Amines asymmetric, using transaminases

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. 

Transaminases are important enzymes in the synthesis of chiral amines, amino acids, and amino alcohols, hi this chapter the properties of transaminases, the reaction mechanisms, and their selectivity and substrate specificity are presented. The synthesis of chiral building blocks for pharmaceutically relevant substances and fine chemicals with transaminases as biocatalysts is discussed. Enzymatic asymmetric synthesis and dynamic resolution are discussed using transaminases. Protein engineering by directed evolution as well as rational design of transaminases under process condition is presented to develop efficient bioprocesses. 

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. 

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. 

A drawback of using lactate dehydrogenase as a biocatalyst to remove pyruvate from the reaction equilibrium is the need for the NADH cofactor. Another possibility to eliminate the coproduct is the application of a p5uruvate decarboxylase (Scheme 29.6b). A cofactor is not required, and the resulting products of pyruvate decarboxylation, acetaldehyde, and CO are highly volatile, shifting the equilibrium toward the product [68]. Several pyruvate decarboxylases from yeast and bacteria are commercially available and are active at the same pH value as the transaminase required for the asymmetric synthesis of chiral amines. 

Studies of transaminases in kinetic resolution elucidated several benefits. Compared to asymmetric synthesis the equilibrium favors product formation if pyruvate is used as an amino acceptor. To get enantiopure amines, kinetic resolution is an acceptable choice with yields of 50%. Various (R)- and (S)-selective transaminases are well established nowadays, leading to enantiopure (S)- and (R)-amines, with high ee. Unfortunately product inhibition is one major disadvantage of kinetic resolution. If a critical concentration of product is achieved, the maximum conversion is prevented. Based on kinetic modeling in a previous study from Shin and Kim, the inhibitory effects were based on the strong binding of the product to the PLP cofactor. Consequently the binding of the amino acceptor is hindered, and conversion of the substrate is not possible [72]. 

Kroutil and co-workers reported on a very efficient biocatalytic asymmetric reductive amination of ketones using co-transaminase as the enzyme for the synthesis of optically active a-chiral amines.Three enzyme systems were used for the successful operation of this methodology based on ... 

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