The phenylpropanoids are a diverse family of organic compounds that are synthesized by plants from the
amino acid
Amino acids are organic compounds that contain both amino and carboxylic acid functional groups. Although hundreds of amino acids exist in nature, by far the most important are the alpha-amino acids, which comprise proteins. Only 22 alpha ...
s
phenylalanine
Phenylalanine (symbol Phe or F) is an essential α-amino acid with the formula . It can be viewed as a benzyl group substituted for the methyl group of alanine, or a phenyl group in place of a terminal hydrogen of alanine. This essential amin ...
and
tyrosine
-Tyrosine or tyrosine (symbol Tyr or Y) or 4-hydroxyphenylalanine is one of the 20 standard amino acids that are used by cells to synthesize proteins. It is a non-essential amino acid with a polar side group. The word "tyrosine" is from the G ...
.
Their name is derived from the six-carbon, aromatic phenyl group and the three-carbon propene tail of
coumaric acid Coumaric acid is a phenolic derivative of cinnamic acid having a hydroxy group
In chemistry, a hydroxy or hydroxyl group is a functional group with the chemical formula and composed of one oxygen atom covalently bonded to one hydrogen atom. I ...
, which is the central intermediate in
phenylpropanoid biosynthesis. From
4-coumaroyl-CoA
Coumaroyl-coenzyme A is the thioester of coenzyme-A and coumaric acid. Coumaroyl-coenzyme A is a central intermediate in the biosynthesis of myriad natural products found in plants. These products include lignols (precursors to lignin and ligno ...
emanates the biosynthesis of myriad natural products including
lignols (precursors to
lignin
Lignin is a class of complex organic polymers that form key structural materials in the support tissues of most plants. Lignins are particularly important in the formation of cell walls, especially in wood and bark, because they lend rigidity a ...
and
lignocellulose),
flavonoids
Flavonoids (or bioflavonoids; from the Latin word ''flavus'', meaning yellow, their color in nature) are a class of polyphenolic secondary metabolites found in plants, and thus commonly consumed in the diets of humans.
Chemically, flavonoids ...
,
isoflavonoids
Isoflavonoids are a class of flavonoid phenolic compounds, many of which are biologically active. Isoflavonoids and their derivatives are sometimes referred to as phytoestrogens, as many isoflavonoid compounds have biological effects via the estro ...
,
coumarins,
aurones
An aurone is a heterocyclic chemical compound which is a type of flavonoid. There are two isomers of the molecule, with (''E'')- and (''Z'')-configurations. The molecule contains a benzofuran element associated with a benzylidene linked in pos ...
,
stilbenes
Stilbenoids are hydroxylated derivatives of stilbene. They have a C6–C2–C6 structure. In biochemical terms, they belong to the family of phenylpropanoids and share most of their biosynthesis pathway with chalcones. Most stilbenoids are produce ...
,
catechin, and phenylpropanoids.
The coumaroyl component is produced from
cinnamic acid.
Phenylpropanoids are found throughout the plant kingdom, where they serve as essential components of a number of structural polymers, provide protection from
ultraviolet light
Ultraviolet (UV) is a form of electromagnetic radiation with wavelength from 10 nm (with a corresponding frequency around 30 PHz) to 400 nm (750 THz), shorter than that of visible light, but longer than X-rays. UV radiatio ...
, defend against
herbivores and
pathogen
In biology, a pathogen ( el, πάθος, "suffering", "passion" and , "producer of") in the oldest and broadest sense, is any organism or agent that can produce disease. A pathogen may also be referred to as an infectious agent, or simply a g ...
s, 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
Enzymes () are proteins that act as biological catalysts by accelerating chemical reactions. The molecules upon which enzymes may act are called substrates, and the enzyme converts the substrates into different molecules known as products ...
phenylalanine ammonia-lyase (PAL). Some plants, mainly
monocotyledonous, use
tyrosine
-Tyrosine or tyrosine (symbol Tyr or Y) or 4-hydroxyphenylalanine is one of the 20 standard amino acids that are used by cells to synthesize proteins. It is a non-essential amino acid with a polar side group. The word "tyrosine" is from the G ...
to synthesize
p-coumaric acid by the action of the bifunctional enzyme
Phenylalanine/tyrosine ammonia-lyase (PTAL). A series of enzymatic
hydroxylation
In chemistry, hydroxylation can refer to:
*(i) most commonly, hydroxylation describes a chemical process that introduces a hydroxyl group () into an organic compound.
