Hydrogen peroxide complexes, with catalase

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. 

Catalase was found to form an intermediate compound in the presence of hydrogen peroxide (Chance, 69). The spectrum was measured from 380-430 nqi and is slightly shifted toward the visible as compared with free catalase. The complex shows no similarities to cyan-catalase or the compound formed when peroxide is added to azide catalase. Its formation is very rapid, the bimolecular velocity constant having a value of about 3 X 107 M.-1 sec.-1. In the absence of added hydrogen donors, the complex decomposes slowly according to a first order reaction with a velocity constant of about 0.02 sec.-1. This catalase complex thus resembles the green primary hydrogen peroxide complex of peroxidase. 

A property all the primary complexes have in common is the decomposition giving free catalase which does follow first order kinetics (Chance, 71). The velocity constants for ethyl and methyl hydroperoxides are 0.04 and 0.016 sec.-1 as compared with 0.02 sec.-1 for the hydrogen peroxide complex. The secondary complexes decompose far more slowly, the first order velocity constants for ethyl and methyl hydroperoxides having the values 2.3 X 10-4 and 4 X 10-6 sec.-1, respectively. 

Although, salen Mn complexes for therapeutic use were originally conceived as SOD mimetics, it soon became clear that EUK-8 also exhibited catalase activity, the ability to metabolize hydrogen peroxide (75). The catalase activity of EUK-8 was not unexpected, since Mn porphyrins had been studied as catalase models by the Meunier laboratory (16) and, like the porphyrins, salen ligands form stable complexes with Mn(III) (6). As described previously (77), similar to that of mammalian heme-iron based catalases (78), the catalase activity of salen Mn complexes is not saturable with respect to hydrogen peroxide. As has been reported for protein catalases (18), salen Mn complexes exhibit peroxidase activity, in the presence of an electron donor substrate, as an alternative to a catalatic pathway. This supports the analogy between the behavior of these mimetics and that of catalase enzymes, and is consistent with the following mechanistic scheme (76,17) ... 

Heme (C34H3204N4Fe) represents an iron-porphyrin complex that has a protoporphyrin nucleus. Many important proteins contain heme as a prosthetic group. Hemoglobin is the quantitatively most important hemoprotein. Others are cytochromes (present in the mitochondria and the endoplasmic reticulum), catalase and peroxidase (that react with hydrogen peroxide), soluble guanylyl cyclase (that converts guanosine triphosphate, GTP, to the signaling molecule 3, 5 -cyclic GMP) and NO synthases. 

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. 

If the diagram is analyzed in the context of the principles of conjugated reaction, it may be concluded that conjugated biooxidation with hydrogen peroxide consists of the basic (primary) catalase reaction of H202 dissociation (reaction (6.17)). Owing to the Chance complex formation [116, 117], this primary reaction induces the secondary non-classical peroxidase reaction (6.18). 

The presently accepted mechanism (52) involves the oxidation of an Fe(III) porphyrin by hydrogen peroxide to form an (FeIV=0)P+ analogous to the previously mentioned compound 1 of the heme catalase. This highly oxidized enzyme form subsequently reacts with an equivalent of Mn(II) to give compound 2, (FeIV=0)P, and Mn(III), which can diffuse off of the enzyme and into the medium. There is little restriction for the type of Mn(II) required in the first reductive step however, the subsequent reduction of compound 2 to resting enzyme requires an Mn(II) dicarboxylate or a-hydroxyacid complex. Studies suggest that the enzyme prefers the 1 1 Mn(II) oxalate complex as substrate. The... 

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