Read this article to learn about Flavonoid. After reading this article you will learn about: 1. Classification of Flavonoid 2. Biosynthesis of Flavonoid.

Classification of Flavonoid:

“Flavonoid is a colorless crystalline compound, C15H10O2, the parent substance of a number of important yellow pigments, occurring in the leaves or in the stems and seed capsules of many prim­roses.” The term flavonoid refers to a class of plant secondary metabolites.

According to the IUPAC nomenclature, they can be classified into:

i. Flavonoids, derived from 2-phenylchromen- 4-one (2-phenyl-l ,4-benzopyrone) structure.

ii. Iso-flavonoids, derived from 3-phenylchromen- 4-one (3-phenyl-1,4- benzopyrone) structure.

iii. Neo-flavonoids, derived from 4-phenylcoumarine (4-phenyl-1,2- benzopyrone) structure.

Flavonoids can be regarded as C6-C3-C6 com­pounds where C6 moiety is benzene ring and it is classified according to the state of oxygenation of the C3 unit i.e. C-2, 3, 4 (Evans, 2005).

Various Classes of Flavonoids along with their ExamplesVarious Classes of Flavonoids along with their Examples Flavonoids are most commonly known for their antioxidant activity. However, it is now known that the health benefits they provide against cancer and heart disease are the result of other mechanisms.

Flavonoids are also commonly re­ferred to as bioflavonoids in the media-the terms are equivalent and interchangeable, for flavonoids are biological in origin. The flavonoids are poly-phenolic compounds possessing 15 carbon atoms; two benzene rings joined by a linear three- carbon chain.

Skeleton

Flavonoids constitute one of the most charac­teristic classes of compounds in higher plants. Many flavonoids are easily recognized as flower pigments in most angiosperm families (flowering plants). However, their occurrence is not restrict­ed to flowers but include all parts of the plant. The chemical structure of flavonoids are based on a C15 skeleton with a chromane ring bearing a second aromatic ring B in position 2, 3 or 4.

C15 Skeleton of FlavonoidsIn few cases, the six-membered heterocyclic ring C occurs in an isomeric open form or is re­placed by a five-membered ring. Biogenetically, the A ring usually arises from a molecule of resorcinol or phloroglucinol synthesized from the ace­tate pathway and has a characteristic hydroxylation pattern at the 5 and 7 position.

The B ring comes from the Shikimate pathway and is usually 4, 3, 4′ or 3, 4, 5-hydroxylated. Based on their chem­ical structure, flavonoids are divided into several subclasses, of which flavonols are the most abun­dant in the plant kingdom.

Biosynthesis of Flavonoid:

Flavonoids are synthesized by the phenylpropanoid metabolic pathway in which the amino acid phenylalanine is used to produce 4-coumaroyl- CoA. This can be combined with malonyl-CoA to yield the true backbone of flavonoids, a group of compounds called chalcones, which contain two phenyl rings.

Conjugate ring-closure of chalcones results in the familiar form of flavonoids, the three- ring structure of a flavone. The metabolic path­way continues through a series of enzymatic mod­ifications to yield flavanones, di-hydro-flavonols, anthocyanins. Along this pathway, many products can be formed, including the flavonols, flavanols, pro-anthocyanidins (tannins) and a host of other polyphenolics.