Aromatic amino acid biosynthesis in higher plants

Regulation of biosynthesis in the multibranched shikimic acid pathway appears to rely heavily on feedback mechanisms, some of which have been demonstrated both in vivo as well as in vitro. It would seem from the composite of plant studies described herein, which by no means are extensive enough to permit inclusive generalization, that adequate control measures have been observed to account for the maintenance of pool sizes at reasonably low levels by the various metabolites. Furthermore, negative evidence for repression of pathway enzymes tends to emphasize the role of enzyme inhibition and activation in the regulation of aromatic amino acid biosynthesis in higher plants which at the present is depicted in Fig. 6. 

Aromatic amino acid biosynthesis in higher plants appears to follow the same reaction sequence previously elucidated in microorganisms. With the... 

This review deals with the following questions. (i) What enzyme steps are utilized for aromatic amino acid biosynthesis in higher plants (ii) What is the subcel-lular location of these enzymes (iii) What regulatory mechanisms govern the output of these enzyme networks ... 

The tightly regulated pathway specifying aromatic amino acid biosynthesis within the plastid compartment implies maintenance of an amino acid pool to mediate regulation. Thus, we have concluded that loss to the cytoplasm of aromatic amino acids synthesized in the chloroplast compartment is unlikely (13). Yet a source of aromatic amino acids is needed in the cytosol to support protein synthesis. Furthermore, since the enzyme systems of the general phenylpropanoid pathway and its specialized branches of secondary metabolism are located in the cytosol (17), aromatic amino acids (especially L-phenylalanine) are also required in the cytosol as initial substrates for secondary metabolism. The simplest possibility would be that a second, complete pathway of aromatic amino acid biosynthesis exists in the cytosol. Ample precedent has been established for duplicate, major biochemical pathways (glycolysis and oxidative pentose phosphate cycle) of higher plants that are separated from one another in the plastid and cytosolic compartments (18). Evidence to support the hypothesis for a cytosolic pathway (1,13) and the various approaches underway to prove or disprove the dual-pathway hypothesis are summarized in this paper. 

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). 

Biosynthesis. Full details of the isolation and characterization of intermediates in the biosynthesis of ubiquinone-8 (233) in E. coli have been described. Two review articles deal with the compartmentation of the biosynthesis of aromatic amino-acids and prenylquinones in higher plants and the regulation of prenylquinone biosynthesis by tyrosine. ... 

The conversion of chorismate into prephenate is an example of a biologically relevant Claisen rearrangement. It is the key intermediate in the biosynthesis of aromatic amino acids (tyrosine, phenylalanine, and tryptophan) in bacteria, fungi, and higher plants (Figure 1.23) [23]. 

The metabolic pathway responsible for biosynthesis of aromatic amino acids and for vitamin-like derivatives such as folic acid and ubiquinones is a major enzyme network in nature. In higher plants this pathway plays an even larger role since it is the source of precursors for numerous phenylpropanoid compounds, lignins, auxins, tannins, cyano-genic glycosides and an enormous variety of other secondary metabolites. Such secondary metabolites may originate from the amino acid end products or from intermediates in the pathway (Fig. 1). The aromatic pathway interfaces with carbohydrate metabolism at the reaction catalyzed by 3-deoxy-D-arabino-heptulosonate 7-phosphate (DAHP) synthase, the condensation of erythrose-4-phosphate and PEP to form... 

Evidence for the operation of the shikimate pathway in higher plants has been obtained mainly from tracer experiments and from a limited range of enzyme studies. These have shown that not only do the pathways of biosynthesis of L-phenylalanine, L-tyrosine and L-tryptophan involve the same intermediates as in bacteria and fungi Figures 1.2, 1.13, 1.14), but that higher plants can also convert the aromatic amino acids into a plethora of characteristic natural products or secondary metabolites ... 

Fieure I.IS Biosynthesis of aromatic amino acids in higher plants ... 

Present evidence, albeit incomplete, strongly supports the concept that pathways utilized for the synthesis of amino acids in higher plants are similar or identical to those utilized by microorganisms. However, examples of species-specific mechanisms for the synthesis of identical compounds have been identified among bacterial species, particularly with respect to aromatic amino acids. Similar examples of biochemical diversity within the plant kingdom might also be anticipated as additional plant species are examined and individual pathways of amino acid biosynthesis are studied in greater detail. 

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