*(ii) the ''degree of hydroxylation'' refers to the number of OH groups in a ...
s and
methylation
In the chemical sciences, methylation denotes the addition of a methyl group on a substrate, or the substitution of an atom (or group) by a methyl group. Methylation is a form of alkylation, with a methyl group replacing a hydrogen atom. These ...
s leads to
coumaric acid Coumaric acid is a phenolic derivative of cinnamic acid having a hydroxy group
In chemistry, a hydroxy or hydroxyl group is a functional group with the chemical formula and composed of one oxygen atom covalently bonded to one hydrogen atom. I ...
,
caffeic acid,
ferulic acid,
5-hydroxyferulic acid, and
sinapic acid. Conversion of these acids to their corresponding
ester
In chemistry, an ester is a compound derived from an oxoacid (organic or inorganic) in which at least one hydroxyl group () is replaced by an alkoxy group (), as in the substitution reaction of a carboxylic acid and an alcohol. Glycerides ...
s produces some of the volatile components of herb and flower
fragrance
An aroma compound, also known as an odorant, aroma, fragrance or flavoring, is a chemical compound that has a smell or odor. For an individual chemical or class of chemical compounds to impart a smell or fragrance, it must be sufficiently v ...
s, which serve many functions such as attracting
pollinator
A pollinator is an animal that moves pollen from the male anther of a flower to the female stigma of a flower. This helps to bring about fertilization of the ovules in the flower by the male gametes from the pollen grains.
Insects are the m ...
s.
Ethyl cinnamate is a common example.
Cinnamic aldehydes and monolignols
Reduction of the
carboxylic acid
In organic chemistry, a carboxylic acid is an organic acid that contains a carboxyl group () attached to an R-group. The general formula of a carboxylic acid is or , with R referring to the alkyl, alkenyl, aryl, or other group. Carboxyli ...
functional groups in the cinnamic acids provides the corresponding aldehydes, such as
cinnamaldehyde
Cinnamaldehyde is an organic compound with the formula(C9H8O) C6H5CH=CHCHO. Occurring naturally as predominantly the ''trans'' (''E'') isomer, it gives cinnamon its flavor and odor. It is a phenylpropanoid that is naturally synthesized by the shi ...
. Further reduction provides
monolignol
Monolignols, also called lignols, are the source materials for biosynthesis of both lignans and lignin and consist mainly of paracoumaryl alcohol (H), coniferyl alcohol (G) and sinapyl alcohol (S). These monolignols differ in their degree of meth ...
s including
coumaryl alcohol
Paracoumaryl alcohol is a phytochemical, one of the monolignols. It is a white solid. ''p''-Coumaryl alcohol is a major precursor to lignin or lignans.
Biosynthesis and occurrence
It is synthesized via the phenylpropanoid biochemical pathway. ...
,
coniferyl alcohol
Coniferyl alcohol is an organic compound with the formula HO(CH3O)C6H3CH=CHCH2OH. A colourless or white solid, it is one of the monolignols, produced via the phenylpropanoid biochemical pathway. When copolymerized with related aromatic compounds, ...
, and
sinapyl alcohol, which vary only in their degree of
methoxylation. The monolignols are monomers that are
polymer
A polymer (; Greek '' poly-'', "many" + '' -mer'', "part")
is a substance or material consisting of very large molecules called macromolecules, composed of many repeating subunits. Due to their broad spectrum of properties, both synthetic a ...
ized to generate various forms of
lignin
Lignin is a class of complex organic polymers that form key structural materials in the support tissues of most plants. Lignins are particularly important in the formation of cell walls, especially in wood and bark, because they lend rigidity a ...
and
suberin, which are used as a structural component of plant cell walls.
The
phenylpropene
Phenylpropene is the organic compound with the formula C6H5CH2CH=CH2. It is a colorless liquid. The compound consists of a phenyl group attached to allyl. Phenylpropene isomerizes to trans-propenylbenzene.
In plant biochemistry, the phenylpro ...
s, including
eugenol
Eugenol is an allyl chain-substituted guaiacol, a member of the allylbenzene class of chemical compounds. It is a colorless to pale yellow, aromatic oily liquid extracted from certain essential oils especially from clove, nutmeg, cinnamon, bas ...
,
chavicol,
safrole and
estragole, are also derived from the monolignols. These compounds are the primary constituents of various
essential oil
An essential oil is a concentrated hydrophobic liquid containing volatile (easily evaporated at normal temperatures) chemical compounds from plants. Essential oils are also known as volatile oils, ethereal oils, aetheroleum, or simply as the o ...
s.
