Phenylpropanoids are a class of plant-derived
organic compounds that are biosynthesized from the amino acid phenylalanine. They have a wide variety of functions, including defense against herbivores, microbial attack, or other sources of injury; as structural components of cell walls; as protection from ultraviolet light; as pigments; and as signaling molecules.
Phenylalanine is first converted to
cinnamic acidby the action of the enzyme phenylalanine ammonia-lyase(PAL). A series of enzymatic hydroxylations and methylations leads to coumaric acid, caffeic acid, ferulic acid, 5-hydroxyferulic acid, and sinapic acid. Conversion of these acids to their corresponding esters produces some of the volatile components of herb and flower fragrances which serve many functions such as attracting pollinators. Ethyl cinnamateis a common example.
Cinnamic aldehydes and monolignols
Reduction of the
carboxylic acidfunctional groups in the cinnamic acids provides the corresponding aldehydes, such as cinnamaldehyde. Further reduction provides monolignols including coumaryl alcohol, coniferyl alcohol, and sinapyl alcohol. The monolignols are monomers that are polymerized to generate various forms of ligninand suberin, which are used as a structural component of plant cell walls.
The phenylpropenes, including
eugenol, chavicol, safroleand estragole, are also derived from the monolignols. The compounds are the primary constituents of various essential oils.
Coumarins and flavonoids
cinnamic acidin the 4-position leads to "p"-coumaric acid, which can be further modified into hydroxylated derivatives such as umbelliferone. Another use of "p"-coumaric acid via its thioesterwith coenzyme A, i.e. 4-coumaroyl-CoA, is the production of chalcone. This is achieved with the addition of 3 malonyl-CoAmolecules and their cyclization into a second phenylgroup "(see polyphenols)". Chalcone is the precursor of all flavonoids, a diverse class of phytochemicals.
Stilbenoids, such as resveratrol, are hydroxylated derivatives of stilbene. They are formed through an alternative cyclization of cinammoyl-CoA or 4-coumaroyl-CoA.
author = K Hahlbrock, D Scheel
title = Physiology and Molecular Biology of Phenylpropanoid Metabolism
journal = Annual Review of Plant Physiology and Plant Molecular Biology
year = 1989
volume = 40
pages = 347–69
url = http://arjournals.annualreviews.org/doi/pdf/10.1146/annurev.pp.40.060189.002023
doi = 10.1146/annurev.pp.40.060189.002023
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