Thephenylpropanoids are a diverse family of organic compounds that are biosynthesized by plants from theamino acidsphenylalanine andtyrosine in theshikimic acid pathway.[1] Their name is derived from the six-carbon, aromatic phenyl group and the three-carbon propene tail ofcoumaric acid, which is the central intermediate inphenylpropanoid biosynthesis. From4-coumaroyl-CoA emanates the biosynthesis of myriad natural products includinglignols (precursors tolignin andlignocellulose),flavonoids,isoflavonoids,coumarins,aurones,stilbenes,catechin, and phenylpropanoids.[2] The coumaroyl component is produced fromcinnamic acid.
Phenylpropanoids are found throughout the plant kingdom, where they serve as essential components of a number of structural polymers, provide protection fromultraviolet light, defend againstherbivores andpathogens, and also mediateplant-pollinator interactions as floral pigments and scent compounds.
Phenylalanine is first converted tocinnamic acid by the action of theenzymephenylalanine ammonia-lyase (PAL). Some plants, mainlymonocotyledonous, usetyrosine to synthesizep-coumaric acid by the action of the bifunctional enzymephenylalanine/tyrosine ammonia-lyase (PTAL). A series of enzymatichydroxylations andmethylations leads tocoumaric acid,caffeic acid,ferulic acid,5-hydroxyferulic acid, andsinapic acid. Conversion of these acids to their correspondingesters produces some of the volatile components of herb and flowerfragrances, which serve many functions such as attractingpollinators.Ethyl cinnamate is a common example.
Reduction of thecarboxylic acid functional groups in the cinnamic acids provides the corresponding aldehydes, such ascinnamaldehyde. Further reduction providesmonolignols includingcoumaryl alcohol,coniferyl alcohol, andsinapyl alcohol, which vary only in their degree ofmethoxylation. The monolignols are monomers that arepolymerized to generate various forms oflignin andsuberin, which are used as a structural component of plant cell walls.
The phenylpropenes, phenylpropanoids withallylbenzene (3-phenylpropene) as theparent compound, are also derived from the monolignols. Examples includeeugenol,chavicol,safrole, andestragole. These compounds are the primary constituents of variousessential oils.
Hydroxylation ofcinnamic acid in the 4-position bytrans-cinnamate 4-monooxygenase leads top-coumaric acid, which can be further modified into hydroxylated derivatives such asumbelliferone. Another use ofp-coumaric acid via itsthioester withcoenzyme A, i.e.4-coumaroyl-CoA, is the production ofchalcones. This is achieved with the addition of threemalonyl-CoA molecules and their cyclization into a secondphenyl group.Chalcones are the precursors of allflavonoids, a diverse class ofphytochemicals.
Stilbenoids, such asresveratrol, are hydroxylated derivatives ofstilbene. They are formed through an alternative cyclization ofcinnamoyl-CoA or4-coumaroyl-CoA.
Phenylpropanoids and otherphenolics are part of the chemical composition ofsporopollenin. It is related tocutin andsuberin.[2] This ill-defined substance found in pollen is unusually resistant to degradation. Analyses have revealed a mixture ofbiopolymers, containing mainly hydroxylatedfatty acids, phenylpropanoids, phenolics and traces ofcarotenoids. Tracer experiments have shown thatphenylalanine is a major precursor, but other carbon sources also contribute. It is likely that sporopollenin is derived from several precursors that are chemically cross-linked to form a rigid structure.
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