Degradation of pyrimidines in Saccharomyces kluyveri: transamination of beta-alanine.

Beta-alanine is an intermediate in the reductive degradation of uracil. Recently we have identified and characterized the Saccharomyces kluyveri PYD4 gene and the corresponding enzyme beta -alanine aminotransferase ((Sk)Pyd4p), highly homologous to eukaryotic gamma-aminobutyrate aminotransferase (GABA-AT). S. kluyveri has two aminotransferases, GABA aminotransferase ((Sk)Uga1p) with 80% and (Sk)Pyd4p with 55% identity to S. cerevisiae GABA-AT. (Sk)Pyd4p is a typical pyridoxal phosphate-dependent aminotransferase, specific for alpha-ketoglutarate (alpha KG), beta-alanine (BAL) and gamma-aminobutyrate (GABA), showing a ping-pong kinetic mechanism involving two half-reactions and substrate inhibition. (Sk)Uga1p accepts only alpha KG and GABA but not BAL, thus only (Sk)Pydy4p belongs to the uracil degradative pathway.