Ferrous peroxidase

Hemoglobin and myoglobin in their ferric forms show rudimentary peroxidatic and catalatic activity, but ferrous peroxidase does not combine reversibly with molecular oxygen. Ionic iron also gives the hydrogen peroxide reactions but not the combination with oxygen. 

Since compound III is the dioxygen complex of ferrous peroxidase, it has similarities to oxymyoglobin and oxyhemoglobin (9, 54). From that point of view it can be proposed that compound III and hydroxylaminodinitrotoluene co-oxidize to yield ferric enzyme and nitrosodinitrotoluene (Fig. 13). Therefore, two independent mechanisms may exist to oxidize hydroxylaminodinitrotoluene one via the catalytic cycle mediated by veratryl alcohol, and the second via oxidation by compound III. 

Inhibition by carbon monoxide was presumed to indicate a role for ferrous peroxidase in the reaction. As pointed out by Mason (1958), Theorell has continued to emphasize this particular difference between the oxidatic and peroxidatic reactions. But in Chance s experiments, an effect of CO was only found on the reaction in absence of Mn++, and even then the inhibition was comparatively feeble. And Yamazaki... 

If the reaction does not involve ferrous peroxidase the effect of CO is difficult to interpret, but the trapping of an intermediate by an inhibitor does not necessarily imply that that intermediate is on the main reaction pathway. As in the case of cyanide, it appears that the CO inhibition is variable and depends on the reaction conditions it may therefore be acting on a subsidiary rather than the main reaction. 

Other substances can activate hydrogen peroxide, for example, ferrous iron (p 352)- The products however are usually less well defined and different from those obtained when using peroxidase. 

The abihty of iron to exist in two stable oxidation states, ie, the ferrous, Fe ", and ferric, Fe ", states in aqueous solutions, is important to the role of iron as a biocatalyst (79) (see Iron compounds). Although the cytochromes of the electron-transport chain contain porphyrins like hemoglobin and myoglobin, the iron ions therein are involved in oxidation—reduction reactions (78). Catalase is a tetramer containing four atoms of iron peroxidase is a monomer having one atom of iron. The iron in these enzymes also undergoes oxidation and reduction (80). 

Besides Fe-, other reducing agents that may be used in conjunction with H2O2 are aliphatic amines, Na2S203 thiourea, ascorbic acid, glyoxal, sulfuric acid, NaHSOs, sodium nitrite, ferric nitrate, peroxidase, AgNOs, tartaric acid, hydroxylamine, ethylene sulfate, sodium phosphite, formic acid, ferrous ammonium sulphate, acetic acid, ferrous sulphate, and HNO2, etc,... 

Luminescence reaction. Pholasin undergoes an oxidative luminescence reaction in the presence of any of the following substances Pholas luciferase, ferrous ions, H2O2, peroxidases, superoxide anions, hypochlorite and other oxidants. In all cases, molecular oxygen is required and pholasin is converted into oxypholasin in the reaction. 

How does nature prevent the release of hydrogen peroxide during the cytochrome oxidase-mediated four-electron reduction of dioxygen It would appear that cytochrome oxidase behaves in the same manner as other heme proteins which utilize hydrogen peroxide, such as catalase and peroxidase (vide infra), in that once a ferric peroxide complex is formed the oxygen-oxygen bond is broken with the release of water and the formation of an oxo iron(IV) complex which is subsequently reduced to the ferrous aquo state (12). Indeed, this same sequence of events accounts for the means by which oxygen is activated by cytochromes P-450. 

Both classical type A enzymes (clade III) and the heme d family (clade II) show a comparatively high sensitivity to azide inhibition and are reduced to ferrous forms in the presence of peroxide and azide (47). In contrast, the catalase-peroxidase (type B) enzymes (see below) are only weakly azide-sensitive. 

