Investigation of beta-alanine aminotransferase at the crossroad of pyrimidine, polyamine, coenzyme A and branched chain amino acid metabolism

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Plants are able to degrade nucleic acids and the nucleotides they contain to keep metabolism in balance and to obtain nutrients and energy. The ring of pyrimidine nucleotides (uracil and thymine) is reduced and opened in three reactions, forming beta-alanine (BA) from uracil and beta-aminoisobutyrate (BAIBA) from thymine. How these products are further degraded and how pyrimidine ring degradation is linked to central metabolism is as yet unknown. BA can not only flow into further degradation, but is also an essential precursor for the synthesis of coenzyme A in the cytosol. However, since pyrimidine ring degradation mutants do not have a defect in CoA biosynthesis, plant metabolism must still be able to provide BA by other means. This may involve the degradation of polyamines, which occurs in the cytosol and peroxisomes. We have discovered the enzyme that metabolizes BA and BAIBA by transamination with pyruvate as acceptor to the corresponding aldehydes and to alanine - beta-alanine aminotransferase (BAAT). BAAT is located in peroxisomes and mitochondria. Based on our preliminary data, literature data, and plausibility assumptions, we developed a working model of the metabolism surrounding BA and BAIBA, which postulates inter alia, that BA and BAIBA can be degraded in the mitochondria to acetyl-CoA by the enzymes of branched-chain amino acid degradation. By quantifying metabolites using also isotope labeling techniques in a broad variety of mutants of pyrimidine and polyamine metabolism, we aim to convert our working model into an experimentally proven metabolic model. This will integrate pyrimidine and polyamine catabolism as well as CoA synthesis and parts of the degradation pathway of branched-chain amino acids, considering reactions in the cytosol, mitochondria and peroxisomes. Several enzymes, including BAAT, with previously unknown physiological roles will be investigated and ideally their function in metabolism will be elucidated. For BAAT, we will also perform crystal structure analysis. In addition, BA sources for CoA synthesis will be elucidated and the pathway of nitrogen derived from BAAT-mediated transamination will be traced. Furthermore, it will be investigated whether pyrimidines are more strongly degraded under short-term carbon limitation, which transcriptional data suggest. Thus, pyrimidine ring degradation (and possibly also polyamine degradation) could contribute to the short-term provision of acetyl-CoA during carbon starvation. Through genome sequencing, the genomes of many plants are now known, but many of the gene functions and interconnections in metabolism are still unclear. This research will contribute to the elucidation of several gene functions and will expand the understanding of plant metabolism in many ways.