Amino acid aromatic, synthesis

Glyphosate also inhibits the activity of a form of 3-deoxy-D-arabino-heptulosonate-7-phosphate synthase (13), however, this inhibition is relatively weak. Effects of glyphosate on the enzymes of aromatic amino acid synthesis have been described in more detail in recent reviews (14-17). 

Figure 2.9 Part of the biosynthetic pathway of aromatic amino acid synthesis...

Figure 4. Enzymes of Rhizobium (a) and Lemna (b) proposed as sites of glyphosate inhibition of aromatic amino acid synthesis. Abbreviations CM, chorismate mutase PDH, prephenate dehydrogenase and PD, prephenate dehydratase.

Chorismate is an intermediate in the biosynthesis of the aromatic amino acids tryptophan, phenylalanine, and tyrosine. Mammals do not synthesize these amino acids bom chorismate. Instead, they obtain the essential aromatic amino acids tryptophan and phenylalanine from the diet, and they can synthesize tyrosine from phenylalanine. Glyphosate is an effective herbicide because it prevents synthesis of aromatic amino acids in plants. But the compound has no effect on mammals because they have no active pathway for de novo aromatic amino acid synthesis. 

The role of quinic acid in the shikimic acid pathway is not understood. In some plants, compared with shikimate, quinate administered exogenously is more effectively incorporated into the aromatic amino acids. These results have been interpreted to mean that quinate occurs on the pathway of aromatic amino acid synthesis and is not a shunt metabolite as depicted in Fig. 2. Different routes of metabolism for quinate and shikimate are also suggested by other experiments. For example, when p CJglucose and [ C]ery-throse are compared for eflSciency of their conversion into shikimate and... 

Another hypothesis of aromatic amino acid synthesis, based on the distribution of the label in tyrosine of yeast grown on radioactive pyruvate or acetate, is that it involves the cyclic condensation of two unsym-metric 4-carbon acids, e.g., oxalacetate. The side chain of tyrosine appears to be formed from pyruvate as an intact 3-carbon unit. 

The well-known cytoplasmic shikimate pathway is important to aromatic amino acid synthesis. It takes a long metabolic pathway to reach to an aromatic amino acid from D-glucose. Two metabolic intermediates, phosphoenolpymvate from the glycolytic pathway and D-erythrose-4-phosphate from the pentose-phosphate pathway, must combine to form 3-deoxy-7-phospho-D-arabinoheptulosonate before formation of 3-dehydroquinate (Fig. 13.4). Thus, there are still several barriers difficult to overcome in the already existing technologies of shikimate fermentation from D-glucose (Draths et al. 1999). 

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