Several metabolic pathways are involved in the synthesis of the mixture of components constituting phenolic resin. The shikimic acid pathway is the source of aromatic amino acids such as phenylalanine (Figures 1-1 and 1-7). A key step toward the formation of many components of phenolic resin is enzymatic conversion of phenylalanine to cinnamic acid, a reaction catalyzed by the important regulatory enzyme phenylalanine ammonia-lyase (PAL). Cin-namic acid is a simple C9 phenolic compound called a phenylpropane because it contains a six-carbon (phenyl) benzene ring and a three-carbon (propyl) side chain (C6-C3). Phenylpropanoids derived from cinnamic acid are building blocks for many other phenolic compounds produced by the phenylpropanoid biosynthetic pathway. For example, benzoic acid derivatives, with a skeleton of a six-carbon benzene ring with a one-carbon substituent (C6-C1), are formed from phenylpropanoids by cleavage of a two-carbon fragment from the side chain. Cinnamic acid, benzoic acid, and its derivative, benzaldehyde (Figure 1-8), occur in the internally produced phenolic resins of such different plants as Myroxylon, leguminous trees that yield Peru balsam (Figure 8-2), and Xanthorrhoea, the Australian grass tree (Plate 12 and Chapter 9). Eugenol, found in numerous angiosperm resins, is synthesized in some cases from a phenylpropane derivative. Lignans (e.g., nordihydroguaiaretic acid, NDGA, Figure 1-8) are relatively common dimeric phenylpropanes that occur in surface leaf resins in desert shrubs such as Larrea, creosote bush (Plate 14).
Flavonoid components of phenolic resin are based on a structure of two benzene rings connected by a C3 bridge (C6-C3-C6), which is synthesized from components of two distinct pathways. One benzene ring and the C3 bridge arise from the shikimic acid-phenylpropanoid pathway whereas the other benzene ring is formed from acetate units via the malonic acid pathway (Figures 1-1 and 1-7). Often, the C3 chain cyclizes with an adjacent hydroxyl
Figure 1-7. Generalized biosynthetic outline of phenylpropanoids and lipophilic flavonoids, two major groups of compounds characterizing phenolic resins.
group to form an oxygen-heterocyclic ring, and various classes of flavonoids are distinguished by the oxidation state of the C3 chain or heterocyclic ring.
Many flavonoids occur as water-soluble glycosides. Those lacking a sugar substituent, called aglycones, often are lipophilic components of phenolic resin; these components frequently are members of flavonoid structural subclasses called flavones, flavanones, or flavonols. Flavonoid aglycones bearing fewer than five hydroxyl groups are hardly soluble in water. O-Methylation or methylenedioxy ring formation are also ways to mask reactivity of phenolic groups, at the same time increasing lipid solubility and volatility (Har-borne 1980). Significant chemical features of the phenolics constituting resins are a reduced number of hydroxyl substituents (Figure 1-8) and a variable number of phenolic groups that are O-methyl substituted (methoxylated). Such lipophilic flavonoids commonly co-occur with terpenoids, particularly sesqui- and triterpenes (Wollenweber and Dietz 1981, Wollenweber and Jay 1988). Lipophilic flavonoids are less well studied than water-soluble ones and are best characterized in bud exudates from northern temperate zone angiosperm trees such as birches (Betula, Plate 33) and poplars (Populus) and in leaf resins of arid-zone shrubs such as monkey flower (Mimulus, Plate 22) and yerba santa (Eriodictyon, Figure 2-15). Diplacone and diplacol, lipophilic flavonoids from Mimulus, are shown in Figure 1-8.
Another way of introducing lipid solubility into phenolic compounds is to attach a hydrophobic side chain. The allergenic phenolic (catechol) compounds found in resins of Anacardiaceae are of this type (Figure 1-8; Chapters 9 and 10). Alternatively, one or more terpene (prenyl) residues may be attached to phenolic compounds to form prenylated phenolics (e.g., tetrahydrocannabinol, Figure 1-8). Most frequently, the terpenoid substituent is attached directly to the benzene ring, but it also can be attached to a phenolic group. Paseshnichenko (1995) summarized numerous prenylated pheno-lics, pointing out that they occur in nearly every phenolic structural class. He suggested that terpenoid and phenolic metabolism are linked through a control mechanism that regulates the distribution of precursors such as acetate, required for biosynthesis of both terpenoids and phenolics. Phenylpropanoid and flavonoid biosynthetic enzymes appear to form assemblies or complexes that cluster at the ER (Hrazdina and Wagner 1985, Winkel-Shirley 1999). Because the ER also appears to be the site of synthesis of most sesqui- and triterpenes, the opportunity exists for close localization of the synthesis of flavonoids and terpenoids and for their subsequent transport to a common storage site. These findings provide a plausible biochemical explanation for Wollenweber and Dietz's (1981) and Wollenweber and Jay's (1988) observation that lipophilic flavonoids frequently co-occur with sesqui- or triterpenes, as well as explaining the existence of so many prenylated phenolics. Much remains to be learned about the biosynthesis and co-occurrence of phenolics with terpenoids, and doubtless many mixtures of terpenoids and phenolics in resins will be reported.
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