Tryptophan biosynthetic pathway

The tryptophan biosynthetic pathway in E. coli. There are five enzymatically catalyzed reactions involved in tryptophan biosynthesis and five different polypeptides associated with these reactions. Polypeptides E and D normally make a tetrameric complex, which... 

A certain bacteria that was a tryptophan auxotroph was observed to grow well when it was supplied with tryptophan, but as soon as the tryptophan in the environment was exhausted it started to excrete a metabolite on the tryptophan biosynthetic pathway. Why didn t it excrete the metabolite before it exhausted the environmental tryptophan ... 

Demonstration That Indole Is Not a True Intermediate in the Tryptophan Biosynthetic Pathway... 

Incorporation studies with isotopes showed that when anthranijate was converted to tryptophan, the carboxyl group df anthranilate was lost as carbon dioxide, but the nitrogen was retained. Because the enzymes in the tryptophan biosynthetic pathway have only a limited specificity, it was possible to substitute 4-methyl-anthranilate in E. coli extracts that could convert anthranilate to indole. This nonisotope label was conserved during the conversion to yield 6-methyl indole. 

A second level of control of the tryptophan biosynthetic pathway was discovered by Charles Yanofsky when he characterized mutants in the trp operon that did not affect Trp repressor binding. Yanofsky and his colleagues characterized a novel form of transcriptional control they called attenuation, which depends on the unique linkage between transcription and translation in prokaryotes. As shown in Figure 28.11, the intracellular concentration of TRP-tRNATrp determines if the ribosome will pause at a set of codons in the trp mRNA that specify consecutive Trp residues. When tryptophan levels are high, and TRP-tRNATrp is available, then the transcriptional termination hairpin loop forms and RNA polymerase disengages from the DNA template just downstream of a polyuridine... 

Taken together, the combined effect of Trp repressor-mediated transcriptional repression, and TRP-tRNATrp sensitive transcriptional attenuation provides a highly sensitive mechanism to control utilization of the tryptophan biosynthetic pathway. 

In B. subtilis, the pathway from chorismate to tryptophan is feedback-inhibited by tryptophan, which suppresses anthranilate synthase activity. Mutant B. subtilis that lacks tryptophan synthetase can grow on minimal medium only when supplemented with exogenous tryptophan. Under these conditions, none of the intermediates in the tryptophan biosynthetic pathway from anthranilate to indole 3-glycerol phosphate are produced. However, when the bacteria have depleted the medium of tryptophan, the levels of those intermediates increase, even though there is no net production of tryptophan. Why ... 

The tryptophan biosynthetic pathway in microorganisms is one of the branches from a common pathway for the biosynthesis of the aromatic substances. Some regulation of tryptophan synthesis occurs at the level of the common aromatic pathway as well as at the level of the synthesis of glutamine [1,2], a tryptophan precursor. This chapter will be... 

The early history of the analyses which disclosed the major features of the tryptophan biosynthetic pathway was reviewed by Yanofsky [7] and by Umbarger and Davis [8], and the more recent history by Gibson and Pittard [3]. The pathway is shown in Fig. 1. Chorismate, a substrate of the first reaction in the tryptophan pathway, is the branch-point intermediate at the end of the common aromatic pathway [9,10]... 

Fig. 1. The tryptophan biosynthetic pathway. Abbreviations AS, anthranilate synthetase PRPP, 5-phosphoribosyI-l-pyrophosphate PRT, phosphoribosyi transferase PRAI, phosphoribosyi anthranilate isomerase InGPS, indoleglycerol phosphate synthetase TS, tryptophan synthetase TS-a, tryptophan synthetase a-chain subunit TS- z, tryptophan synthetase -chain dimer subunit.

III. GENE-ENZYME RELATIONSHIPS IN THE TRYPTOPHAN BIOSYNTHETIC PATHWAY... 

Tentative gene-enzyme relationships in the tryptophan biosynthetic pathway of B. subtilis are shown in Fig. 3. Early studies indicating that... 

The four bacterial species described above display a variety of organizations of the genes and proteins involved in the tryptophan biosynthetic pathway. In one species an enzyme may consist of two different protein subunits solely devoted to catalyzing a single reaction, whereas in another species the second subunit also mediates another enzymatic reaction by itself. Some species utilize a single polypeptide to carry out two of the reactions for which other species provide two different and independent proteins. As a result of these differences the number of genes involved in the tryptophan pathway varies from five to seven. 

An early indication that the enzymes of the tryptophan biosynthetic pathway were subject to repression control by the end product was a report by Monod and Cohen-Bazire [129] that tryptophan synthetase formation in A. aerogenes was inhibited by tryptophan. Yanofsky [7] described much of the early work on repression of the tryptophan biosynthetic enzymes. 

Smolen, G. and Bender, J. (2002) Arabidopsis cytochrome P450 cyp83Bl mutations activate the tryptophan biosynthetic pathway. Genetics... 

It was mentioned above that the carboxyl carbon is lost in the conversion of anthranilic acid to indole. Consequently, two additional carbon atoms must be supplied to complete the pyrrole ring of the indole. The observation that various ribose derivatives could be the source of these two carbons provided the clue that led to the elucidation of the mechanism of indole synthesis in the tryptophan biosynthetic pathway (232). Yanofsky determined that sonic extracts of a tryptophan auxotroph of E. cdi (that also grew on anthranilic acid or indole) could utilize ribose, ribose 5-phosphate, and 5-phosphoribosylpyrophosphate to form indole from anthranilic acid. With the two former compounds, ATP was essential for the reaction, with the latter compound it was not. This result made it appear evident that 5-phosphoribosylpyrophosphate was the more immediate reactant in the condensation with anthranilic acid. 

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