Hydroxycinnamic acid biosynthesis

As discussed in Section 7, the general phenylpropanoid pathway originally included the biosynthesis of the hydroxycinnamic acids caffeic acid (3.32), femlic acid (3.33), 5-hydroxyferulic acid (3.34), and sinapic acid (3.35) from / coumaric acid (3.30), as well as the corresponding CoA-esters 

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The key enzymes involved in the formation of the hydroxycinnamic acids (HCAs) from phenylalanine and malonyl-CoA are now discussed in detail, while later sections address the branches of the flavonoid pathway leading to anthocyanins, aurones, flavones, flavonols, PAs, and isotlavonoids. This is followed by brief reviews of the regulation of flavonoid biosynthesis and the use of flavonoid genes in plant biotechnology. To assist the reader. Figure 3.1 presents the carbon numbering for the various flavonoid types discussed. 

Proanthocyanidins and Procyanidins - In a classical study Bate-Smith ( ) used the patterns of distribution of the three principal classes of phenolic metabolites, which are found in the leaves of plants, as a basis for classification. The biosynthesis of these phenols - (i) proanthocyanidins (ii) glycosylated flavonols and (iii) hydroxycinnamoyl esters - is believed to be associated with the development in plants of the capacity to synthesise the structural polymer lignin by the diversion from protein synthesis of the amino-acids L-phenylalanine and L-tyro-sine. Vascular plants thus employ one or more of the p-hydroxy-cinnarayl alcohols (2,3, and 4), which are derived by enzymic reduction (NADH) of the coenzyme A esters of the corresponding hydroxycinnamic acids, as precursors to lignin. The same coenzyme A esters also form the points of biosynthetic departure for the three groups of phenolic metabolites (i, ii, iii), Figure 1. 

Their biosynthesis derives from the condensation of three acetyl units and of a derivative of hydroxycinnamic acid leading to the formation of a common intermediate, tetrahydroxychalcone. This chalcone is precursor of several compounds, the most important being the 4-oxo-flavonoids [19]. 

Coumarins and isocoumarins appear to be of varied origins. Simple coumarins, such as umbelliferone, are formed by the shikimic acid pathway in which hydroxylation of p-hydroxycinnamic acid occurs. Other coumarins, for example alternariol (690), are derived from a polyketide unit, as are a number of chromanones, chromones, pyranones and isocoumarins (B-78MI22400). The biosynthesis of 5-hydroxy-2-methylchromone has been shown to involve the chromanone (60JCS654). However, isocoumarins are also derived from the mixed acetate-shikimate route, through initial cyclization of the polyketide and subsequent lactonization. 

In plants, biosynthesis and exudation of allelochemicals follows developmental, diurnal, and abiotic/biotic stress-dependent dynamics. Compounds from 14 different chemical classes have been linked to allelopathic interactions, including several simple phenolic acids (e.g., benzoic and hydroxycinnamic acids) and flavonoids [Rice, 1984 Macias et al., 2007]. The existence of several soil biophysical processes that can reduce the effective concentration and bioactivity of these compounds casts doubts on their actual relevance in allelopathic interactions [Olofsdotter et al., 2002]. However, there are well-documented examples of phenylpropanoid-mediated incompatible interactions among plants. Several Gramineae mediate allelopathic interactions by means of... 

Bokern, M., Heuer, S. and Strack, D. (1992) Hydroxycinnamic acid transferases in the biosynthesis of acylated betacyanins purification and characterization from cell cultures of Chenopodium rubrum and occurrence in some other members of the Caryophyllales. Bot. Acta, 105, 146-51. 

A cross-talk between the two activation pathways of hydroxycinnamic acids by CoA or glucose was shown in an Arabidopsis mutant with hyperfluorescence upon reduction of UDP-glucose sinapate glucosyltransferase acfivify a part of fhe sinapic acid was activated as CoA thioester and directed into flavonoid biosynthesis (Sinlapadech et al, 2007). 

Davin, F.B. and Fewis, N.G. (2003) An historical perspective on lignan biosynthesis monolignol, allylphenol and hydroxycinnamic acid coupling and downstream metabolism. Phytochemistry Rev., 2, 257-88. 

Biosynthesis of Flavonoids (58) from 4-Hydroxycinnamic Acid (14) and Acetic Acid... 

One mole 4-hydroxycinnamic acid (14) might react with 3 mol acetic acids to yield a precursor chalcone (57) and then flavonoids (58) (Fig. 11) [25,26]. The proposed biosynthesis of flavonoids was confirmed in a Antirrhinum majus... 

Fig. 11 Biosynthesis of flavonoids (58) from 4-hydroxycinnamic acid (14) and acetic acid...

Fig. 12 Biosynthesis of epicatechin (55), cyanidin (51) and proanthocyanidins (44) by two enzymes of leucoanthocyanidin dioxygenase (LDOX) and leucoanthocyanidin reductase (LAR) from 4-hydroxycinnamic acid (p-coumaric acid, 3)...

Coumarins are 1,2-benzopyrones that are derived from the phenylpropanoid pathway however, major details of their biosynthesis are still largely unknown.187 Coumarins can also be produced, through the cleavage of meliotoside (the /3-glucopyranoside of o-hydroxycinnamic acid) in dead plant material, or hay, where in the... 

The biosynthesis of O-glucosides and O-glucose esters of hydroxycinnamic acids Phytochemistry 2 (1963) 1-6. 

Lunarine.— The biosynthesis of lunarine (109) has been studied in Lunaria biennis. Although [3 - C]phenylalanine was incorporated, [3 - C]tyrosine was not Consequently, phenylalanine is converted first into cinnamic acid, which is then hydroxylated. Oxidative coupling of p-hydroxycinnamic acid and cyclization would give (110), the analogue of Pummerer s ketone, and thence to... 

The role of CYP98A3 in phenylpropanoid biosynthesis was validated by the analysis of the refS mutant. As a result of the lack of C3 H activity, and as would be expected based upon its re.f phenotype, the refS mutant lacks sinapoylmalate. Further, saponification of leaf extracts yielded high levels of p-coumaric acid, indicating that refS accumulates />-coumaric acid esters that are not normally found in wild-type plants. The presence of these novel compounds demonstrates a degree of plasticity in hydroxycinnamic acid ester synthesis in... 

Biosynthesis. The primary precursors of L. are con-iferyl, sinapyl and p-coumaiyl alcohoi, which are derived from 4-hydroxycinnamic acid. L. from conifers (i. e. from softwood) is derived chiefly from conifeiyl alcohol with variable but small proportions of sinapyl and p-coumaryl alcohol. L. from dicotyledonous an-giosperms (i.e. from hardwood), particularly deciduous trees, is formed chiefly from sinapyl (-44%) and coniferyl (-48 %) alcohol, with about 8 % p-cou-maryl alcohol. L. in grasses is formed from p-coumar-yl (-30%), coniferyl (-50%) and sinapyl (-20%) alcohol. These primary L. precursors are formed from the aromatic amino acids L-phenylalanine and L-tyro-sine by a series of reactions shown (Rg.). The first reaction is catalysed by L-Phenylalanine ammonia-lyase (EC 4.3.1.5) (see) this enzyme is induced by light in a process involving phytochrome, and it is of general importance in the synthesis of plant phenolic compounds from phenylalanine and tyrosine. 

The utilization of phenylalanine for the biosynthesis of the mesembrine alkaloids was shown to involve its conversion to cinnamic acid (151) and 4-hydroxycinnamic acid (152), intermediates which are characteristic of the phenylpropanoid pathway (74). 

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