Phenylpropanoid
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.
Cinnamic acids
Phenylalanine is first converted to cinnamic acid by 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 cinnamate is a common example.
Cinnamic aldehydes and monolignols
Reduction of the carboxylic acid functional 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 lignin and suberin, which are used as a structural component of plant cell walls.
The phenylpropenes, including eugenol, chavicol, safrole and estragole, are also derived from the monolignols. The compounds are the primary constituents of various essential oils.
Coumarins and flavonoids
Hydroxylation of cinnamic acid in 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 thioester with coenzyme A, i.e. 4-coumaroyl-CoA, is the production of chalcone. This is achieved with the addition of 3 malonyl-CoA molecules and their cyclization into a second phenyl group (see polyphenols). Chalcone is the precursor of all flavonoids, a diverse class of phytochemicals.
Stilbenoids
Stilbenoids, such as resveratrol, are hydroxylated derivatives of stilbene. They are formed through an alternative cyclization of cinammoyl-CoA or 4-coumaroyl-CoA.
References
- K Hahlbrock, D Scheel (1989). "Physiology and Molecular Biology of Phenylpropanoid Metabolism". Annual Review of Plant Physiology and Plant Molecular Biology. 40: 347–69. doi:10.1146/annurev.pp.40.060189.002023.