Identification and Characterization of Three Putative Beta-Alanine Aminotransferases in Arabidopsis thaliana

Amino acids serve many cellular processes outside of proteinogenesis that influence the growth and development of the plant, as well as the generation of metabolic energy. Here, we examine the contribution of branched-chain amino acid degradation to the production of β-alanine in the model organism, Arabidopsis thaliana. In plants, the non-proteinogenic amino acid, β-alanine, serves as a key intermediate in the synthesis of the essential molecules vitamin B5 and coenzyme A (CoA). The production of vitamin B5 via the condensation of pantoate and β-alanine is highly conserved among plants and bacteria. The synthesis of the former from valine is similarly conserved among plants and bacteria while that of the latter is not. In bacteria, the decarboxylation of aspartate yields β-alanine. In higher plants, despite the importance, the current knowledge surrounding β-alanine lacks strong evidence for the primary biosynthetic pathways. Previous studies found that polyamines, uracil, and propionate can all serve as intermediates in pathways leading to β-alanine. Given that the catabolism of branched-chain amino acids (BCAAs) valine and isoleucine result in the production of propionyl-CoA (a derivative of propionate), we hypothesized that these BCAAs could also serve as a source of β-alanine. Propionyl-CoA is typically converted to acetyl-CoA via beta-oxidation, however, one of the intermediates of propionyl-CoA degradation, malonate semialdehyde, can react to form β-alanine via transamination. To characterize the interconnection between BCAAs and β-alanine, we have bioinformatically identified three potential candidate aminotransferases from A. thaliana. Here, we describe the recombinant expression in Escherichia coli, purification, and the functional characterization of three putative β-alanine aminotransferases