Apigenin (4′,5,7-trihydroxyflavone), found in many plants, is a

natural product A natural product is a natural compound or substance produced by a living organism—that is, found in nature. In the broadest sense, natural products include any substance produced by life. Natural products can also be prepared by chemical syn ...

belonging to the

flavone Flavone is an organic compound with the formula . A white solid, flavone is a derivative of chromone with a phenyl (Ph) substituent adjacent to the ether group. The compound is of little direct practical importance, but susbstituted derivatives, t ...

class that is the

aglycone An aglycone (aglycon or genin) is the compound remaining after the glycosyl group on a glycoside is replaced by a hydrogen atom. For example, the aglycone of a cardiac glycoside would be a steroid molecule. Detection A way to identify aglycone ...

of several naturally occurring

glycosides In chemistry, a glycoside is a molecule in which a sugar is bound to another functional group via a glycosidic bond. Glycosides play numerous important roles in living organisms. Many plants store chemicals in the form of inactive glycosides. ...

. It is a yellow crystalline solid that has been used to dye wool.

Sources in nature

Apigenin is found in many fruits and vegetables, but

parsley Parsley, or garden parsley (''Petroselinum crispum'') is a species of flowering plant in the family Apiaceae that is native to the central and eastern Mediterranean region (Sardinia, Lebanon, Israel, Cyprus, Turkey, southern Italy, Greece, Por ...

celery Celery (''Apium graveolens'') is a marshland plant in the family Apiaceae that has been cultivated as a vegetable since antiquity. Celery has a long fibrous stalk tapering into leaves. Depending on location and cultivar, either its stalks, lea ...

celeriac Celeriac (''Apium graveolens'' var. ''rapaceum''), also called celery root, knob celery, and turnip-rooted celery (although it is not a close relative of the turnip), is a variety of celery cultivated for its edible stem or hypocotyl, and shoo ...

, and

chamomile Chamomile (American English) or camomile (British English; see spelling differences) ( or ) is the common name for several plants of the family Asteraceae. Two of the species, ''Matricaria recutita'' and ''Anthemis nobilis'', are commonly us ...

tea are the most common sources. Apigenin is particularly abundant in the flowers of chamomile plants, constituting 68% of total

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. Dried parsley can contain about 45 mg apigenin/gram of the herb, and dried chamomile flower about 3-5 mg/gram. The apigenin content of fresh parsley is reportedly 215.5 mg/100 grams, which is much higher than the next highest food source, green celery hearts providing 19.1 mg/100 grams.

Biosynthesis

Apigenin is biosynthetically derived from the general

phenylpropanoid pathway The phenylpropanoids are a diverse family of organic compounds that are synthesized by plants from the amino acids phenylalanine and tyrosine. Their name is derived from the six-carbon, aromatic phenyl group and the three-carbon propene tail of ...

and the flavone synthesis pathway. The phenylpropanoid pathway starts from the aromatic amino acids L-phenylalanine or L-tyrosine, both products of the

Shikimate pathway The shikimate pathway (shikimic acid pathway) is a seven-step metabolic pathway used by bacteria, archaea, fungi, algae, some protozoans, and plants for the biosynthesis of folates and aromatic amino acids (tryptophan, phenylalanine, and tyrosine). ...

. When starting from L-phenylalanine, first the amino acid is non-oxidatively deaminated by

phenylalanine ammonia lyase The enzyme phenylalanine ammonia lyase (EC 4.3.1.24) catalyzes the conversion of L-phenylalanine to ammonia and ''trans''-cinnamic acid.: :L-phenylalanine = ''trans''-cinnamate + NH3 Phenylalanine ammonia lyase (PAL) is the first and committed ...

(PAL) to make cinnamate, followed by oxidation at the ''para'' position by

cinnamate 4-hydroxylase 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, ...

(C4H) to produce ''p''-coumarate. As L-tyrosine is already oxidized at the ''para'' position, it skips this oxidation and is simply deaminated by tyrosine ammonia lyase (TAL) to arrive at ''p''-coumarate. To complete the general phenylpropanoid pathway, 4-coumarate CoA ligase (4CL) substitutes coenzyme A (CoA) at the carboxy group of ''p''-coumarate. Entering the flavone synthesis pathway, the type III

polyketide synthase Polyketides are a class of natural products derived from a precursor molecule consisting of a chain of alternating ketone (or reduced forms of a ketone) and methylene groups: (-CO-CH2-). First studied in the early 20th century, discovery, biosynth ...

enzyme

chalcone synthase Chalcone synthase or naringenin-chalcone synthase (CHS) is an enzyme ubiquitous to higher plants and belongs to a family of polyketide synthase enzymes (PKS) known as type III PKS. Type III PKSs are associated with the production of chalcones, a ...

