Flavan derivatives biosynthesis

Chalcones are not flavan derivatives since they lack the characteristic central heterocycle. They are converted spontaneously into true flavan derivatives, the flavanones, the reacjtion occurring particularly readily in acidic medium. They play a central role in the biosynthesis of the flavan derivatives. Chalcone glycosides oceur in relatively high quantities in the flowers of several of the Compositae and Leguminosae, which, as a result. 

Modifications of the Heterocycle (Fig, 770). In the next step the open chain structure is closed to form ring A. We obtain the kind of substance which we recognize, namely, a chalcone. Isotope experiments, especially those of Grisebach, have removed all doubt that such chalcones are the parent substances of all flavan derivatives. The chalcones are converted to flavones and all of the subsequent reaction sequences up to the anthocyanidins consist only of appropriate modifications of the central ring system. We should realize that the elucidation of the biosynthesis of the anthocyanins implies at the same time an understanding of the biosynthesis of all flavan derivatives. This is because the individual flavan derivatives are either precursors of the anthocyanins or they can be derived from such precursors. 

The biosynthesis of the proanthocyanidins is believed to proceed by addition of an electrophilic extension unit derived from a flavan-3,4-dioP or a flavan-3-oP to a nucleophilic starter unit, most likely a flavan-3-ol, with sequential addition of further chain-extension units. Although the genetics of interflavanyl bond formation in the proanthocyanidin polymerization process are not yet defmed, " the search for the elusive condensing enzyme continues unabated. ... 

The biosynthesis of anthocyanidins has been elucidated more recently than most other flavonoids. These positively charged compounds are derived from flavan-3,4-diols via dihydroflavonols. The hydroxyl group of the 3-position of anthocyanidins is introduced by flavanone 3-hydroxylase which converts flavanones to dihydroflavonols (see above). (25)-Flavanone 3-hydroxylase from flowers of Petunia hy-brida catalyzes the conversion of (25)-naringenin (10) [but not (2/ )-naringenin] to (2/ ,3i )-dihydrokaempferol (13). The hydrolase is a soluble enzyme that requires oxygen, 2-oxoglutarate, Fe, and ascorbate as cofactors (Britsch and Grisebach, 1986). (2S)-Eriodictyol (17) is converted to (2/ ,3/ )-dihydroquercetin. 

Havanols are a wide group of polyphenols that include flavan-3-ols (e.g., catechin and proanthocyanidins), flavan-4-ols, and flavan-3,4-diols. They arise from plant secondary metabolism through condensation of phenylalanine derived from the shikimate pathway with malonyl-CoA obtained from citrate that is produced by the tricarboxylic acid cycle, leading to the formation of the key precursor in the flavonoids biosynthesis the naringenin chalcone. The exact nature of the molecular species that undergo polymerization and the mechanism of assembly in proanthocyanidins are still unknown. From a structural point of view, flavanols... 

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