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 am ...
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 amino a ...
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 Gr ...
.
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. In ...
, 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 Monolignol, lignols (precursors to lignin ...
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 ...
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 posi ...
,
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 pro ...
,
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 radiation i ...
, 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 germ ...
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. A ...
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 Gr ...
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 t ...
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. In ...
,
caffeic acid,
ferulic acid,
5-hydroxyferulic acid, and
sinapic acid
Sinapinic acid, or sinapic acid (Sinapine - Origin: L. Sinapi, sinapis, mustard, Gr., cf. F. Sinapine.), is a small naturally occurring hydroxycinnamic acid. It is a member of the phenylpropanoid family. It is a commonly used matrix in MALDI mass ...
. 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 ar ...
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 vol ...
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 maj ...
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. Carboxylic ...
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 shik ...
. 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,
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 ...
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 phenylprop ...
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, basil ...
,
chavicol
Chavicol (''p''-allylphenol) is a natural phenylpropene, a type of organic compound. Its chemical structure consists of a benzene ring substituted with a hydroxy group and a propenyl group. It is a colorless liquid found together with terpenes i ...
,
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 ch ...
. Another use of ''p''-coumaric acid via its
thioester
In organic chemistry, thioesters are organosulfur compounds with the functional group . They are analogous to carboxylate esters () with the sulfur in the thioester playing the role of the linking oxygen in the carboxylate ester, as implied by t ...
with
coenzyme A
Coenzyme A (CoA, SHCoA, CoASH) is a coenzyme, notable for its role in the synthesis and oxidation of fatty acids, and the oxidation of pyruvate in the citric acid cycle. All genomes sequenced to date encode enzymes that use coenzyme A as a subs ...
, 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 Monolignol, lignols (precursors to lignin ...
, is the production of
chalcones
Chalconoids Greek: χαλκός ''khalkós'', "copper", due to its color), also known as ''chalcones'', are natural phenols related to chalcone. They form the central core for a variety of important biological compounds.
They show antibacterial, a ...
. This is achieved with the addition of 3
malonyl-CoA
Malonyl-CoA is a coenzyme A derivative of malonic acid.
Functions
It plays a key role in chain elongation in fatty acid biosynthesis and polyketide biosynthesis.
Fatty acid biosynthesis
Malonyl-CoA provides 2-carbon units to fatty acids and commi ...
molecules and their cyclization into a second
phenyl group.
Chalcones
Chalconoids Greek: χαλκός ''khalkós'', "copper", due to its color), also known as ''chalcones'', are natural phenols related to chalcone. They form the central core for a variety of important biological compounds.
They show antibacterial, a ...
are the precursors of all
flavonoid
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 ...
s, a diverse class of
phytochemical
Phytochemicals are chemical compounds produced by plants, generally to help them resist fungi, bacteria and plant virus infections, and also consumption by insects and other animals. The name comes . Some phytochemicals have been used as poisons ...
s.
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. Sources ...
, 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 + NADP ...
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 Monolignol, lignols (precursors to lignin ...
.
Sporopollenin
Phenylpropanoids and other
phenolic
Phenolic is an adjective and a substantive (noun) that may apply to :
* Phenol (or carbolic acid), a colorless crystalline solid and aromatic compound
* Phenols, a class of chemical compounds that include phenol
* Phenolic content in wine
* Phenol ...
s 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 ]biopolymer
Biopolymers are natural polymers produced by the cells of living organisms. Like other polymers, biopolymers consist of monomeric units that are covalently bonded in chains to form larger molecules. There are three main classes of biopolymers, cl ...
s, 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, fr ...
s, phenylpropanoids, phenolics and traces of carotenoid
Carotenoids (), also called tetraterpenoids, are yellow, orange, and red organic compound, organic pigments that are produced by plants and algae, as well as several bacteria, and Fungus, fungi. Carotenoids give the characteristic color to pumpki ...
s. 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 amino a ...
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 Christoph ...
*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
Chavicol (''p''-allylphenol) is a natural phenylpropene, a type of organic compound. Its chemical structure consists of a benzene ring substituted with a hydroxy group and a propenyl group. It is a colorless liquid found together with terpenes i ...
*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, basil ...
* 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 pre ...
* Eleutheroside
Eleutherosides are a diverse group of chemical compounds that were isolated from roots of the herb ''Eleutherococcus senticosus'' which is commercially offered mostly as extracts. Eleutheroside A is a saponin and sterol glycoside while other eleut ...
s
References
*
{{Phenylpropanoids
*