Oxygenase models

On the other hand, regardless of the different types of ligands, functional models have contributed to an understanding of the reaction processes of oxygenation and to 

The above two types of model studies may be explained by Fig. 6. Functional models are specially important for development of catalytic processes by mimicking enzymes. 

Formation of metal-oxo species first became realistic in model studies on heme oxygenases (shunt path), and it has also become popular in nonheme systems. As for the metal-jd-peroxo species, the distinctive example of model study has appeared in the copper enzymes. The type 7 peroxo species has been proposed first based on models for dicopper oxy-hemocyanin [19, 20] before demonstration of the same oxygen bonding in enzyme [13]. The type 7 x-peroxo structure has been applied to diiron species in sMMO [14] in addition to 6 [21]. (see Chapter 8) 

The other important contribution of the model systems to the progress in enzymatic studies is to obtain information about structures and reactivities of substrate-metal intermediates. One of the characteristic examples is the isolation of mono- and bi-dentate catecholatoiron(III) complexes in relevance to catechol dioxygenases. Reactivities of these 

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Biological Function of Heme Oxygenase Heme Oxygenase Model Systems Heme Oxygenase The Protein... 

Role of Ascorbic Acid in a Mono-oxygenase Model (Udenfriend s System)... 

J. Razenberg, A. W. Vandermade, J. W. H. Smeets, R. J. M. Nolte, Cyclohexene epoxidation by the mono-oxygenase model (tetraphenylporphyrinato)manganese(in) acetate-sodium hypochlorite, J. Mol. Catal. 31 (1985) 271. 

Iley, J., L. Constantino, F. Norberto, and E. Rosa (1990). Oxidation of the methyl groups of N,N-dimethylbenzamides by a cytochrome P450 mono-oxygenase model system. Tetrahedron Lett. 31,4921 922. 

An interesting historical quirk is the fact that the first oxygenase model system, the Udenfriend reagent (Figure 2), was reported in 1954 [10], one year prior to the discovery of the enzymes it emulates. Udenfriend and coworkers found that a mixture of Fe(II), EDTA, ascorbic acid and dioxygen is able to hydroxylate aromatic rings at neutral pH and under mild conditions. It was later found that the ascorbic acid can be replaced by a variety of hydrogen donors... 

Despite detailed and repeated measurements by various techniques, almost no evidence can be found for the formation of (LFe=0) in the cyclidene systems, despite the fact that peroxo complexes form readily and by one route that constitutes the reverse of the reaction of equation 6. The results reported here also stand in contrast to recent studies based on non-heme oxygenase model systems that indicate the possibility of multiple pathways, implying that (hydroperoxo)iron(III) species might have catalytic activity e.g., a) Y.-D. Wu, K. N. Houk, J. S. Valentine, and W. Nam, Inorg, Chem, 31 718 (1992) b) W. Nam, R. Ho, and J. S. Valentine, J, Am,... 

T. Funabiki, T. Toyoda, H. Ishida, M. Tsujimoto, S. Ozawa, and S. Yoshida, Oxygenase model... 

The totally synthetic superstmctured cobalt(II) cyclidene complexes (Figure 61) CoA (vaulted), CoB (unbridged) and CoC (lacunar), function as both oxidase and oxygenase models in oxygenation of substituted phenols . 

More recently, research concerning catalytic oxidation reactions has emerged on one side from bioinorganic chemistry with cytochrome P450 models and non-por-phyrinic methane mono-oxygenase models, and on the other side, from organic chemistry with asymmetric epoxidation and dihydroxylation. 

This mechanism represents a shortcut in biological mono-oxygenase enzyme systems that use O2 as oxygen atom sources and whose mechanism is much more complex because of the requirement to cleave O2. Non-porphyrinic binuclear methane mono-oxygenase model complexes are also able to activate methane in the same way (see Chap. 19). When S = RH, Groves originally proposed the well-known rebound mechanism in which the Fe =0 species removes an H atom from RH, then transfers OH to produce the alcohol ROH ... 

Nothing is known about the identity of the iron species responsible for dehydrogenation of the substrate. Iron-oxo species such as FeIV=0 or Fem-OOH are postulated as the oxidants in most heme or non-heme iron oxygenases. It has to be considered that any mechanistic model proposed must account not only for the observed stereochemistry but also for the lack of hydroxylation activity and its inability to convert the olefinic substrate. Furthermore, no HppE sequence homo-logue is to be found in protein databases. Further studies should shed more light on the mechanism with which this unique enzyme operates. 

T Funabiki (ed.). Oxygenases and Model Systems. Dordrecht Kluwer, 1997. 

Copper enzymes are involved in reactions with a large number of other, mostly inorganic substrates. In addition to its role in oxygen and superoxide activation described above, copper is also involved in enzymes that activate methane, nitrite and nitrous oxide. The structure of particulate methane mono-oxygenase from the methanotrophic bacteria Methylococcus capsulatus has been determined at a resolution of 2.8 A. It is a trimer with an a3P33 polypeptide arrangement. Two metal centres, modelled as mononuclear and dinuclear copper, are located in the soluble part of each P-subunit, which resembles CcOx subunit II. A third metal centre, occupied by Zn in the crystal, is located within the membrane. 

The differences between the myoglobin model and heme oxygenase that account for the increased efficiency of heme cleavage by the latter... 

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