Phenolics biosynthetic pathways

Figure 1.35 Schematic diagram of the phenolic biosynthetic pathway accompanied by the key enzymes involved. Enzyme abbreviations PAL, phenylalanine ammonia-lyase BA2H, benzoic acid 2-hydroxylase C4H, cinnamate 4-hydroxylase COMT-1, caffeic/5-hydroxyferulic acid O-methy I transferase 4CL, p-co um a ra te C o A ligase F5H, ferulate 5-hydroxylase GT, galloyltransferase ACoAC, acetylCoA carboxylase.

Phenolic compounds are commonplace natural products Figure 24 2 presents a sampling of some naturally occurring phenols Phenolic natural products can arise by a number of different biosynthetic pathways In animals aromatic rings are hydroxylated by way of arene oxide intermediates formed by the enzyme catalyzed reaction between an aromatic ring and molecular oxygen... 

Many phytotoxic compounds produced by higher plants are phenolic compounds. Several of these have been implicated in allelopathy. Based on the biosynthetic pathway from which they are derived, phenolic compounds produced by higher plants fall into two general categories 1) terpenoid phenolic compounds derived from five... 

Figure 1. Biosynthetic pathway for production of shikimic acid pathway-derived phenolic compounds in higher plants.

This chapter provides an overview of the biosynthetic pathways leading to the major classes of phenolic compounds as outlined in Chapter 1. This overview is intended to show the origin of the different families of phenolic compounds, many of which share precursors. In addition to an overview of established pathways, newly elucidated steps in the biosynthesis of several classes of compounds will be highlighted. 

The focus of this book is centered on structure, nomenclature and occurrence of phenolic compounds (Chapter 1), and their chemical properties (Chapter 2). Chapter 3 describes the biosynthetic pathways leading to the major classes of phenolics. This chapter presents an up-to-date overview of the genetic approaches that have been used to elucidate these pathways. Chapter 4 presents an overview of methods for the isolation and identification of plant phenolic compounds. Given that much of the recent... 

The biosynthesis of flavonoids, stilbenes, hydroxycinnamates, and phenolic acids involves a complex network of routes based principally on the shikimate, phenyl-propanoid, and flavonoid pathways (Figs. 1.35 and 1.36). These biosynthetic pathways constitute a complex biological regulatory network that has evolved in vascular plants during their successful transition on land and that ultimately is essential for their growth, development, and survival [Costa et al., 2003]. 

Comprehensive information on the network of pathways responsible for the synthesis of numerous secondary metabolites can be found in Chapter 21. In addition, information on this aspect is also available in articles by Shimada et al. [2003], Toshiaki [2003], Tanner et al. [2003], Boatright et al. [2004], Hoffmann et al. [2004], Dixon et al. [2005], Niemetz and Gross [2005], Xie and Dixon [2005], and Ferrer et al. [2008], Nonetheless, the complete dissection of phenolic metabolic pathway is far from being complete. For example, recent reports underline that important questions still remain to be answered in the field of protoanthocyanidins and tannins [Xie and Dixon, 2005], and that the exact nature of the biosynthetic pathway(s) leading to lignin monomers has not been fully elucidated [Boudet, 2007]. 

Phenolic compounds and flavonoids are a unique category of plant phytochemicals especially in terms of their vast po ential health-benefiting properties. They represent the most abundant and the most widely represented class of plant natural products. A substantial amount of research has been carried out over the past two decades yet large information gaps still exist. For example, the inventory of these compounds is still incomplete, although there is continuous effort to provide new structures. In addition the dissection of the metabolic pathways for certain phenolic compounds remains to be resolved. Recent reports underline that important questions that still need to be answered in the field of proanthocyanidin and tannin biosynthesis [Xie and Dixon, 2005], and even the exact nature of the biosynthetic pathway(s) leading to lignin monomers is not fully elucidated. 

Although the CNB hypothesis predicts changes in concentrations in all C-based secondary metabolites in relation to the plant C/N status, different classes of these metabolites may respond differently to environmental variation,175 176 since they are derived from different biosynthetic pathways, represent different proportions of a plant s carbon budget, and may have different turnover rates.8 176 177 Early successional herbs and woody plants that produce phenolic compounds conform to the CNB hypothesis178 more predictably than plants that produce other classes of C-based secondary metabolites such as terpenes176 179 and furanocoumarins.180... 

The biosynthetic work on mescaline in the peyote cactus L. williamsii and in the Peruvian cactus T. pachanoi has led to the formulation of biosynthetic pathways according to Scheme 2. A major pathway probably involves decarboxylation of tyrosine followed by hydroxylation to yield dopamine. Dopamine is methylated on the meta hydroxy group to 4-hydroxy-3-methoxyphenethylamine (3-methoxytyramine) which then undergoes hydroxylation to the key intermediate 4,5-dihydroxy-3-methoxyphenethylamine (20). Para-O-methylation of 20 yields 3,4-dimethoxy-5-hydroxyphenethylamine (21), which is the immediate precursor of the main phenolic tetrahydroisoquinolines of peyote. Alternatively, meta-O-methylation yields 3,5-dimethoxy-4-hydroxyphenethylamine (19), which is further efficiently methylated to mescaline. Parallel pathways involving N-methylated compounds probably exist in these cacti (10). 

The peyote cactus contains an O-methyltransferase that has been isolated and characterized (353). By using variously substituted phenolic phenethylamines as substrates for this enzyme, the previously postulated biosynthetic pathways to mescaline in this cactus could be verified (354, 327). 

Phenylpropanoids have an aromatic ring with a three-carbon substituent. Caffeic acid (308) and eugenol (309) are known examples of this class of compounds. Phenylpropanoids are formed via the shikimic acid biosynthetic pathway via phenylalanine or tyrosine with cinnamic acid as an important intermediate. Phenylpropanoids are a diverse group of secondary plant compounds and include the flavonoids (plant-derived dyes), lignin, coumarins, and many small phenolic molecules. They are known to act as feeding deterrents, contributing bitter or astringent properties to plants such as lemons and tea. 

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