Chiral amines using transaminases

Figure 14,45 Synthesis of a range of chiral amines 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. 

Figure 14.22 Resolution of racemic chiral amines using a transaminase from 6. megaterium.

A typical process flow diagram for the production of chiral amines using aminotransferases is shown in Figure 7.8. The buffering agent, pyridoxal 5-phosphate, amine donor, enzyme, and substrate ketone in aqueous solution are mixed in a biotransformation vessel. The desired reaction conditions such as temperature, pH, amine donor, and acceptor concentrations are maintained during transamination reaction. The transamination reaction time depends on the rate at which transaminase is catalyzing the reaction, while the extent of conversion depends on... 

SYNTHESIS OF CHIRAL AMINES USING o-TRANSAMINASES 2.2.1 ct)-Transaminases Definition and General Facts... 

A transaminase patented by Celgene Corporation (Warren, NJ), called an co-aminotransferase [(co-AT)E.C. 2.6.1.18] does not require an a-amino acid as amino donor instead it requires a primary amine and hence has the ability to produce chiral amines.125 126 A similar co-AT from Vibrio fluvialis has been described for the production of chiral amines along with chiral alcohols when coupled with AdH or chiral amino acids when coupled with an a-amino acid aminotransferase.127130 Another co-AT, ornithine (lysine) aminotransferase (E.C. 2.6.1.68), has been described for the preparation of a chiral pharmaceutical intermediate used in the synthesis of Omapatrilat, a vasopep-tidase inhibitor developed by Bristol-Myers Squibb, as well as the UAA A1 -piperidinc-6-carboxylic acid.131-132... 

The aminotransferase, or transaminase class of enzymes, are ubiquitous, PLP-requiring enzymes that have been used extensively to prepare natural L-amino acids [84,85]. They catalyze the general reaction shown in Scheme 15, where an amino group from one L-amino acid is transferred to an a-keto acid to produce a new L-amino acid and the respective a-keto acid. Those enzymes most commonly used have been cloned, overexpressed, and generally used as whole cell or immobilized preparations. These include the following branched chain aminotransferase (SCAT) (EC 2.6.1.42), aspartate aminotransferase (AAT) (EC 2.6.1.1), and tyrosine aminotransferase (TAT) (EC 2.6.1.5). A transaminase patented by Celgene Corporation (Warren. NJ), called an co-aminotransferase, does not require an a-amino acid as amino donor and hence is used to produce chiral amines [86,87]. Another useful transaminase, n-amino acid transaminase (DAT) (EC 2.6.1.21), has been the subject of much study [37,88,89]. This enzyme catalyzes the reaction using a n-amino acid donor, either alanine or aspartate (Scheme 16). 

Celgene developed transaminase technology for the enantioselective conversion of chiral amines to ketones [25]. Low molecular weight aldehydes, such as propional dehyde, were used as the amine group acceptor. This process has been used on a... 

Outline the problems associated with transaminase catalyzed amination of ketones to produce chiral amines, particularly with respect to product inhibition and equilibrium. Suggest two different methods that can be used to overcome these problems. 

Depending on the substrate preference of the employed transaminase, the following couples of sacrificial amine donor/keto acceptors were used, which are often derived from the a-aminoacid pool (such as alanine/pyruvate, phenylalanine/ phenylpyruvate, glutamic acid/a-ketoglutarate, aspartic acid/a-ketosuccinate) or constitute simple amines/ketones, such as 2-propylamine/acetone and 2-butyl-amine/2-butanone. It should be kept in mind that the absolute configuration of a chiral amine-donor has to match the stereospecificity of the co-TA in order to be accepted. 

Figure 15.6 Chiral amines accessible from ketones using o-transaminases and amine donors (typically alanine, isopropyl amine, a-methylbenzylamine).

If the transaminase is used in the form of whole cells instead of purified enzyme, the cost of production of chiral amines decreases, since enzyme purification adds... 

The enzyme class which has received the most increase in attention in industrial and synthetic applications since 2000 is (o-transaminases, in which a suitable inexpensive amino-donor 16 is used to convert a C=0 group of a prochiral substrate 15 into chiral amine 17. In order to shift the equilibrium to the desired amine product, the keto by-product 18 is usually converted in a second irreversible enzymatic step into an iimocent side product 19 (Scheme 15.3). 

Cofactor regeneration is not only a task when using oxidoredurtases but also for some representatives from other enzyme classes, for example, transferases and lyases. Transaminases require pyridoxal phosphate and transform a ketone into the chiral amine on the expense of a donor amine that is the cosubstrate needed in stoichiometric amount [17]. A representative example for pyridoxal phosphate regeneration is shown in Scheme 2.5, exemplified for the use of isopropylamine as an amine donor. This reagent is especially useful for two reasons. First, it is a small (and thus atom economical), cheap, and readily available amine. 

While first examples regarding the biocatalytic preparation of chiral amines mainly involved kinetic resolution processes using hydrolases [15], in the last years, the identification of novel biocatalysts including amine oxidases (AOs), amino acid oxidases (AAOs), lyases, or ra-transaminases (co-TAs) has provided new catalytic tools for the production of chiral amines with high stereoselectivity [16-18]. 

Chiral amines have been attracting attention as an important composition, particularly for pharmaceutical products. The organic synthetic methods of optically active amine compounds have been developed through the traditional resolution of racemic amines with the formation of diastereomer salts using an optically active mandelic acid or tartaric acid. Enzymatic synthesis has mainly used lipase and S- or R-stereoselective amine transaminase (AT) [29-31] (Figure 19.7). Turner et al. successfully synthesized chiral (R)- and (S)-amines by kinetic resolution using a combination of stereoselective AT and d- or L-amino acid oxidase (AAOx) [32] (Figure 19.7). However, the theoretical yield of the products has been limited to 50% in the kinetic resolution. 

Orthogonal cascades have been used to date for the removal of by-products in order to shift equilibrium. An excellent example of an orthogonal cascade is based on using alanine as an amino donor for the co-transaminase-catalyzed synthesis of chiral amines [49]. 

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. 

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