Catalase catalytic mechanism

Catalases have proven to be a treasure trove of unusual modifications. The first noted modification was the oxidation of Met53 of PMC to a methionine sulfone (77). Met53 is situated in the distal side active site adjacent to the essential His54 in a location where oxidation by a molecule of peroxide would not be unexpected. Among the catalases whose structures have been solved, PMC is unique in having the sulfone because valine is the more common replacement in other catalases. The sulfone does not seem to have a role in the catalytic mechanism and is clearly generated as a posttranslational modification. A small number of catalases from other sources, principally bacteria, have Met in the same location as PMC, and it is a reasonable prediction that the same oxidation occurs in those enzymes as well, although this has not been demonstrated. 

Catalases continue to present a challenge and are an object of interest to the biochemist despite more than 100 years of study. More than 120 sequences, seven crystal structures, and a wealth of kinetic and physiological data are currently available, from which considerable insight into the catalytic mechanism has been gained. Indeed, even the crystal structures of some of the presumed reaction intermediates are available. This body of information continues to accumulate almost daily. 

Fita and Rossmann [100] presented a comprehensive analysis of the catalase active site and discussed probable catalytic mechanisms with the participation of acid-base catalytic groups in the redox transformations of the substrate. Figure 6.3 is a diagram of catalase redox transformation with formation of intermediate complexes A, III and IV. Note that in this work the experimentally found analogy of complex II formation for catalase and cytochrome-c-peroxidase complex is applied to particular simulations [101, 102],... 

The importance of reactions 1-3 in the biosphere is clear. However, relatively little is known about the catalytic mechanisms of these reactions, particularly reactions 2 and 3. In order to better understand the catalytic mechanisms of these enzymes, it is important to establish the correlation between metal site structure and enzymatic function. X-ray absorption spectroscopy is one of the premier tools for determining the local structural environment of metalloprotein metal sites. In the following, we summarize our results using X-ray absorption spectroscopy to characterize the structure of the Mn active site environments in manganese catalase and in the OEC and show how these structural results can be used to deduce details of the catalytic mechanism of these enzymes. 

RSSF techniques have been used extensively to study the catalytic mechanisms of both peroxidases and catalases (117-127). This class of enzymes are ferri(Fe III)-hemoproteins that exhibit characteristic a, 3, and Soret spectral bands in the 400- to 600-nm region of the visible spectrum. These spectral bands are generally indicative of a high-spin nonplanar Fe heme prosthetic group. Peroxidases and catalases utilize either alkyl peroxides (Equation 5, where AH2 represents a wide variety of reducing substrates) or hydrogen peroxide (Equation 6) as substrates, respectively. 

FIGURE 6.11 Typical current—time responses of Fe-SOD/MPA-modified Au electrode toward 02 in 25 mM phosphate buffer (02-saturated, pH 7.5) containing 0.002 unit of XOD upon the addition of 50 nM xanthine at +300 (a) and —lOOmV (b). The arrows represent the addition of 10 j,M of Cu, Zn-SOD (a) and 580 units of catalase and 10 pM of Cu, Zn-SOD to the solution (b). The solution was stirred with a magnetic stirrer at 200rpm. Inset mechanism for the amperometric response of SODs/MPA-modified Au electrodes to 02, based on enzymatic catalytic oxidation (a) and reduction (b) of 02 (M metal ions of SODs). (Reprinted from [138], with permission from the American Chemical Society.)... 

It follows from the above that MPO may catalyze the formation of chlorinated products in media containing chloride ions. Recently, Hazen et al. [172] have shown that the same enzyme catalyzes lipid peroxidation and protein nitration in media containing physiologically relevant levels of nitrite ions. It was found that the interaction of activated monocytes with LDL in the presence of nitrite ions resulted in the nitration of apolipoprotein B-100 tyrosine residues and the generation of lipid peroxidation products 9-hydroxy-10,12-octadecadienoate and 9-hydroxy-10,12-octadecadienoic acid. In this case there might be two mechanisms of MPO catalytic activity. At low rates of nitric oxide flux, the process was inhibited by catalase and MPO inhibitors but not SOD, suggesting the MPO initiation. 

The catalase-peroxidases present other challenges. More than 20 sequences are available, and interest in the enzyme arising from its involvement in the process of antihiotic sensitivity in tuherculosis-causing bacteria has resulted in a considerable body of kinetic and physiological information. Unfortunately, the determination of crystallization conditions and crystals remain an elusive goal, precluding the determination of a crystal structure. Furthermore, the presence of two possible reaction pathways, peroxidatic and catalatic, has complicated a definition of the reaction mechanisms and the identity of catalytic intermediates. There is work here to occupy biochemists for many more years. 

Many variations of dinuclear i-phenoxo-bis( j.-carbox Tato)dimanga-nese(II) complexes of L5 [lOOe] show catalase-like activity (more than 1000 catalytic turnovers based on the measurement of evolved dioxygen) via the mechanism shown in Scheme 4 (R1 = R2 = CH3 = 29). 

Action mechanisms and kinetic features of catalases, peroxidases and monooxygenases. Catalytic system selection for conjugated dehydrogenation, epoxidation and monooxygenation reactions. 

It is shown that with the model system [38, 59] heme iron reaction with hydrogen peroxide is promoted by acidic catalytic sites, which are replaced by distal amino acid group bound to heme [60], Here experimentally observed two-electron oxidation substrate in one stage and corresponded hydride-ion transfer is confirmed [61, 62], In the example of catalase reaction the transfer mechanism of two electrons simultaneously was discussed above... 

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