Ferric catalase complexes

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. 

Figure 16-11 (A) Stereo drawing showing folding pattern for beef liver catalase and the positions of the NADPH (upper left) and heme (center). From Fita and Rossmann.198 (B) Diagram of proposed structure of an Fe(III)-OOH ferric peroxide complex of human catalase (see also Fig. 16-14).

Inhibitor Molarity to Give 50% Inhibition Relative Affinity for Ferric Catalase Dissociation Constant of Ferric Complex... 

The inhibition of catalase by cyanide shows none of the characteristics of the azide or hydroxylamine inhibition as is to be expected if cyanide combines with the ferric form. At low peroxide concentrations about 10-3 M the equilibrium constant for the formation of the cyan-catalase complex (Ki) determined from kinetic data using the expression... 

Answers to these questions were initiated over a decade ago during our studies on catalase (CAT) and horseradish peroxidase (HRP) (30). Both native enzymes are ferric hemoproteins and both are oxidized by hydrogen peroxide. These oxidations cause the loss of two electrons and generate active enzymatic intermediates that can be formally considered as Fe + complexes. 

Fig. 2. The Bonnichsen, Chance, and Theorell 34) mechanism for the dismutation of hydrogen peroxide by catalase. (A) The simple ping-pong mechanism (ferric-peroxide compound (ycle) involves only the successive formation and decomposition of the compound 1 intermediate by two successive molecules of H2O2. (B) Reversible ES(Fe -H202) and ternary (compound I-H2O2]) complexes are added to the mechanism in A.

The higher stability of ferrous heme nitrosyl complexes compared to the corresponding ferric species is well documented [see Refs (44-46, 69) for reviews]. Since cytochrome P450, catalase, and cytochrome c oxidase are commonly associated with reducing agents, a mechanism exists for longer term inhibition of the activity of these proteins. 

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. 

It is noteworthy that Aust and coworkers have shown that in vitro lipid peroxidation initiated by ferrous-ADP or ferrous-AMP complexes was strongly stimulated by the presence of the analogous ferric complexes, with no effect of either SOD, catalase or OH scavengers [156,157]. This suggests that a ferrous-dioxygen-ferric chelate may serve as a potent free radical initiator of its own. 

The reaction sequence at the heme active site starts with the binding of unactivated triplet dioxygen forming the so-called oxy-heme complexes. The iron center in 02-activating heme enz5maes is then thought to be converted into a peroxo anion species. It can be protonated to form a ferric hydroperoxo intermediate usually termed compormd 0 (183), which is a crucial reactive species in catalase and peroxidase enz5nne catalysis (Fig. 21). These hydroperoxo intermediates of hemoproteins are important... 

Oxoiron(IV) porphyrins, one oxidizing equivalent above the resting ferric state, are known as compound II in the catalytic cycle of peroxidases and catalases. The (Porp)Fe =0 complexes can be generated by (i) the homolytic O—O bond cleavage of (Porp)Fe -0-0-Fe (Porp), which is formed by the addition of dioxygen to iron(II) porphyrins in the presence of a nitrogen base, (ii) the chemical oxidation of iron(III) porphyrins by m-CPBA and PhIO under certain circumstances, (iii) the electrochemical oxidation of hydroxoiron(III) porphyrins, and (iv) the reactions of iron(III) porphyrins with hydroperoxides (ROOH) in aqueous or organic solvents. More detailed preparation methods and physical properties of various oxoiron(IV) porphyrin complexes are summarized in recent reviews. ... 

The reaction cycle of the catalases (Fig. 6), like that of the peroxidases, begiris with the high-spin ferric state (7i) which reacts with a molecule of hydrogen peroxide to form the Compound I intermediate (14). Next, however, oxidation of a second hydrogen peroxide molecule yields dioxygen, with the concomitant return of catalase Compound I to the native resting state. Catalases can be made to produce a Compound II intermediate that is generally described as an Fe =0 complex like Compound II of the peroxidases. 

An interesting range of compounds with varying catalase or peroxidase properties are found in the various aquorferric ions near neutral pH. Magnetic measurements on these hydrous ferric oxides show that they vary from high spin to low spin complexes 173). The oxidation of... 

In a more general context of hemoproteins some further studies appear worth mentioning. A coral allene oxide synthase has been characterized which employs a heme in the conversion of 8R-hydroperoxyeicosatetraenoic acid into the corresponding allene oxide. EPR of the ferric enzyme and its cyanide and azide complexes strongly suggested tyrosinate ligation, as in catalase, but the access of small molecules to the heme as well as the interaction with the protein environ-... 

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