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

植物能够降解核酸和它们所含的核苷酸，以保持新陈代谢的平衡，并获得营养和能量。嘧啶核苷酸（尿嘧啶和胸腺嘧啶）的环在三个反应中被还原和打开，从尿嘧啶形成β-丙氨酸（BA），从胸腺嘧啶形成β-氨基异丁酸（BAIBA）。这些产物如何进一步降解以及嘧啶环降解如何与中枢代谢相关联尚不清楚。BA不仅可以流入进一步降解，而且还是细胞质中合成辅酶A的必需前体。然而，由于嘧啶环降解突变体在CoA生物合成中没有缺陷，植物代谢必须仍然能够通过其他方式提供BA。这可能涉及多胺的降解，这发生在细胞质和过氧化物酶体。我们发现了通过转氨作用代谢BA和BAIBA的酶，丙酮酸作为相应醛和丙氨酸-β-丙氨酸氨基转移酶（BAAT）的受体。BAAT位于过氧化物酶体和线粒体中。基于我们的初步数据，文献数据，和可扩展性假设，我们开发了一个工作模型的代谢周围BA和BAIBA，其中阿利亚假设，BA和BAIBA可以在线粒体中降解为乙酰辅酶A的支链氨基酸降解酶。通过在各种各样的嘧啶和多胺代谢突变体中使用同位素标记技术定量代谢物，我们的目标是将我们的工作模型转换为实验证明的代谢模型。这将整合嘧啶和多胺催化剂以及CoA合成和支链氨基酸的降解途径的一部分，考虑到细胞溶质，线粒体和过氧化物酶体中的反应。几种酶，包括BAAT，与以前未知的生理作用将被调查，理想的代谢功能将被阐明。对于BAAT，我们还将进行晶体结构分析。此外，辅酶A合成的BA来源将被阐明，氮来源于BAAT介导的转氨作用的途径将被追踪。此外，将研究嘧啶类是否在短期碳限制下更强烈地降解，转录数据表明。因此，嘧啶环降解（也可能是多胺降解）可能有助于在碳饥饿期间短期提供乙酰辅酶A。通过基因组测序，许多植物的基因组现在是已知的，但许多基因功能和代谢中的相互联系仍然不清楚。这项研究将有助于阐明几个基因的功能，并将在许多方面扩大对植物代谢的理解。

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.