Abstract
Flavonoids and biochemically-related chalcones are important secondary metabolites, which are ubiquitously present in plants and therefore also in human food. They fulfill a broad range of physiological functions in planta and there are numerous reports about their physiological relevance for humans. Flavonoids have in common a basic C6-C3-C6 skeleton structure consisting of two aromatic rings (A and B) and a heterocyclic ring (C) containing one oxygen atom, whereas chalcones, as the intermediates in the formation of flavonoids, have not yet established the heterocyclic C-ring. Flavonoids are grouped into eight different classes, according to the oxidative status of the C-ring. The large number of divergent chalcones and flavonoid structures is from the extensive modification of the basic molecules. The hydroxylation pattern influences physiological properties such as light absorption and antioxidative activity, which is the base for many beneficial health effects of flavonoids. In some cases antiinfective properties are also effected.
Highlights
Flavonoids were the first secondary metabolites to be studied, with some flavonoid classes, starting with the visibly detectable anthocyanins, having been known for a long time [1]
Stereospecific cyclization of the chalcone molecule leads to the formation of flavanone, the first flavonoid, which is the immediate precursor of flavones, isoflavones, flavan 4-ols and dihydroflavonols
Hydroxylation in position 3 of flavanone, which is catalyzed by the 2-oxoglutarate-dependent dioxygenase FHT, provides dihydroflavonol, which is the direct intermediate for flavonol and flavan-3,4-diol formation
Summary
Flavonoids were the first secondary metabolites to be studied, with some flavonoid classes, starting with the visibly detectable anthocyanins, having been known for a long time [1]. The distribution in the plant kingdom and the presence of flavonoids in different plant tissues has been widely studied [2]. In the seventies and eighties of the past century, most steps in the biosynthetic pathway that lead to the main flavonoid classes were successfully elucidated [3]. In the nineties there was an explosion in the amount of literature focusing on different aspects of flavonoids, including genetics, horticulture, plant biochemistry, biotechnology, nutrition, pharmacognosy and phytomedicine. The aim of this article is to summarize the knowledge about the introduction of hydroxyl groups in the flavonoid pathway and to review possible structure-activity relationships clearly dependant on the hydroxylation pattern of flavonoids
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