- Phenylpropanoid
Phenylpropanoids are a class of plant-derived
organic compound s that are biosynthesized from theamino acid phenylalanine . They have a wide variety of functions, including defense againstherbivore s, microbial attack, or other sources of injury; as structural components ofcell wall s; as protection fromultraviolet light ; aspigment s; and assignaling molecule s.Cinnamic acids
Phenylalanine is first converted to
cinnamic acid by the action of theenzyme phenylalanine ammonia-lyase (PAL). A series of enzymatichydroxylation s andmethylation s leads tocoumaric acid ,caffeic acid ,ferulic acid ,5-hydroxyferulic acid , andsinapic acid . Conversion of these acids to their correspondingester s produces some of the volatile components of herb and flowerfragrance s which serve many functions such as attractingpollinator s.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 ascinnamaldehyde . Further reduction providesmonolignol s includingcoumaryl alcohol ,coniferyl alcohol , andsinapyl alcohol . The monolignols are monomers that arepolymer ized to generate various forms oflignin andsuberin , which are used as a structural component of plant cell walls.The phenylpropenes, including
eugenol ,chavicol ,safrole andestragole , are also derived from the monolignols. The compounds are the primary constituents of variousessential oil s.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 asumbelliferone . Another use of "p"-coumaric acid via itsthioester withcoenzyme A , i.e. 4-coumaroyl-CoA, is the production ofchalcone . This is achieved with the addition of 3malonyl-CoA molecules and their cyclization into a secondphenyl group "(seepolyphenols )". Chalcone is the precursor of allflavonoid s, a diverse class ofphytochemical s.tilbenoids
Stilbenoid s, such asresveratrol , are hydroxylated derivatives ofstilbene . They are formed through an alternative cyclization of cinammoyl-CoA or 4-coumaroyl-CoA.References
*cite journal
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
issue =
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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