Primary metabolism aromatic amino acids

It is generally accepted that chloroplasts possess an intact pathway of aromatic amino acid biosynthesis that is tightly regulated. In addition, the subcellular location of some aromatic-pathway isozymes has been shown to be in the cytosol, but whether an intact pathway exists in the cytosol has not yet been proven. The evidence bearing on aromatic amino acid compartmentation and regulation is reviewed, with particular emphasis given to the relationship between primary biosynthesis and secondary metabolism in the cytosol. 

Although phenylalanine and tyrosine are closely related in chemical structure and in mammalian metabolism, these amino acids form separate sections of the Amaryllidaceae alkaloids. Phenylalanine contains no oxygen in the aromatic ring yet it serves as a primary precursor of the C-6—C-1 fragment in alkaloids which may contain as many as three oxygenated substituents in ring A. Tjrosine is a universal precursor of ring C and the two-carbon side chain (Ce—C2). Phenylalanine is not... 

The indole moiety of the terpenoid indole alkaloids originates from tryptophan, an aromatic amino acid, which is derived from chorismate via anthranilate. Chorismate is a major branching point in plant primary and secondary metabolism. Here the shikimate pathway (Fig. 6) branches into different pathways (Fig. 7), among others leading to the aromatic amino acids tyrosine, phenylalanine, and tryptophan. 

The biosynthesis of shikimate, the direct precursor for chorismate, has been reviewed elsewhere 138-141). The shikimate pathway leading to chorismate is located in the plastids. For the two branches from chorismate leading to the aromatic amino acids, it has been postulated that both occur in a plastidial and a cytosolic form 142). The plastidial form is responsible for the aromatic amino acids for primary metabolism, and the cytosolic one for the biosynthesis of the aromatic amino acids used as precursors in secondary metabolism (for a review, see refs. 141,143). 

Plant secondary metabolites are biosynthesized from rather simple building blocks supplied by primary metabolism. Two important metabolic routes in this are the shikimate pathway and the isoprenoid biosynthesis. The shikimate pathway leads to the synthesis of phenolic compounds and the aromatic amino acids phenylalanine, tyrosine and tryptophan. The isoprenoid biosjmthesis is a heavily branched pathway leading to a broad spectrum of compounds (fig. 1). From plants and microorganisms more than 37,000 isoprenoid compounds have been isolated so far [1]. 

The precursor substrates and enzymes necessary for the first committed steps often appear to have been recruited from primary metabolic pathways, such as glycolysis, the Krebs cycle, the pentose phosphate pathway and the shikimate pathway [32], For example, the aromatic amino acid s L-phenylalanine and L-tyrosine, produced by the shikimate pathway, are precursors for a wide spectrum of natural products including phenylpropanoids, flavonoids, lignins, coumarins, cyanogenic glycosides, glucosinolates, and alkaloids [33],... 

Primary Metabolism of Shikimic Acid and Aromatic Amino Acids... 

Glyphosate is phloem mobile and accumulates in accordance with source-sink relationships in meristematic areas of the treated plants. " The general physiological effects of glyphosate are consistent with its acting as a metabolic poison. Death is a slow process, requiring days or weeks much more rapid effects, within hours, can be demonstrated at the biochemical level. These effects can be explained if the turnover rates of specific proteins, and therefore sensitivity to amino acid synthesis inhibition, differ. Inhibition of phenolic biosynthesis and chlorophyll formation and effects on the levels of the phytohormone 3-indoleacetic acid have been considered as target sites but can be viewed as necessary consequences of the primary interference with aromatic amino acid biosynthesis. " ... 

---