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Enzymatyczne utlenianie typu Baeyera-Villgera

Kołek T., Świzdor A., Panek A.

Wiadomości Chemiczne

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2009

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[Z] 63, 9-10

819-845

EN

Baeyer-Villiger (BV) reaction is oxidation of ketones, leading to cleavage of one of the C-CO-C bonds with simultaneous insertion of an oxygen atom into the cleaved bond. Resulting products obtained from cyclic ketones are lactones, while esters are obtained from acyclic ketones. Numerous strains of microorganisms produce enzymes catalyzing BV oxidation. These enzymes participate in the processes of degradation of natural and synthetic ketones, which can be used by the microorganisms as carbon source (Scheme 3 and 4). The enzymes are monooxygenases (Baeyer-Villigerases, BVMOs), usually containing flavinoadenine nucleotide and cooperating with NAD(P)H reductases. Research on the role of BV oxidation in degradation processes has evolved into intensive studies on the mechanism of this reaction and its use in synthesis, especially after isolation (in 1976) of cyclohexane monooxygenase from Acinetobacter sp. NCIB9871(CHMOAcineto 1) [9]. Subsequently, further strains were identified which produced BVMOs catalyzing oxidation of ketones of diverse structures. In addition to the best-characterized cyclohexane monooxygenase, there are: cyclopentanone, phenylacetone, cyclododecanone, aliphatic ketone and 2-oxo-3-en-4,5,5-trimethyl-cyclopentenylacetic acid monooxygenases. The adjective characterizing a given BVMO is derived from the ketone constituting carbon source, or from the ketone which is oxidized with the highest yield. In addition to these ketones, the enzymes accept their various structural analogues, therefore it is possible to select a biocatalyst which carries out oxidation of a given substrate. Among BVMOs, particularly selective are the enzymes carrying out BV oxidation of steroidal ketones: these enzymes operate only on steroid substrates, mostly containing 3-oxo--4-en moiety, but also they exhibit regioselectivity as well - the oxidation of ketones takes place only at C-17. During the regioselective enzymatic oxidation, "atypical" lactones [2, 4], which are not produced in chemical BV oxidation processes, are sometimes formed. Products of stereoselective enzymatic reactions are optically pure lactones and esters, which are starting points for further asymmetric synthesis of biologically active compounds, including medicines. The application of genetic engineering allows obtaining recombinant microbial strains, which are non-pathogenic and produce larger amounts of the enzyme than the wild-type strains. The recombinants are also able to produce mutated BVMOs exhibiting higher selectivity and/or lifetime, as well as activity towards different spectrum of substrates than the parent enzymes [2]. However, reaction yields of transformations carried out by the recombinants are still not significantly better than the results obtained with the wild-type strains. The recombinants usually require an expensive reagent, isopropyl-? --D-tiogalactopyranoside (ITPG), to induce their BVMOs [23]. Enzymatic BV oxidation, due to the selectivity of the enzymatic action, is competitive to the chemical oxidation. The enzyme selectivity allows for using pure products, including enantiopure compounds, with high yield. The process is environmentally friendly, because the oxidizer in the enzymatic BV reaction is molecular oxygen, and the amount of byproducts is limited. Synthetic application of BVMOs is limited by three principal factors: isolation of the enzyme in amounts suitable for large-scale applications, decrease in the enzymatic activity in the presence of the substrate and/or the product, and the isolation of the product. Separation of the unreacted enantiomer of the substrate in the process of kinetic separation of the racemic mixture, or separation of regioisomeric lactones are costly and time-consuming operations. Another possible problem is the fact that some strains producing useful BVMOs are pathogenic. Therefore, multidirectional research efforts are devoted to overcome the aforementioned barriers with the goal of establishing economically viable methods of biotransformations [4].

2

Reakcje hydroksylacji z udziałem dehydrogenez : zastosowanie w syntezie

Kołek T., Świzdor A., Szpineter A.

Wiadomości Chemiczne

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2004

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[Z] 58, 3-4

245--262

EN

Although the majority of enzymatic hydroxylation reactions is catalysed by monooxygenases, dehydrogenases also play an important role in many reactions of this type. For example, dehydrogenases take part in hydroxylation of alifatic acids or nicotinic acid and its analogue. These reactions are important for degradation, biosynthesis and metabolism processes. Also, enzymic hydroxylation has been succesfully applied to the synthesis of L-carnitine, which is pharmacologically important compound. Another synthetic application involves enantioselective hydroxylation of isobutyric acid, where the proper catalyst species selection may lead to each enantiomer of the product selectively. Both enantiomers of b-hydroxyisobutyric acid are known as valuable chiral synthons for synthesis of many biologically active compounds, i.e. drugs, vitamins and others. The mechanism of alifatic compounds hydroxylation is well known - all the steps have been well documented. The reaction described were carried out by means of induced enzymes. The proof of dehydrogenases mediation in hydroxylation of N-heterocyclic substrates is the fact, that the oxygen in hydroxyl group derives from water, not from the air. Some of these reactions proceed quantitatively, affording very clean products. The reaction that found practical application of considerable importance is the hydroxylation of nicotinic acid (being precursor of a new generation insccticide) and its analogues. It is highly probable that the microbial hydroxylation of this type can find application in transformations of so called "enewable resource" (i.e. nicotine) in order to obtain important biologically active products.

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Hydroksylacje enzymatyczne. Zastosowanie preparatywne i mechanizm reakcji z udziałem monooksygenaz cytochromowych P450

Kołek T.

Wiadomości Chemiczne

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2002

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[Z] 56, 5-6

449-468

EN

Enzymatic conversion of a carbon - hydrogen to a carbon - hydroxyl bond is a key part of the biosynthesis and oxidative metabolism in living organisms from bacteria to humans. Biocatalytic hydroxylations are also widely used in organic synthesis, especially to introduce the desired function to the site of the molecule, which is difficult to obtain by chemical methods. Biocatalytic hydroxylations are most often performed by cytochrome P450 dependent monooxygenases. In a catalytic cycle, one oxygen atom from oxygen molecule is incorporated to water and the other is incorporated into the substrate. Cytochrome P450 is known to catalyze hydroxylations, epoxidations, N-, S-, and O-oxidations, dealkylation, and dehalogenations. The mechanism of the hydroxylation reaction has been explored in the context of synthetic applications, selection of the substrate, and enzyme - substrate interaction. Models involving defined spatial relationships between the mode of substrate-enzyme binding and hydroxylation sites have rationalized the regio- and stereochemistry of hydroxylation. Many of the steps in sequence of the oxidation reactions are well characterized, but the identity of the final oxidant and the reaction mechanism are still not well understood. Recent developments in area biohydroxylation have been focused on the use of new biocatalysts and substrates for C-H activation, the use of recombinant strains of yeast and bacteria expressing desired hydroxylase enzymes.