Coumarins and flavonoids
Hydroxylation of
cinnamic acid in the 4-position by
trans-cinnamate 4-monooxygenase
In enzymology, a trans-cinnamate 4-monooxygenase () is an enzyme that catalyzes the chemical reaction
:trans-cinnamate + NADPH + H+ + O2 \rightleftharpoons 4-hydroxycinnamate + NADP+ + H2O
The 4 substrates of this enzyme are trans-cinnamate, N ...
leads to
''p''-coumaric acid, which can be further modified into hydroxylated derivatives such as
umbelliferone
Umbelliferone, also known as 7-hydroxycoumarin, hydrangine, skimmetine, and ''beta''-umbelliferone, is a natural product of the coumarin family.
It absorbs ultraviolet light strongly at several wavelengths. There are some indications that this c ...
. Another use of ''p''-coumaric acid via its
thioester with
coenzyme A, i.e.
4-coumaroyl-CoA
Coumaroyl-coenzyme A is the thioester of coenzyme-A and coumaric acid. Coumaroyl-coenzyme A is a central intermediate in the biosynthesis of myriad natural products found in plants. These products include lignols (precursors to lignin and ligno ...
, is the production of
chalcones. This is achieved with the addition of 3
malonyl-CoA molecules and their cyclization into a second
phenyl group.
Chalcones are the precursors of all
flavonoids, a diverse class of
phytochemicals.
Stilbenoids
Stilbenoids, such as
resveratrol
Resveratrol (3,5,4′-trihydroxy-''trans''-stilbene) is a stilbenoid, a type of natural phenol, and a phytoalexin produced by several plants in response to injury or when the plant is under attack by pathogens, such as bacteria or fungi. Sourc ...
, are hydroxylated derivatives of
stilbene. They are formed through an alternative cyclization of
cinnamoyl-CoA
Cinnamoyl-Coenzyme A is an intermediate in the phenylpropanoids metabolic pathway.
Enzymes using Cinnamoyl-Coenzyme A
* Cinnamoyl-CoA reductase, an enzyme that catalyzes the chemical reaction cinnamaldehyde + CoA + NADP+ → cinnamoyl-CoA + NAD ...
or
4-coumaroyl-CoA
Coumaroyl-coenzyme A is the thioester of coenzyme-A and coumaric acid. Coumaroyl-coenzyme A is a central intermediate in the biosynthesis of myriad natural products found in plants. These products include lignols (precursors to lignin and ligno ...
.
Sporopollenin
Phenylpropanoids and other
phenolics 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 biopolymers, containing mainly hydroxylated ]fatty acid
In chemistry, particularly in biochemistry, a fatty acid is a carboxylic acid with an aliphatic chain, which is either saturated or unsaturated. Most naturally occurring fatty acids have an unbranched chain of an even number of carbon atoms, f ...
s, phenylpropanoids, phenolics and traces of carotenoids. Tracer experiments have shown that phenylalanine
Phenylalanine (symbol Phe or F) is an essential α-amino acid with the formula . It can be viewed as a benzyl group substituted for the methyl group of alanine, or a phenyl group in place of a terminal hydrogen of alanine. This essential amin ...
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
*Apiole
Apiole is a phenylpropene, also known as apiol, parsley apiol, or parsley camphor. Its chemical name is 1-allyl-2,5-dimethoxy-3,4-methylenedioxybenzene. It is found in the essential oils of celery leaf and all parts of parsley. Heinrich Christo ...
*Asarone
Asarone is chemical compound of the phenylpropanoid class found in certain plants such as ''Acorus'' and ''Asarum''. There are two isomers, α (or ''trans'') and β (or ''cis''). As a volatile fragrance oil, it is used in killing pests and bacte ...
* Chavicol
*Dillapiole
Dillapiole is an organic chemical compound and essential oil commonly extracted from dill weed, though it can be found in a variety of other plants such as fennel root. This compound is closely related to apiole, having a methoxy group positioned ...
*Elemicin
Elemicin is a phenylpropene, a natural organic compound, and is a constituent of several plant species' essential oils.
Natural occurrence
Elemicin is a constituent of the oleoresin and the essential oil of ''Canarium luzonicum'' (also referred ...
* Estragole
*Eugenol
Eugenol is an allyl chain-substituted guaiacol, a member of the allylbenzene class of chemical compounds. It is a colorless to pale yellow, aromatic oily liquid extracted from certain essential oils especially from clove, nutmeg, cinnamon, bas ...
* Methyl eugenol
*Myristicin
Myristicin is a naturally occurring compound found in common herbs and spices, the most well known being nutmeg. It is an insecticide, and has been shown to enhance the effectiveness of other insecticides in combination. Myristicin is also a prec ...
* Eleutherosides
References
*
{{Phenylpropanoids
*