The interaction of dioxygen has been observed in several systems, mostly due to autooxidation of ferrous hemes with dioxygen, but only characterized in a few instances. Sakamoto et al. (119) have illustrated peroxidase-type activity using a helix-disulfide-helix system that binds a single heme as shown in Fig. 13. The initial communication illustrated that the addition of an organic cosolvent, trifiuoroethanol, increases the helical content of the peptide, the affinity for heme (1.7 DM IQ at maximal affinity, 15% TFE), and the peroxidase activity (conversion of... 

Studies on the effect of pH on peroxidase catalysis, or the heme-linked ionization, have provided much information on peroxidase catalysis and the active site structure. Heme-linked ionization has been observed in kinetic, electrochemical, absorption spectroscopic, proton balance, and Raman spectroscopic studies. Kinetic studies show that compound I formation is base-catalyzed (72). The pKa values are in the range of 3 to 6. The reactions of compounds I and II with substrates are also pH-dependent with pKa values in a similar range (72). Ligand binding (e.g. CO, O2 or halide ions) to ferrous and ferric peroxidases is also pH-dependent. A wide range of pKa values has been reported (72). The redox potentials of Fe3+/Fe2+ couples for peroxidases measured so far are all affected by pH. The pKa values are between 6 and 8, indicative of an imidazole group of a histidine residue (6, 31-33),... 

There is some experimental evidence showing the effects of heme-linked ionization on lignin peroxidase. The redox potential of the Fe +/Fe2+ couple of lignin and Mn-dependent peroxidases is pH-dependent, as is the absorption spectrum of the ferrous form of the lignin and Mn-dependent peroxidases (6). The pKa values determined from both experiments were 6.6. Detailed studies were performed studying O2 binding to the ferrous lignin peroxidase (79). The pKa for the ionization was measured at different... 

Cyclodextrinyl ditelluride has been recognized as an excellent glutathione peroxidase mimic, revealed in die mitochondrial damage system induced by ferrous sulphate/ascor-bate. ... 

SCHEME 4.3 Cytochrome P450 and peroxidase pathways to hydroperoxo-ferric intermediate or Compound 0 (5). Ferric cytochrome P450 (1) is reduced to the ferrous state (2), which can hind dioxygen to form oxy-ferrous complex (3). Reduction of this complex results in the formation of peroxo-ferric complex (4), which is protonated to give hydroperoxo-ferric complex (5). The same hydroperoxo-ferric complex is formed in peroxidases and catalases via reaction with hydrogen peroxide. 

Conroy CW, Tyma P, Daum PH et al (1978) Oxidation-reduction potential measurements of cytochrome c peroxidase and pH dependent spectral transitions in the ferrous enzyme. Biochim Biophys Acta 537 62-69... 

Compound III designates a complex in which a molecule of oxygen is bound end-on to the ferrous iron of the peroxidase. It is thus a structure that is close to those of the... 

Extensive investigations on the catalytic mechanism of classical peroxidases resulted in a consensus model involving five different iron species [30, 31], These species are ferrous, ferric, Compound I, Compound II, and Compound III (Fig. 11.1). As discussed in Chap. 5, after the reaction of ground state (GS) Femporphyrin with H202, Compound I (Cl) is formed, a cationic oxob e,vpor-phyrin-based Ji-free radical. Electron paramagnetic resonance (EPR) studies established that, in peroxidases of classes I and III, the second oxidation equivalent in Cl is present as a porphyrin-based free radical [32, 33]. In peroxidases from fungal sources, electron abstraction from the protein results in the formation of a different species with the free radical based in a residue close to the porphyrin. 

Aside from the classical examples of hemoglobin and myoglobin, reaction of ferrous heme iron with O2 in hemeperoxidases has been reported for myeloperoxidase [60], horseradish peroxidase C [62], bovine liver catalase [68], lignin peroxidase [46], and lactoperoxidase [61]. With the exception of lactoperoxidase, the binding of O2 is irreversible and CIII engages in one or more of the decay pathways described below. 

Rodriguez-Lopez. IN, Smith AT, Thomeley RNF (1997) Effect of distal cavity mutations on the binding and activation of oxygen by ferrous horseradish peroxidase. J Biol Chem... 

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