ions (Fenton catalysis Kennedy et al., 1989). 

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. 

Another illustration of the power of molecular dynamics simulation can be drawn from the sphere of enzyme catalysis. Many enzyme-catalyzed reactions proceed at a rate that depends on the diffusion-limited association of the substrate with the active site. Sharp et al. [28] have carried out Brownian dynamics simulations of the association of superoxide anions with superoxide dismutase (SOD). The active center in SOD is a positively charged copper atom. The distribution of charge over the enzyme is not uniform, and so an electric field is produced. Using their model, Sharp et al. [28] have shown that the electric field enhances the association of the substrate with the enzyme by a factor of 30 or more. Their calculations also predict correctly the response of the association rate to changes in ionic strength and amino... 

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. 

The catalysis of the disproportionation of the superoxide anion O2 by Pt clusters, in subcolloidal solutions or supported on colloidal Ti02 particles, has also been smdied by time-resolved techniques. The decay of Oj obeys first order kinetics with respect to both Oj" and Pt clusters because the catalysis is governed indeed by the proton concentration adsorbed at the cluster surface. 

SODs accelerate, by four orders of magnitude, the spontaneous reaction of dismutation of superoxide radical anion by a cychc oxidation-reduction mechanism of an active site metal ion. In the case of CuZnSOD, the Cu ion is involved in the catalysis. In the catalytic cycle, the Cu is reversibly oxidized and reduced by successive encounters with OT to yield O2 and H2O2 ... 

The generation of O2 from potassium superoxide was also applied to stop-flow procedures. In this method O2 was dissolved in dimethyl sulfoxide and stabilized in 18-crown-6-polyether. This method is useful for mechanistic studies indeed, McClune and Fee (1976) were able to obtain catalytic rate constants for bovine copper/zinc superoxide dismutase as a function of pH in various buffers. More recently the mechanism of catalysis and of anion inhibition of iron superoxide dismutase from E. coli have been examined by this method using a specially constructed stop-flow spectrophotometer (Bull and Fee, 1985). A limitation of the method is that the pre-equilibrium state cannot be properly investigated because of the time resolution of the stop-flow equipment (== 5 msec). 

Displacement by cyanide works particularly well, but there are many other examples of anion exchange reactions which are enhanced by phase transfer catalysis. Most monovalent anions can be transferred, including alkoxides, phenoxides, thiocyanates, nitrates, nitrites, superoxides and all of the halides, but divalent anions are usually too hydrophilic to be transferred into the organic phase. 

During heme catalysis, a Fe + protoporphyrin complex (P-Fe +), like in myoglobin, will be oxidized by air to P-Fe + as indicated in Formula 3.66. The formed superoxide radical anion O2, whose properties are discussed... 

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