The aspartic acid metabolic pathway, an exciting and essential pathway in plants

Abstract

Aspartate is the common precursor of the essential amino acids lysine, threonine, methionine and isoleucine in higher plants. In addition, aspartate may also be converted to asparagine, in a potentially competing reaction. The latest information on the properties of the enzymes involved in the pathways and the genes that encode them is described. An understanding of the overall regulatory control of the flux through the pathways is undisputedly of great interest, since the nutritive value of all cereal and legume crops is reduced due to low concentrations of at least one of the aspartate-derived amino acids. We have reviewed the recent literature and discussed in this paper possible methods by which the concentrations of the limiting amino acids may be increased in the seeds.

Deciphering the Role of Aspartate and Prephenate Aminotransferase Activities in Plastid Nitrogen Metabolism

“…Nevertheless, the levels of both amino acids clearly increased (approximately 2-fold for Thr and 3-fold for Ile) in plants cosilenced for both aminotransferases when compared with control plants. To further study the impact of plastidic AAT silencing, the expression levels of genes encoding three key enzymes in the Asp metabolic pathway were determined: Asp kinase (AK1), Asp semialdehyde dehydrogenase (ASDH1), and LL-diaminopimelate aminotransferase (LL-DAP; Azevedo et al, 2006;Hudson et al, 2006). The relative abundance of AK1 transcripts was similar in all samples examined; however, the expression of ASDH1 and LL-DAP significantly increased in NbPAT and NbAsp5/NbPAT silenced plants (Fig.…”

“…ALS, Acetolactate synthase; AS, Asn synthetase; ASN, asparaginase; BCAT, branched-chain aminotransferase; CBL, cystathionine b-lyase; CGS, cystathionine g-synthase; DAP-D, diaminopimelate decarboxylase; DAP-F, diaminopimelate epimerase; DHAD, dihydroxy acid dehydratase; DHDPR, dihydrodipicolinate reductase; DHDPS, dihydrodipicolinate synthase; HSDH, homoserine dehydrogenase; HSK, homoserine kinase; KARI, ketol-acid reductoisomerase; MS, Met synthase; TD, Thr deaminase; TS, Thr synthase. main pathways: (1) the biosynthesis of Asn in the cytosol (Lea and Azevedo, 2003;Lea et al, 2007); and (2) the biosynthesis of Lys, Met, Thr, and Ile through the socalled Asp metabolic pathway in the plastids (Azevedo et al, 2006;Jander and Joshi, 2009;Galili, 2011;Fig. 1).…”

Threonine-insensitive Homoserine Dehydrogenase from Soybean

“…In plants and microorganisms, aspartate is the common precursor of the essential amino acids lysine, threonine, methionine, and isoleucine (1)(2). Because this branching set of pathways is critical for central metabolism and not found in mammals, the enzymes of the aspartate family of amino acid biosynthesis present targets for the development of anti-microbial agents and herbicides (1)(2)(3) and for improving the nutritional value of food crops (4 -7).…”

“…Because this branching set of pathways is critical for central metabolism and not found in mammals, the enzymes of the aspartate family of amino acid biosynthesis present targets for the development of anti-microbial agents and herbicides (1)(2)(3) and for improving the nutritional value of food crops (4 -7). In the initial step of the pathway (1), aspartate kinase (AK) 7 catalyzes the phosphorylation of aspartate (Fig.…”

“…Subsequent metabolism of Hse yields methionine and threonine. Both AK and HSD are highly regulated by feedback inhibition through threonine to control flux through the various branches of the pathway (1)(2). The regulation of these enzymes by threonine depends on their structural organization in different organisms.…”