The phenylpropanoids are a diverse family of organic compounds that are biosynthesized by plants from the phenylalanine and tyrosine in the shikimic acid pathway.
Phenylpropanoids are found throughout the plant kingdom, where they serve as essential components of a number of structural polymers, provide protection from ultraviolet light, defend against herbivores and , and also mediate plant-pollinator interactions as floral pigments and scent compounds.
Hydroxycinnamic acids
Phenylalanine is first converted to
cinnamic acid by the action of the
enzyme phenylalanine ammonia-lyase (PAL). Some plants, mainly
monocotyledonous, use
tyrosine to synthesize
p-Coumaric acid by the action of the bifunctional enzyme phenylalanine/tyrosine ammonia-lyase (PTAL). A series of enzymatic
and
leads to
coumaric acid,
caffeic acid,
ferulic acid, 5-hydroxyferulic acid, and
sinapic acid. Conversion of these acids to their corresponding
produces some of the volatile components of herb and flower
, which serve many functions such as attracting
.
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
including
coumaryl alcohol, coniferyl alcohol, and
sinapyl alcohol, which vary only in their degree of
. The monolignols are monomers that are
to generate various forms of
lignin and
suberin, which are used as a structural component of plant cell walls.
The phenylpropenes, phenylpropanoids with allylbenzene (3-phenylpropene) as the parent compound, are also derived from the monolignols. Examples include eugenol, chavicol, safrole, and estragole. These compounds are the primary constituents of various .
Coumarins and flavonoids
Hydroxylation of
cinnamic acid in the 4-position by trans-cinnamate 4-monooxygenase leads to
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
Chalconoid. This is achieved with the addition of three
malonyl-CoA molecules and their cyclization into a second
phenyl group.
Chalcones are the precursors of all
, a diverse class of
.
Stilbenoids
, such as
resveratrol, are hydroxylated derivatives of
stilbene. They are formed through an alternative cyclization of
cinnamoyl-CoA or 4-coumaroyl-CoA.
Sporopollenin
Phenylpropanoids and other
natural phenol are part of the chemical composition of
sporopollenin. It is related to
cutin and
suberin.
[ This ill-defined substance found in pollen is unusually resistant to degradation. Analyses have revealed a mixture of , containing mainly hydroxylated , phenylpropanoids, phenolics and traces of . Tracer experiments have shown that phenylalanine 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.
]
See also
External links
