Transaminases for the synthesis of enantiopure beta-amino acids

Abstract

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.

Figure 1

(a) Examples of pharmaceutically important natural products containing a β-amino acid moiety: paclitaxel from the yew tree Taxus brevifolia and the chromophore of the chromoprotein C-1027 from the Actinobacteria Streptomyces griseus. The β-amino acid moieties are highlighted in grey. (b) Comparison of the backbones of α-, β³- and β²-peptides.

Figure 2

Schematic reaction scheme of the synthesis of β-amino acids catalyzed by transaminases by (a) kinetic resolution of a racemic β-amino acid or (b) asymmetric synthesis starting from a prochiral β-keto acid.

Figure 3

Activity assays for the screening of novel transaminases (a) by formation of a blue copper complex with the produced α-amino acid, (b) by withdrawal of the produced pyruvate in a multi-enzymatic one-pot reaction system which ultimately leads to a pH drop, (c) by direct measurement of the absorbance of acetophenone produced from the transamination of α-methylbenzylamine, (d) by measuring the decrease in conductivity which results from the conversion of the two charged substrates to the uncharged/zwitterionic products and (e) by oxidation of the produced alanine which ultimately leads to the oxidation of the dye pyrogallol red by H₂O₂ in a multi-enzymatic one-pot reaction system. Abbreviations: LDH = lactate dehydrogenase, GDH = glucose dehydrogenase, AAO = amino acid oxidase, HRP = horse radish peroxidase.