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. 2010:8:e0137.
doi: 10.1199/tab.0137. Epub 2010 Aug 23.

Branched-Chain Amino Acid Metabolism in Arabidopsis thaliana

Stefan Binder  1
Affiliations

Branched-Chain Amino Acid Metabolism in Arabidopsis thaliana

Stefan Binder. Arabidopsis Book. 2010.

Abstract

Valine, leucine and isoleucine form the small group of branched-chain amino acids (BCAAs) classified by their small branched hydrocarbon residues. Unlike animals, plants are able to de novo synthesize these amino acids from pyruvate, 2-oxobutanoate and acetyl-CoA. In plants, biosynthesis follows the typical reaction pathways established for the formation of these amino acids in microorganisms. Val and Ile are synthesized in two parallel pathways using a single set of enzymes. The pathway to Leu branches of from the final intermediate of Val biosynthesis. The formation of this amino acid requires a three-step pathway generating a 2-oxoacid elongated by a methylene group. In Arabidopsis thaliana and other Brassicaceae, a homologous three-step pathway is also involved in Met chain elongation required for the biosynthesis of aliphatic glucosinolates, an important class of specialized metabolites in Brassicaceae. This is a prime example for the evolutionary relationship of pathways from primary and specialized metabolism. Similar to animals, plants also have the ability to degrade BCAAs. The importance of BCAA turnover has long been unclear, but now it seems apparent that the breakdown process might by relevant under certain environmental conditions. In this review, I summarize the current knowledge about BCAA metabolism, its regulation and its particular features in Arabidopsis thaliana.

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Figures

Figure 1.
Figure 1.
Structures of the three branched-chain amino acids.
Figure 2.
Figure 2.
Key enzymes (blue) and metabolites (black) of branched-chain amino acid biosynthesis. Allosteric inhibition is indicated by red lines. The restoring effect of Val on threonine deaminase inhibition by Ile is depicted as a dotted green line.
Figure 3.
Figure 3.
Branched-chain amino acid biosynthesis (shaded in green) and Met chain elongation pathway (shaded in blue). Enzymes are given in boxes and are abbreviated as given in the text. Proteins given in green are specific for BCAA biosynthesis. Proteins active in Met chain elongation are given in blue. Proteins active in both pathways are highlighted in red. MAM1/2* indicates that either MAM1 (in Col-0) or MAM2 (in Landsberg erecta) catalyze condensation reactions leading to the preferential accumulation of C4 (in Col-0) and C3 (in Ler) glucosinolates. Abbreviations of metabolites are: Pyr: pyruvate; 2OB: 2-oxobutanoate; 2AL: 2-acetolactate; 2A2HB: 2-aceto-2-hydroxybutanoate; 2,3DH3MB: 2,3-dihydroxy-3-methylbutanoate; 2,3DH3MP: 2,3-dihydroxy-3-methylpentanoate; 3MOB: 3-methyl-2-oxobutanoate; 3MOP: 3-methyl-2-oxopentanoate; 4MOP: 4-methyl-2-oxopentanoate; 2-IPM: 3-isopropylmalate; 3-IPM: 2-isopropylmalate; 4MTOB: 4-methylthio-2-oxobutanoate; MTOA: 4-methylthio-2-oxo acid.
Figure 4.
Figure 4.
Key enzymes (blue) and metabolites (black) of branched-chain amino acid degradation.
See this image and copyright information in PMC

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