Date Published: January 31, 2012
Author(s): Jens Rudat, Birgit R Brucher, Christoph Syldatk.
Optically pure β-amino acids constitute interesting building blocks for peptidomimetics and a great variety of pharmaceutically important compounds. Their efficient synthesis still poses a major challenge. Transaminases (also known as aminotransferases) possess a great potential for the synthesis of optically pure β-amino acids. These pyridoxal 5′-dependent enzymes catalyze the transfer of an amino group from a donor substrate to an acceptor, thus enabling the synthesis of a wide variety of chiral amines and amino acids. Transaminases can be applied either for the kinetic resolution of racemic compounds or the asymmetric synthesis starting from a prochiral substrate. This review gives an overview over microbial transaminases with activity towards β-amino acids and their substrate spectra. It also outlines current strategies for the screening of new biocatalysts. Particular emphasis is placed on activity assays which are applicable to high-throughput screening.
Since the discovery of transamination in biological systems (Braunstein and Kritzmann 1937Moyle Needham 1930) the significance of transaminases (TAs) for amino acid metabolism has been the subject of intensive research. Over the last 15 years, TAs have gained increasing attention in organic synthesis for the biocatalytic production of a wide variety of chiral amines and α-amino acids. This has been discussed in detail in a series of excellent reviews (Höhne and Bornscheuer 2009; Koszelewski et al. 2010; Taylor et al. 1998; Ward and Wohlgemuth 2010). Advantages in the use of TAs lie in mostly low-cost substrates, no necessity for external cofactor recycling and the enzymes’ high enantioselectivity and reaction rate. For the synthesis of enantiopure β-amino acids only a limited number of TAs are available. Therefore efficient screening techniques for TAs with high activities as well as broader substrate specificity and different enantioselectivities are crucial for the successful application of transaminases for the synthesis of β-amino acids. Of particular interest are methods that can be used at small scale compatible with microtiter plates.
TAs possess a great potential for the enzymatic synthesis of enantiopure β-amino acids as these enzymes offer the possibility to gain a 100% yield in contrast to the conventional kinetic resolutions using other biocatalysts (see introduction). Transaminases are commonly used tools in the synthesis of various chemicals and pharmaceuticals. Thus production and purification of these enzymes in bulk quantities is well-established, and so is immobilization. Additionally, several process parameters for biotechnological applications are well investigated for both kinetic resolutions and asymmetric syntheses, e.g. the usage of different (co-)solvents and variation of pH and PLP concentration as well as different strategies of product removal (Koszelewski et al. 2008). Of special importance are the thoroughly tried and tested methods to shift the equilibrium to the product side by removal of the coproduct (kinetic resolutions) or degradation of the coproduct/recycling of the amino donor by different enzymes in asymmetric synthesis (Koszelewski et al. 2010).
The authors declare that they have no competing interests.