Tryptophan pathway

Phenoxazines — The two main types of phenoxazines are the ommochromes and the microbial phenoxazines. The biosynthesis of ommochromes occurs via the kynurenine pathway. The tryptophan amino acid is converted to formylkynurenine and then to kynurenine and 3-hydroxykynurenine. Not all the steps of ommochrome synthesis are completely elucidated yet. Ommatins are dimers and ommins are oligomers of 3-hydroxykynurenine. - The papiliochromes are derived from tyrosine as well as from the tryptophan pathway. The key intermediate in the formation of papiliochromes is N-beta-alanyldopamine (NBAD). Papiliochromes are synthesized in special wing scale cells, before melanins. " "... 

Nicotinic acid derivatives occur in biologic materials as the free acid, as nicotinamide, and in two coenzymatic forms nicotinamide adenine dinucleotide (NAD), and nicotinamide adenine dinucleotide phosphate (NADP). These coenzymes act in series with flavoprotein enzymes and, like them, are hydrogen acceptors or, when reduced, donors. Several plants and bacteria use a metabolic pathway for the formation of nicotinic acid that is different from the tryptophan pathway used by animals and man (B39). 

In plants and bacteria, phenylalanine and tyrosine are synthesized from chorismate in pathways much less complex than the tryptophan pathway. The common intermediate is prephenate (Fig. 22-19). The final step in both cases is transamination with glutamate. 

The breakdown of glucose to pyruvate. The conversion of glucose to pyruvate requires ten steps and one enzyme for each step. In steps 1 and 3, ATP is consumed. In steps 7 and 10, ATP is produced. The net production of ATP is 2 moles for each mole of glucose consumed. Unlike the intermediates in the tryptophan pathway, many of the intermediates between glucose and pyruvate serve as useful substances for other purposes. A full description of all the steps in this sequence is given in chapter 12. 

Manipulation of the tryptophan pathway continues to provide a range of potential therapeutic targets for disease intervention. Inhibition of the KP and the control of excitotoxic and neuroprotective metabolites are of particular interest in the treatment of HD. Although the discovery of a potent and selective inhibitor of the KP capable of efficacy in vivo in animals is challenging, the true hurdle for researchers is the translation of these lead compounds into viable therapies for the treatment and control of HD. 

Fig. 1. Diagram of the metabolic reactions that determine pool size of the free lAA in plant cells. Inputs include (A) de novo synthesis from non-tryptophan and tryptophan pathways (B) conjugate hydrolysis and (C) transport. Outputs from the lAA pool inelude (D) oxidative catabolism (E) conjugate synthesis (F) transport and (G) possible lAA use during growth and developmental processes.

These developments have certainly changed our concepts of lAA biogenesis from what we knew only a few years ago. It is important to remember, however, that while the establishment of the existence of a non-tryptophan pathway to lAA shows that there is more than one biosynthetic path to lAA, we still know very little about which pathway a plant uses for specific physiological processes or why one pathway is used and not the other. We do know that in the bean seedling, lAA biosynthesis begins even before the stored conjugates are depleted [51] and this biosynthesis comes primarily from tryptophan conversion. Likewise, in carrot callus tissue, the conversion of tryptophan to lAA is also the predominant route [43]. However, when carrot cells are induced to form embryos by growth on 2,4-D-free medium, the conversion of tryptophan to lAA decreases and the non-tryptophan pathway predominates. Interactions between these pathways and the role each plays in development remain to be determined. 

Standard microbial methods for investigating tryptophan biosynthesis were applied to plants in order to study auxin biosynthesis [6,212,218]. A series of Arabidopsis mutants with lesions in four sites in the pathway from chorismate to tryptophan has been obtained, but so far no comparable array of mutations exists for any other plant species [18]. Normanly et al. [44] used these Arabidopsis mutants to dissect lAA biosynthesis and showed that the non-tryptophan pathway to lAA branches from tryptophan biosynthesis at the point of indole or indole-3-glycerol phosphate. 

The next enzyme in the tryptophan pathway is anthranilate-5-phosphoribosyl transferase (EC 2.4.2.18). In some microbial enzymes it is part of the AS enzyme complex, but this transferase activity could not be measured in the purified C. roseus enzyme. 

Also at a subcellular level, compartmentation plays an important role, as was described earlier for the terpenoid and tryptophan pathways. Still, some questions remain concerning this aspect of the biosynthesis. 

For the tryptophan pathway it is a matter of debate whether there is only one pathway that is localized in the plastids, or whether a plastidial and cytosolic pathway occurs in the plant. In the latter hypothesis, the cytosolic pathway is believed to be responsible for the secondary metabolism, whereas the plastidial pathway takes care of the tryptophan production for primary metabolism (for a review, see ref. 143). So far no evidence for such a dual pathway has been found in C. roseus (156 R. Bongaerts et al., unpublished results). 

Individual measurement of all five tryptophan pathway enzymes have been reported from tobacco and carrot cell cultures, wheat, corn, and peas (Widholm, 1973 Singh and Widholm, 1974 Hankinsa/, 1976). The relative amounts of the enzymes assayed in vitro differ with the various sources. However, Singh and Widholm (1974) reported extractable quantities of each enzyme in wheat, regardless of the tissue source or plant age, sufficient to synthesize the amount of tryptophan present within the same tissue in 48 h. No in vitro aggregation of any of the tryptophan branch enzymes was ob-... 

Dubouzet JG, Matsuda F, Ishihara A, Miyagawa H, Wakasa K. Production of indole alkaloids by metabolic engineering of the tryptophan pathway in rice. Plant Biotech-no//2013 ll(9) 1103-11. 

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]... 

Fio. 3. Gene-enzyme relationships in the tryptophan pathway of bacteria. The gene designated by a question mark represents a possible location (inferred from mapping distances) of the gene coding for the smaller protein component of AS in P. putida. The gene has not yet been identified by a specific mutational defect. Dashed lines represent relationships whose existence are still in question. See the text for further explanations. 

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. 

The aromatic biosynthetic pathways do not exist in isolation. Some interactions among the branches of the common aromatic pathway have been mentioned in this chapter. Substrates which are common to several different pathways can serve to interconnect the functioning of the pathways, although they may appear to be quite independent. Jensen has coined the term metabolic interlock to describe regulatory interactions among different metabolic pathways [146a,243,244]. A number of reports have appeared of interactions between the histidine and tryptophan pathways in B. subtilis involving effects on the rates of enzyme synthesis [245,246] as well as enzyme activity [244], Evidence for an interaction between the histidine and tryptophan pathways of N. crassa has also been reported [247],... 

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