Bacteria tryptophan synthesis

Precursors in the biosynthesis of niacin In animals and bacteria, tryptophan and in plants, glycerol and succinic acid. Intermediates in the synthesis include kynurenine, hydroxyanthranilic acid, and quinolinic acid. In animals, the niacin storage sites are liver, heart, and muscle. Niacin supplements are prepared commercially by (1) Hydrolysis of 3-cyanopyndine or (2) oxidation of nicotine, quinoltne, or collidine. 

We noted earlier the coordinate repression of the enzymes for tryptophan synthesis in the presence of tryptophan. When the level of tryptophan is depleted or reduced, trp mRNA is synthesized in 3-4 minutes and rapidly degraded it has a half-life of some 3 minutes. This allows the bacteria to respond quickly to changing requirements for tryptophan. 

V. for the rat, which can satisfy its total requirement by the degradation of tryptophan. Synthesis of the nicotinamide moiety of NAD(P) in bacteria and plants occurs by a different pathway, in which aspartic acid and dihydroxyacetone phosphate act as precursors. 

Precursors of phenylpropanoids are synthesized from two basic pathways the shikimic acid pathway and the malonic pathway (see Fig. 3.1). The shikimic acid pathway produces most plant phenolics, whereas the malonic pathway, which is an important source of phenolics in fungi and bacteria, is less significant in higher plants. The shikimate pathway converts simple carbohydrate precursors into the amino acids phenylalanine and tyrosine. The synthesis of an intermediate in this pathway, shikimic acid, is blocked by the broad-spectrum herbicide glyphosate (i.e., Roundup). Because animals do not possess this synthetic pathway, they have no way to synthesize the three aromatic amino acids (i.e., phenylalanine, tyrosine, and tryptophan), which are therefore essential nutrients in animal diets. 

Attenuation. A major mechanism of feedback repression, known as attenuation, depends not upon a repressor protein but upon control of premature termination. It was first worked out in detail by Yanofsky et al. for the trp operon of E. coli and related bacteria.184 186 Accumulation of tryptophan in the cell represses the trp biosynthetic operon by the action of accumulating tryptophanyl-tRNATlP, which specifically induces termination in the trp operon. Other specific "charged" arnino-acyl-tRNA molecules induce termination at other amino acid synthesis operons. 

Bacterial De-Novo Synthesis. The basic idea behind this variant is to use the synthetic potential of bacteria to produce the indole precursor (Scheme 3). Although indole (23) does not occur as an intermediate in bacterial metabolism, it appears as an enzyme-linked intermediate in the biocatalytic transformation ofD-glucose (17) to L-tryptophan (22). The crucial biosynthetic step is the conversion of indole-3-glycerine phosphate (21) to L-tryptophan (22) by the enzyme tryptophan synthase. 

Shikimic acid pathway chemical pathway common in plants, bacteria, and fungi, where aromatic amino acids (e.g., tryptophan, phenylalanine, tyrosine) are synthesized, thereby providing the parent compounds for the synthesis of the phenylpropanoid units in lignins. 

Tryptophan (fig. 8) is one of the twenty amino acids used by all of life on Earth to build proteins. Although plants, fungi, bacteria, and some other organisms can biosynthesize tryptophan from smaller carbon molecules, humans cannot and must ingest tryptophan as part of their diet. That is, tryptophan is one of the essential amino acids. In fungi and plants, tryptophan is the chemical precursor for the biosynthesis of tryptamines such as DMT and psilocybin. In humans and other animals, tryptophan is the precursor for the synthesis of the neurotransmitter serotonin, 5-hydroxytryptamine (5-HT fig. 9). 

Plants and some bacteria synthesize all 20 amino acids (see also Chapter 2). Humans (and other animals) can synthesize about half of them (the nonessential amino acids) but require the other half to be supplied by the diet (the essential amino acids). Diet must also provide a digestible source of nitrogen for synthesis of the nonessential amino acids. The eight essential amino acids are isoleucine, leucine lysine, methionine, phenylalanine, threonine, tryptophan, and valine. In infants, histidine (and possibly arginine) is required for optimal development and growth and is thus essential. In adults, histidine is nonessential, except in uremia. Under certain conditions. 

Ordering of Biosynthetic Pathways A simple example of the first type of process Is the biosynthesis of a metabolite such as the amino acid tryptophan In bacteria. In this case, each of the enzymes required for synthesis of tryptophan catalyzes the conversion of one of the intermediates in the pathway to the next. In E. coli, the genes encoding these enzymes lie adjacent to one another in the genome, constituting the... 

Another example of translational control in eukaryotes is the inhibition of yeast GCN4 protein synthesis by stem-loop structures present in the 50 end of the mRNA. GCN4 control, and an analogous situation in bacteria, links amino-acid biosynthesis to ribosome pausing in the 50 end of the mRNA. This mechanism was first described for the tryptophan operon in E. coli and it is often referred to as attenuation. Transcriptional and translational control of the tryptophan biosynthetic enzymes are described in Chapter 28. 

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