Phenylalanine pathway

Auxotrophic mutant lack one or more enzymes involved in the synthesis of amino acids (such as tyrosine). This prevents accumulation of the amino acid and thus avoids feedback inhibition of enzymatic steps in the L-phenylalanine pathway. 

Thirty-four naturally occurring compounds that incorporate the oxazole moiety have been isolated thus far. The sources are diverse—plants of the families Gramineae and Rutaceae, nudibranch egg masses, and microorganisms, the latter having furnished the majority of the compounds. With three exceptions, the marine and bacterial oxazoles appear to have been formed from peptides of aliphatic amino acids, while the oxazoles of the Gramineae and Rutaceae arise from the chorismic acid-phenylalanine pathway. The oxazoles have not been... 

SA in plants is thought to be derived from the phenylalanine pathway by the cinnamic acid chain oxidative pathway [54]. 

Dehydroshikimate reductase which reversibly interconverts dehydroshikimate and shikimate is a key enzyme in the biosynthesis of phenolic compounds via the phenylalanine pathway. Phenylalanine ammonia-lyase which catalyzes the cleavage of phenylalanine into transcin-namate and NH3, is equally important for the biosynthesis of phenols. Its activity in tea leaves parallels the content of catechins and epicatechins. 

Sun, Z., et al. Metabolic engineering of the L-phenylalanine pathway in Escherichia coli for the production of S- or R-mandelic acid. Microb. Cell Fact., 2011.10 71. 

Some people lack the enzymes necessary to convert L phenylalanine to L tyrosine Any L phenylalanine that they obtain from their diet is diverted along a different meta bolic pathway giving phenylpyruvic acid... 

Some chemicals such as iadigo, tryptophan, and phenylalanine are overproduced ia bacteria by pathway engineering (36—38). In this method, the enzymes iavolved ia the entire pathway are overproduced. In addition, the host bacterium is also altered such that the carbon flow is directed toward the engiaeered pathway (38). E. colih.2LS been modified to overproduce iadigo and tryptophan and phenylalanine. CoTjnebacteriumglutamicum has been engiaeered to overproduce tryptophan from 28 to 43 g/L. Similarly, attempts are underway to overproduce several vitamins by pathway engineering (34,38). 

Herbicides also inhibit 5- (9/-pymvylshikiniate synthase, a susceptible en2yme in the pathway to the aromatic amino acids, phenylalanine, tyrosine and tryptophan, and to the phenylpropanes. Acetolactate synthase, or acetohydroxy acid synthase, a key en2yme in the synthesis of the branched-chain amino acids isoleucine and valine, is also sensitive to some herbicides. Glyphosate (26), the sulfonylureas (136), and the imida2oles (137) all inhibit specific en2ymes in amino acid synthesis pathways. 

Biosynthesis of Tea Flavonoids. The pathways for the de novo biosynthesis of flavonoids in both soft and woody plants (Pigs. 3 and 4) have been generally elucidated and reviewed in detail (32,51). The regulation and control of these pathways in tea and the nature of the enzymes involved in synthesis in tea have not been studied exhaustively. The key enzymes thought to be involved in the biosynthesis of tea flavonoids are 5-dehydroshikimate reductase (52), phenylalanine ammonia lyase (53), and those associated with the shikimate/arogenate pathway (52). At least 13 enzymes catalyze the formation of plant flavonoids (Table 4). 

The earliest references to cinnamic acid, cinnamaldehyde, and cinnamyl alcohol are associated with thek isolation and identification as odor-producing constituents in a variety of botanical extracts. It is now generally accepted that the aromatic amino acid L-phenylalanine [63-91-2] a primary end product of the Shikimic Acid Pathway, is the precursor for the biosynthesis of these phenylpropanoids in higher plants (1,2). 

Figure 30.14 Pathway for the bacterial biosynthesis of phenylalanine from chorismate, involving a Claisen rearrangement.

Another way to enhance the production of an amino acid is to make use of DNA-recombinant technology, often in combination with foe mutations already described. In this way foe negative features of foe micro-organisms are avoided. To help explain this, we will consider a well known fermentation of L-phenylalanine using Escherichia coli. We have already seen foe metabolic pathway leading to foe production of L-phenylalanine in Figure 8.4. 

It was proven that the pathway of L-phenylalanine formation involved phenylpyruvic add as intermediate and two steps could be distinguished (see Figure 8.6 section 8.7) ... 

Enzymes in the pathway to L-phenylalanine are subject to feedback inhibition by products (amino adds) arising from pathway intermediates. 

Figure 30-13. Alternative pathways of phenylalanine catabolism in phenylketonuria. The reactions also occur in normal liver tissue but are of minor significance.

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