Transaminase, asymmetric

Stewart, J.D. (2001) Dehydrogenases and transaminases in asymmetric synthesis. Current Opinion in Chemical Biology, 5, 120-129. 

Another class of enzymes that can be used for the enantioselective synthesis of amines and amino acids is the aminotransferases or transaminases (TAs) [29]. As shown in Scheme 6.15, they can be employed in a kinetic resolution or an asymmetric synthesis mode. 

Another example is provided by r>-tert-leucine (4) (Scheme 10), where an asymmetric approach cannot be used. This unnatural amino acid is one of the few that cannot be made by our current o-amino acid transaminase. While this enzymatic method is being researched, small amounts of material have been prepared by a resolution method [24]. 

Examples of uimatural L-amino acids that can be accessed by transaminase methodology are L-2-aminobutyric acid, i.-homophcnylalaninc, and L-lert-leu-cine the latter is not accessible by an asymmetric hydrogenation approach because of the absence of a P-hydrogen. 

Researchers at Celgene developed both (R) and (S) selective transaminases that were active on a range of aliphatic and aromatic ketones and amines [25, 55 57]. Two approaches were employed based upon kinetic resolution, which has been discussed above, and asymmetric synthesis. The asymmetric synthesis approach starts with a... 

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]. 

Stewart, D. (2001) Dehydrogenases and Transaminases in Asymmetric Synthesis, Curr. Opin. Chem. Biol. 5,120-129. 

On the other hand, a-transaminases have been used extensively in the production of amino acids through kinetic resolution and asymmetric synthesis. While many studies rely on the use of an excess of cosubstrate to drive the reaction to completion, some multienzymatic approaches have been developed as well. As an example, aspartate has been used as an amino donor in a multienzymatic synthesis of L-2-aminobutyrate from L-threonine (Scheme 4.8). ° The rather complex multistep sequence started with the in situ formation of 2-ketobutyrate from L-threonine catalysed by threonine deaminase (ThrDA) from E. coli. A tyrosine transaminase (lyrAT) from E. coli converted 2-ketobutyrate and L-aspartie acid to L-2-aminobutyrate and oxaloacetate, which spontaneously decarboiq lated to give pyruvate. Since the... 

Tufvesson, P., lima-Ramos, J., Jensen, J.S., Al-Haque, N., Neto, W., and Woodley, J.M. (2011) Process considerations for the asymmetric synthesis of chiral amines using transaminases. Biotecknol. Bioeng., 108,1479-1493. 

Finally, great effort has been recently devoted to the exploitation of the so-called co-transaminases (co-TAs) for the preparation of optically pure amines [31]. These biocatalysts are generally pyridoxal-5 -phosphate (PLP)-dependent enzymes and are capable of performing reductive amination readions without using either an a-amino add as amine donor or an a-keto add as amino acceptor. Therefore, they find several applications in the asymmetric synthesis of nonracemic aprimary amines, and enzymes with different substrate spedfidty are currently available also from commercial sources. 

Scheme 11.14 Application ofa-transaminases to the asymmetric synthesis of o-amino acids (a) and the herbicide L-phosphothricin (b).

Fuchs, M Koszelewski, D., Tauber, K, Kroutil, W and Faber, K (2010) Chemoenzymatic asymmetric total synthesis of (S)-rivastigmine using (o-transaminases. Chem. Commun.,... 

Hohne, M., Kuhl, S., Karen, R., and Bornscheuer, U.T. (2008) Efficient asymmetric synthesis of chiral amines by combining transaminase and pyruvate decarboxylase. ChemBioChem,... 

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