H2O2 reduction catalysis

Despite the high catalytic activity toward H2O2 reduction and disproportionation, ORR catalysis does not appear to proceed via free H2O2 as inferred from... 

When earlier transition metals are used in the cofacial DPX and DPD porphyrin motifs, oxidation reactions are promoted by a metal 0x0 intermediate. As for reductive catalysis, the oxidative catalytic reactions of the Pacman porphyrins are derived from PCET. The catalase reaction (H2O2 disproportionation to give 0.5 O2 and H2O) is exemplary of this PCET reactivity as the reaction includes both 0 0 bond making and bond-breaking catalysis coupled to proton transport. 

So-called blue multinuclear copper oxidase enzymes, such as laccase and ascorbate oxidase, catalyze the stepwise oxidation of organic substrates (most likely in successive one-electron steps) in tandem with the four-electron reduction of O2 to water, i.e. no oxygen atom(s) from O2 are incorporated into the substrate (Eq. 4) [15]. Catechol oxidase, containing a type 3 center, mediates a two-electron substrate oxidation (o-diphenols to o-chinones), and turnover of two substrate molecules is coupled to the reduction of O2 to water [34,35]. The non-blue copper oxidases, e.g. galactose oxidase and amine oxidases [27,56-59], perform similar oxidation catalysis at a mononuclear type 2 Cu site, but H2O2 is produced from O2 instead of H2O, in a two-electron reduction. 

In contrast to the active site of galactose oxidase, to pre-catalyst 13, and to the system reported by Stack et al., the proposed catalytic species 15 does not imdergo reduction to Cu intermediates, as the oxidation equivalents needed for the catalysis are provided for solely by the phenoxyl radical Hgands. Since the conversion of alcohols into aldehydes is a two-electron oxidation process, only a dinuclear Cu species with two phenoxyl ligands is thought to be active. Furthermore, concentrated H2O2 is formed as byproduct in the reaction instead of H2O, as in the system described by Marko et al. [159]. 

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. 

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 ... 

Flavin-dependent oxynitrilase has many properties common to flavoprotein oxidases. The enzyme binds sulfite in an N5 adduct, and one-electron reduction produces anionic semiquinone.The sulfite adduct and one- or two-electron reduced enzyme are inactive, as is apoenzyme, suggesting that the flavin is involved in catalysis. When the flavin is replaced by 5-carba-5-deaza FAD, a low level of activity is retained. " " However, when the enzyme containing the artificial flavin is exposed to H2O2, the isoalloxazine ring system is partially degraded to a redox-inactive heterocyclic system. " After the formation of this redox-inactive flavin derivative, enzyme activity actually increases dramatically, suggesting that the flavin plays a structural role. It is possible that the redox state of the flavin serves a regulatory role, since enzyme activity decreases when the natural flavin is reduced however, this has yet to be proven. 

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