Charakterisierung der Funktion von Branched-Chain Aminotransferasen und Isopropylmalat Isomerasen im Primär- und/oder Sekundärmetabolismus in Arabidopsis thaliana

Abstract

The main topic of this thesis was to investigate in detail the function of different branched-chain aminotransferases (AtBCATs) in A. thaliana. These characterizations should help to understand the relevance of AtBCATs in primary and secondary metabolism. In vitro enzyme activity tests showed highest affinity of AtBCAT4 to the main substrate in the methinone-derived glucosinolate biosynthesis. In addition, detailed metabolite profiling with corresponding AtBCAT4 knock out mutants revealed significant decrease in all methionine-derived glucosinolates, and thus a 50 % reduction in total amount of methionine-derived glucosinolates. These results indicate that AtBCAT4 is responsible for the initial transamination reaction in the very beginning of the methionine-derived glucosinolate biosynthesis. AtBCAT3, a plastid member of the AtBCAT family, seems to have a dual function in primary and secondary metabolism. In enzyme activity tests AtBCAT3 accepts substrates of the branched-chain amino acid (BCAA) metabolism, as well as ketomethylthiobutyrate (MTOB) from which the methionine-derived glucosiolates are built. A further important part of this thesis was to identify potential candidates for the isomerization reactions involved in the chain elongation cycle of glucosinolate biosynthesis and in the leucine biosynthesis. These analyses lead to the identification of an aconitase gene family in A. thaliana which comprises seven members. Due to DNA homology one large subunit and three small subunits of the isopropylmalate isomerase, as well as three aconitase genes, were found. Metabolite profiling of corresponding T-DNA insertion lines indicate that IPMI LSU, IPMI SSU2 and IPMI SSU3 are involved in both glucosinolate and leucine biosynthesis. Since ipmi ssu1 is embryolethal, a participation in leucine biosynthesis might be possible.