(CHS) uses consecutive condensations of three equivalents of

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 ...

followed by aromatization to convert ''p''-coumaroyl-CoA to chalcone.

Chalcone isomerase In enzymology, a chalcone isomerase () is an enzyme that catalyzes the chemical reaction :a chalcone \rightleftharpoons a flavanone Hence, this enzyme has one substrate, a chalcone, and one product, a flavanone. This enzyme belongs to the fam ...

(CHI) then isomerizes the product to close the pyrone ring to make naringenin. Finally, a flavanone synthase (FNS) enzyme oxidizes naringenin to apigenin. Two types of FNS have previously been described; FNS I, a soluble enzyme that uses 2-oxogluturate, Fe²⁺, and ascorbate as cofactors and FNS II, a membrane bound, NADPH dependent cytochrome p450 monooxygenase.

Glycosides

The naturally occurring

glycoside In chemistry, a glycoside is a molecule in which a sugar is bound to another functional group via a glycosidic bond. Glycosides play numerous important roles in living organisms. Many plants store chemicals in the form of inactive glycosides. ...

s formed by the combination of apigenin with sugars include: *

Apiin Apiin is a natural flavonoid, a diglycoside of the flavone apigenin found in the winter-hardy plants parsley and celery, and in banana leaf. The glycoside moiety at carbon-7 of apigenin, ''O''-β-D-apiofuranosyl(→)2-β-D-glucosyl, is carried by ...

(apigenin 7-''O''-apioglucoside), isolated from

and celery *

Apigetrin Apigetrin is a chemical compound that can be found in dandelion coffee and in ''Teucrium gnaphalodes ''Teucrium gnaphalodes'' is a plant species in the genus ''Teucrium''. It is endemic to the Iberian Peninsula and grows at altitudes between 20 ...

(apigenin 7-glucoside), found in

dandelion coffee Dandelion 'coffee' (also dandelion tea) is a tisane made from the root of the dandelion plant. The roasted dandelion root pieces and the beverage have some resemblance to coffee in appearance and taste, and it is thus commonly considered a coffee ...

Vitexin Vitexin is an apigenin flavone glucoside, a chemical compound found in the passion flower, ''Vitex agnus-castus'' (chaste tree or chasteberry), in the ''Phyllostachys nigra'' bamboo leaves, in the pearl millet (Pennisetum millet), and in Hawthorn. ...

(apigenin 8-''C''-glucoside) *

Isovitexin Isovitexin (or homovitexin, saponaretin) is a flavone. the apigenin-6-''C''-glucoside. In this case, the prefix 'iso' does not imply an isoflavonoid (the position of the B-ring on the C-ring), but the position of the glucoside on the flavone. Na ...

(apigenin 6-''C''-glucoside) *

Rhoifolin Rhoifolin is a chemical compound. It is first isolated from plant ''Rhus succedanea''. The term "Rhoi" derived from generic name of plant Rhus. It is a flavone, a type of flavonoid isolated from '' Boehmeria nivea'', China grass or ramie (leaf), ...

(apigenin 7-''O''-

neohesperidoside Neohesperidose is the disaccharide which is present in some flavonoids. It can be found in species of ''Typha.'' ''Delphinidin-3-neohesperidoside and cyanidin-3- neohesperidoside from receptacles of Podocarpus species, Oyvind M. Andersen, Phytoche ...

) * Schaftoside (apigenin 6-''C''-glucoside 8-''C''-arabinoside)

References

{{Progestogenics Aromatase inhibitors Delta-opioid receptor antagonists Flavones GABAA receptor positive allosteric modulators Resorcinols Kappa-opioid receptor antagonists Mu-opioid receptor antagonists NMDA receptor antagonists Phytoestrogens Progestogens Vinylogous carboxylic acids

Sources in nature

Biosynthesis

Glycosides

See also

References