Shikimate pathway common branch

Bacteria, fungi, and plants share a common pathway for the biosynthesis of aromatic amino acids with shikimic acid as a common intermediate and therefore named after it—the shikimate pathway. Availability of shikimic acid has proven to provide growth requirements to tryptophan, tyrosine, and phenylalanine triple auxotrophic bacterial strains. Chorismate is also the last common precursor in the aromatic amino acid biosynthetic pathway, but the pathway is not named after it, as it failed to provide growth requirements to the triple auxotrophs. The aromatic biosynthetic pathway starts with two molecules of phosphoenol pyruvate and one molecule of erythrose 4-phosphate and reach the common precursor, chorismate through shikimate. From chorismate, the pathway branches to form phenylalanine and tyrosine in one and tryptophan in another. Tryptophan biosynthesis proceeds from chorismate in five steps in all organisms. Phenylalanine and tyrosine can be produced by either or both of the two biosynthetic routes. So phenylalanine can be synthesized from arogenate or phenylpyruvate whereas tyrosine can be synthesized from arogenate or 4-hydroxy phenylpyruvate. 

An outline of the shikimate pathway from carbohydrate through chorismate to the aromatic amino acids and other metabolically important compounds is shown in Figure 1.1. The major branch point occurs at chorismate and that part of the metabolic sequence from carbohydrate to chorismate is generally referred to as the common pathway. 

Structural considerations and known enzymic reactions suggest that the reaction of shikimate 5-phosphate with enolpyruvate phosphate is related to prephenate formation, and that the reactions of shikimate 5-phosphate with glutamine are involved in the synthesis of anthranilate and p-amino-benzoate. A common intermediate for these two branches of the main pathway, after shikimate 5-phosphate, appears to be unlikely, since it would require the further transformation of the latter compound before reaction with enolpyruvate phosphate. If shikimate 5-phosphate is the branch-point intermediate, quintuple auxotrophs should be completely blocked in any one reaction before shikimate 5-phosphate. In the absence of secondary metabolic effects, mutants blocked immediately after it should not be quintuple auxotrophs. They should show a requirement for either phenylalanine plus tyrosine, or tryptophan plus p-aminobenzoate (since anthranilate and p-aminobenzoate may have a common intermediate ). p-Hydroxybenzoate might be derived from this intermediate, or, independently, from shikimate 5-phosphate. This possibility is illustrated on page 264, X being the possible common intermediate. 

Herbicides that inhibit enzymes important for amino acid synthesis account for 28% of the herbicide market. Just three enzymes are involved the enzyme that adds phosphoenolpyruvate to shikimate-3-phoshate in the pathway leading to aromatic compounds, the enzyme that makes glutamine from glutamate and ammonia, and the first common enzyme in the biosynthesis of the branched-chain amino acids. 

Extensive studies support the hypothesis that these phenazine precursors are derived from the shikimic acid pathway, as outlined in Scheme 1, with chorismic acid (51) as the most probable branch point intermediate. Shikimic acid (50) is converted to chorismic acid (51) in known transformations that are part of the common aromatic amino acid biosynthetic pathway. The transformation from chorismic acid (51) to the phenazine precursors has been discussed and investigated through intensive biochemical studies so far, no intermediates have been identified and little is known about the genetic origin and details of the phenazine biosynthesis. ... 

Shikimate and chorismate are common precursors of all aromatics synthesized by the shikimate/choris-mate pathway (Fig. 2). Figure 3 shows the sequence of conversions up to chorismate, which is the branching point in the biosynthesis of different aromatics. 

The precursors of flavonoid biosynthesis include shikimic acid, phenylalanine, cinnamic acid, and p-coumaric acid. Shikimic acid acts as an intermediate in the biosynthesis of aromatic acid. The basic pathways to the core isoflavonoid skeletons have been established both enzymatically and genetically [16]. The synthesis of isoflavones can be broadly divided into three main synthetic pathways the formylation of deoxybenzoins, the oxidative rearrangement of chalcones and flavanones, and the arylation of a preformed chromanone ring. In leguminous plants, the major isoflavonoids are produced via two branches of the isoflavonoid biosynthetic pathway, and the different branches share a majority of common reactions [1]. Unlike the common flavonoid compotmds, which have a 2-phenyl-benzopyrone core structure, isoflavones, such as daidzein and genistein, are 3-phenyl-benzopyrone compounds. Biochemically, the synthesis of isoflavones is an offshoot of the flavonoids biosynthesis pathway. Several attempts have aimed to increase... 

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