https://www.ncbi.nlm.nih.gov/pmc/articles/PMC4931407
Untitled Document
Flavonoids represent a large subgroup of the phenolic class of plant specialized metabolites. They are widely distributed throughout the plant kingdom. The basic flavan skeleton that forms all flavonoids is a 15-carbon phenylpropanoid core (C6-C3-C6 system), which is arranged into two aromatic rings (A and B) linked by a heterocyclic pyran ring (C). According to the oxidation status and saturation of the heterocyclic ring, flavonoids are divided into several groups, which include flavones, flavanones, isoflavones, flavonols, 3-deoxy flavonoids, and anthocyanins. Flavones comprise one of the largest groups, which are characterized by the presence of a double bond between C-2 and C-3, and the attachment of the B ring to C-2. As is the case with other flavonoids, flavones have a diversity of functions that have contributed in making plants adapt to a terrestrial environment including: (i) protection against UV radiation and oxidative stress; (ii) interspecies interactions (pathogen resistance, symbiosis, protection from herbivory, and allelopathy); and (iii) plant development (copigmentation with anthocyanins and lignification). In addition to their physiological, biochemical, and ecological functions to plants, flavones also exert biological activities on animals, providing important nutritional value. Key to fulfilling these multiple roles, flavones are characterized by a high degree of chemical diversity provided by modifications of the chemical backbone, which include hydroxylation, O-/C-glycosylation, O-methylation, and acylation. This review focuses on the multiple and specialized metabolic pathways responsible for flavone biosynthesis, discusses their roles in plant biology and briefly summarizes the recent progress towards establishing the mechanisms by which flavones provide health benefits.
Structures of some important flavonoids discussed in this review: (A) flavones, including aglycones, C-/O-glycosyl flavones, and O-methylated flavones; and (B) selected flavonoids mentioned in the text belonging to the flavanone, isoflavone, and flavonol groups.
