Shikimate pathway regulation

The general phenylpropanoid pathway begins with the deamination of L-phenylalanine to cinnamic acid catalyzed by phenylalanine ammonia lyase (PAL), Fig. (1), the branch-point enzyme between primary (shikimate pathway) and secondary (phenylpropanoid) metabolism [5-7]. Due to the position of PAL at the entry point of phenylpropanoid metabolism, this enzyme has the potential to play a regulatory role in phenolic-compound production. The importance of this is illustrated by the high degree of regulation both during development as well as in response to environmental stimuli. 

Pyocyanin (160) is derived from the shikimate pathway, and one protein, PhzC, is equivalent to enzymes that catalyze the first step in this pathway, converting erythrose 4-phosphate (162) and phosphoenolpyruvic acid (163) to 3-deoxy-D-arabinoheptulosonate 7-phosphate (164) (Fig. 28). The equivalent enzyme in the shikimate pathway is thought to be feedback regulated, and PhzC is likely to shunt intermediates toward the shikimate pathway in preparation for pyocyanin (160)... 

R. A. Jensen, Tyrosine and Phenylalanine Biosynthesis Relationship between Alternative Pathways, Regulation and Subcellular Location. In Recent Advances in Phytochemistry, Vol. 20 The Shikimic Acid Pathway E. E. Conn, Ed. Plenum Press New York, 1985 pp 57-81. 

Figure 18 Allosteric regulation in the shikimate pathway for biosynthesis of aromatic amino acids in coli. Molecules that cause feedback inhibition are shown in red.

Tryptophan is a product of the shikimate pathway and is converted into tryptamine by tryptophan decarboxylase (TDC), Fig. (4). TDC is a cytosolic soluble enzyme that occurs as a dimeric protein, and it was shown to exhibit a high substrate specificity and to be under post-translational control [46-52]. A cDNA clone encoding TDC was isolated by DeLuca et al. [53] and the full gene was characterized by Gooddijn et al. [54, 55] who found that TDC is encoded by a single copy gene without introns. The Tdc promoter has also been cloned and its regulation characterized [56, 57]. 

S ATP + shikimate <3, 17, 19> (<3> SK2 is the isoenzyme that normally functions in aromatic biosynthesis in the cell, SKI functions only when high intracellular levels of shikimate occurs [3] <19> energy charge plays a role in regulating shikimate kinase, thereby controlling the shikimate pathway [10] <17> the enzyme catalyzes the committed step in the seven-step biosynthesis of chorismate [18]) (Reversibility <3, 17, 19> [3, 10, 18]) [3, 10, 18]... 

The enzyme DAHP synthase regulates the carbon flow in the shikimate pathway. Different biotic and abiotic stresses, including mechanical wounding and fungal elicitation, induce the accumulation of DAHP synthase mRNA, and, therefore, of phenolic metabolites. 

The Shikimate Pathway, Conn, E. E., Ed. Plenum New York, NY, 1974. Herrmann, K.M. In Amino Acids Biosynthesis and Genetic Regulation Herrmann, K.M. Somerville, R.L., Eds. Addison-Wesley Reading, MA, 1983, Chap. 17. 

EPSPS is the sixth enzyme in the shikimate pathway that leads to the biosynthesis of aromatic amino acids, tryptophan, tyrosine, and phenylalanine. These aromatic amino adds along with intermediates of the pathway give rise to important secondary metabolites commonly referred to as phenylpropanoids that include phenolics, lignins, tannins, phytoalexins, etc. [1]. The shikimate pathway is localized in plastids and EPSPS is a key enzyme in regulating the flux through the pathway. 

One of the more recent applications of carbene insertion into 0-H bond of alcohols includes the synthesis of chorismic (and pseudochoris-mic) acid [81], which occupies in microorganisms and plants, a strategic position in the shikimate pathway [82] as the key branch point intermediate governing the biosynthesis of aromatic aminoacids, isoprenoid qui-nones, bacterial and plant growth regulators, and other vital compounds. 

Jensen, R. A., Tyrosine and phenylalanine biosynthesis Relationship between alternative pathways, regulation and subcellular location, in The Shikimic Acid Pathway (E. E. Conn, ed.). Recent Advances in Phytochemistry Vol. 20, 57-81, Plenum Press, New York, 1986. 

SCHAB, A. SCHULZ, H.C. STEINRUCKEN. 1981. Interference of glyphosate with the shikimate pathway. Proc. Plant Growth Regul. Soc. Am. 8 99-106. 

Regulation of the shikimate pathway of carrot cells in suspension culture. Plant Physiol. 75 369-371. 

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). 

Specific Control of Phytoalexin Accumulation by "Metabolite Shunting" of Biosynthetic Pathways. Graham and coworkers (personal communication), at the Monsanto Laboratories, St. Louis, have developed techniques to selectively shunt defensive metabolites, particularly of the shikimic acid cycle. Through various techniques, certain compounds are applied to plant aerial or root parts, and these compounds have the property of inducing specific accumulations of secondary metabolites. The directions of these accumulations are under known enzymic control (48), and the regulation of these enzymes is achieved by selecting appropriate inducers. Such inducers seem to provide a novel approach to the control of insects by magnifying the ability of plants to produce and concentrate antiherbivory compounds. 

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