Hemoproteins catalase reactions

The donor type D5 comprises the two species azide and hydroxy-lamine. These both react with the enz5mie in the presence of peroxide to give rise to ferrous forms of catalase, otherwise normally inaccessible (catalase is the only common hemoprotein that is nonreducible by dithionite). The final inhibited form of catalase in the presence of azide and peroxide is NO-ferrocatalase, but not every azide molecule becomes an NO only in the presence of CO is there a stoichiometric inhibition of enzyme by peroxide with formation of 1 equiv of CO-ferrocatalase for every peroxide molecule added (43). This suggested a three-electron reduction of compound I either to give ferrocatalase, N2, and NO (10-20% total) or to give ferrocatalase, N, and N2O (80-90% total). However, Kalyanaraman et al. (45) have demonstrated the formation of the azidyl (N=N=N ) radical in the reaction, and Lardinois... 

A quarter of a century has passed since the first contribution on catalase to The Enzymes Enzyme substrate compounds Mechanism of action of hydroperoxidases (I). In this perspective, we can identify a sequence of steps in the development of ideas on the mechanism of enzymic action and the nature of enzyme-substrate compounds. The identification of these compounds and the approach to enzymic reactions at concentrations stoichiometric with the substrate caused a principal transition of viewpoint on hemoprotein catalysis from free radical mechanisms (2) unrelated to an active center toward the acceptance of catalysis occurring at the iron atom of the porphyrin (S-5). The latter concept followed natu-... 

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... 

McCarthy and White have noted the ability of a series of seven hemoproteins, P-450lm2, P-450lm4, P-420lm2, HRP, CPO, catalase, and metmyoglobin, as well as hemin to catalyze a set of five oxidative reactions (57, 55). These reactions are typical of the peroxidase reaction [oxidation of pyrogallol to purpuro-gallin, Eq. (8)] and three characteristic P-450 reactions (aliphatic hydroxylation, aromatic hydroxylation, and olefin epoxidation). 

In addition, the ability to decarboxylate a peroxy acid was measured. All hemoproteins were able to carry out peroxidation, but three (HRP, CPO, and catalase) were much better catalysts than the others. Only P-450 enzymes were competent catalysts for hydroxylation and epoxidation. Furthermore, the decarboxylation reaction was strictly limited to the P-450 enzymes (Table VI). 

In the editorial preface to the first volume of Advances in Catalysis the decision was made known not to publish reviews of specialized topics in biocatalysis but from time to time to bring reports in which the relationship and parallelism between this special field and normal catalysis are discussed. This is the first of these reports. Its purpose is to examine the reactions of four hemoproteins, hemoglobin, myoglobin, peroxidase, and catalase, which all contain the same coordination compound of iron—ferrous or ferric protoporphyrin attached to different protein molecules, with oxygen, hydrogen peroxide, and in a few cases additional reducing substances. Some of these reactions are specific ... 

It has been shown above that the catalytic action of catalase and peroxidase is intimately connected with the ability of these hemoproteins to form complexes with hydrogen peroxide (or alkyl peroxides). By choosing the experimental conditions the existence of three different complexes can be demonstrated spectroscopically. The chemical nature of these complexes is as yet unknown, and mechanisms have been represented as bimolecular reactions between substrate and complex. 

TPO is a hemoprotein enzyme tvith binding sites for both iodine and tyrosine. In model systems, TPO has no catalytic activity in the absence of H2O2. TPO degrades H2O2 in a catalase-like reaction releasing O2. Several iodination intermediates were postulated for this reaction, for instance TPO-bound iodinium (U) andTPO-bound hypoiodite (I ). 

On the other hand, accurate data on a theoretically sound basis are a prerequisite for the study (rf the reaction mechanism, the chemistry, and the physiological role of catalases and peroxidases. This kind of research employs, as a rule, purified enzyme preparations in transparent solutions, although Chance (99) has worked out rapid and sensitive spectrophoto-metric methods for the detailed kinetic study of hemoproteins in turbid cell su nsions. 

Hemoglobins, cytochromes, pei oxidases, and catalases have this feature in common—that their effect is centered in one or several iron atoms in each molecule. The iron atoms themselves mediate the reactions typical for each hemoprotein. As far as is known, the iron cannot be replaced by any other metal without loss of effect. Gjessing and Sumner (31) state that they once found a peroxidase effect in manganese protoporphyrin + peroxidase protein. This result, however, was not confirmed by Theorell (80). In all these proteins the iron is incorporated in the center of a porphyrin molecule whose four central nitrogen atoms are bound to the iron, thus blocking four of its six co-ordination places. 

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