Oxygenase catalyst

Thus a number of enzymes have been shown to be able to control the oxidation of sulfides to optically active sulfoxides most extensive investigations have concentrated on mono-oxygenases (e.g. from Acinetobacter sp., Pseudomonas putida) and haloperoxidases1 071 (from Caldariomyces fumago and Coral I ina officinalis). A comparison of the methodologies11081 led to the conclusion that the haloperoxidase method was more convenient since the catalysts are more readily available (from enzyme suppliers), the oxidant (H2O2) is cheap and no cofactor recycling is necessary with the haloperoxidases. Typical examples of haloperoxidase-catalysed reactions are described in Scheme 24. 

A broad spectrum of chemical reactions can be catalyzed by enzymes Hydrolysis, esterification, isomerization, addition and elimination, alkylation and dealkylation, halogenation and dehalogenation, and oxidation and reduction. The last reactions are catalyzed by redox enzymes, which are classified as oxidoreductases and divided into four categories according to the oxidant they utilize and the reactions they catalyze 1) dehydrogenases (reductases), 2) oxidases, 3) oxygenases (mono- and dioxygenases), and 4) peroxidases. The latter enzymes have received extensive attention in the last years as bio catalysts for synthetic applications. Peroxidases catalyze the oxidation of aromatic compounds, oxidation of heteroatom compounds, epoxidation, and the enantio-selective reduction of racemic hydroperoxides. In this article, a short overview... 

Biological oxidation of sulfides involves cytochromes P-450 or flavin-dependent oxygenases. A chiral flavin model was prepared by Shinkai etal. and used as the catalyst in the oxidation of aryl methyl sulfides [87]. Flavinophane 30 (Scheme 6C.10) is a compound with planar chirality. It catalyzes the oxidation of sulfides with 35% H202 in aqueous methanol at -20°C in the dark. 

A topic we have investigated for some time is the reaction of olefins towards a O2/ CO2 mixture. We have shown that carbon dioxide acts as regulator of the oxidative properties of dioxygen [7]. In fact, transition metal catalysts (Rh, Cu) behave as "mono-oxygenases" when a O2/ CO2 mixture is used (III in Scheme 3 is formed only in traces), and as "dioxygenases" in the presence of O2 only (III is the major product). We have postulated the intermediacy in such reaction of peroxocarbonates, species known for a long time. 

Recently ascorbic acid has been assigned a significant function in the reactions involving oxygen insertion by model oxygenase and peroxidase systems in which ferric ion or a ferric chelate is considered to be the catalyst. Although reaction mechanisms suggested by earlier workers involved ascorbic acid merely as a reductant to convert Fe(III) to Fe(II), which would then in turn interact with the oxidant, Hamilton... 

Over-oxidation problems are solved efficiently in biological systems by segregating catalysts and products into different environments. So, for instance, the active sites of several oxygenases are buried deeply into hydrophobic pockets where lipophilic substrates are readily oxidized, while the more hydrophilic reaction products, when released into the surrounding aqueous environment, do not have further access to the catalytic site [12]. 

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