Dehydrogenases asymmetric oxidation

The focus is on proteins with redox functionalities hke oxidases, mono- and dioxygenases, and other enzymes able to utilize molecular oxygen as an oxidant, but dehydrogenases and peroxidases will also be discussed. The use of common proteins such as bovine serum albumin with no distinctive redox functionalities as chiral templates for asymmetric oxidations has been reviewed elsewhere [16]. As the oxidations require the transfer of electrons from a substrate to an electron... 

Irwin, A.J. and Jones, J.B. (1977) Asymmetric syntheses via enantiotopicaUy selective horse liver alcohol dehydrogenase catalyzed oxidations of diols containning a prochiral center. Journal of the American Chemical Society, 99, 555-551. 

It also has to be stressed that many useful redox enzymes for asymmetric oxidations are not commercially available as isolated enzymes—alcohol dehydrogenases are the main exception here— but have to be isolated from the wild-type organism or produced in recombinant form. For that reason, applied biocatalysis is a multidisciplinary field where the expertise of biologists, chemists, and engineers is required. 

Musa, M.M., Ziegelmann-Fjeld, K.I., Vieille, C. et al. (2007) Asymmetric reduction and oxidation of aromatic ketones and alcohols using W110A secondary alcohol dehydrogenase from Thermoanaerobacter ethanolicus. The Journal of Organic Chemistry, 72 (1), 30-34. 

Figure 16.2-49. Asymmetric reduction of ethyl-4-chloro-3-oxobutanoate catalyzed by an alcohol dehydrogenase (ADH) in recombinant E. coli. The necessary reduction equivalents were derived from the oxidation of isopropanol with the same enzyme.

Stereoinversion catalyzed by two different alcohol dehydrogenases via enantiospecific oxidation followed by an asymmetric reduction. 

In another application, recombinant E. coli produced 36.6 g/L ethyl-(R)-4-chloro-3-hydroxybutanoate (99% ee) from 40 g/L ethyl-4-chloro-3-oxo-butanoate[210). Here, the secondary alcohol dehydrogenase served as both synthetic (asymmetric reduction) and regenerating (NADH-regeneration via isopropanol oxidation) enzyme (Fig. 16.2-49). 

Stereospecificity Enzymes are also steiicaUy specific when acting on substrates that are stereoisomeric, i.e. isomers in which the atoms are oriented differently in space. The presence of a chiral (asymmetric) center (carbon) in a molecule gives rise to an enatiomeric pair, D and L or R and S (Cahn et al, 1966). Stereospecific enzymes that act on only one enantiomer but not the other are known as chiral stereospecificity. For example, o-lactate dehydrogenase oxidizes only D-lactate to pyruvate and L-lactate dehydrogenase, its c-enantiomer ... 

There are several examples of d to l inversion of amino acids in the literature. D-Phenylalanine may have therapeutic properties in endogenous depression and is converted to L-phenylalanine in humans [145]. o-Leucine is inverted to the L-enantiomer in rats. When o-enantiomer is administered, about 30% of the enantiomer is converted to the L-enantiomer with a measurable inversion from l to o-enantiomer. As indicated in Fig. 13, D-leucine is inverted to the L-enantiomer by two steps. It is first oxidized to a-ketoisocarproate (KIC) by o-amino acid oxidase. This a-keto acid is then asymmetrically reaminated by transaminase to form L-leucine. In addition, KIC may be decarboxylated by branched-chain a-keto acid dehydrogenase, resulting in an irreversible loss of leucine (Fig. 13) [146]. D-Valine undergoes a similar two-step inversion process, and this can be antagonized by other amino acids such as o-leucine. The primary factor appears to be interference with the deamination process [147]. 

The asymmetric transfer hydrogenation of ketones is an effective way to prepare enan-tiopure alcohols." " We were attracted to this reaction as we anticipated that one could exploit the reversibility of the reaction to perform either for the enantioselective reduction or for the kinetic resolution of racemic alcohols via oxidation. This behaviour is reminiscent of alcohol dehydrogenases which can operate either as oxidases or reductases. ... 

It would be well to point out a few examples which illustrate the overlap of asymmetric reduction studies and molecular biochemistry. Diphosphopyridine nucleotide (DPN) and triphospho-pyridine nucleotide (TPN) are important coenzymes in biochemical oxidation reduction reactions. Certain enzymes function as catalysts for the reversible transfer of hydrogen between these nucleotides and a substrate for which the enzyme is specific. For example, DPN and the enzyme, alcohol dehydrogenase (ADH), form a redox system with ethanol. Using deuterium labeled reducing agent and substrate, Westheimer, Vennesland,... 

For a long time, kinetic resolution of alcohols via enantioselective oxidation or via acyl transfer employing, for example, lipases along with dynamic kinetic resolution have been the biocatalytic methods of choice for the preparation of chiral alcohols. In recent years, however, impressive progress has been made in the use of alcohol dehydrogenases (ADHs) and ketor-eductases (KREDs) for the asymmetric synthesis of alcohols by stereoselective reduction of the corresponding ketones. Furthermore, recent remarkable multienzymatic systems have been successfully applied to the deracemisation of alcohols via stereoinversion based on an enantioselective oxidation followed by an asymmetric reduction. 

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