Abstract
Flavonoids are secondary metabolites derived from phenylalanine and acetate metabolism. They fulfil a variety of functions in plants and have health benefits for humans. During the synthesis of the tricyclic flavonoid natural products in plants, oxidative modifications to the central C ring are catalyzed by four of FeII and 2-oxoglutarate dependent (2-ODD) oxygenases, namely flavone synthase I (FNS I), flavonol synthase (FLS), anthocyanidin synthase (ANS) and flavanone 3β-hydroxylase (FHT). FNS I, FLS and ANS are involved in desaturation of C2–C3 of flavonoids and FHT in hydroxylation of C3. FNS I, which is restricted to the Apiaceae species and in rice, is predicted to have evolved from FHT by duplication. Due to their sequence similarity and substrate specificity, FLS and ANS, which interact with the α surface of the substrate, belong to a group of dioxygenases having a broad substrate specificity, while FNS I and FHT are more selective, and interact with the naringenin β surface. Here, we summarize recent findings regarding the function of the four 2-ODD oxygenases and the relationship between their catalytic activity, their polypeptide sequence and their tertiary structure.
Highlights
The flavonoids represent a large group of plant secondary metabolites
Activity has been demonstrated when the enzyme is presented with trans-dihydroquercetin, a natural substrate for flavonol synthase (FLS); this reaction generates quercetin, indicating an overlap between the substrate and product selectivities for the 2-oxoglutarate dependent (2-ODD) oxygenases involved in flavonoid synthesis [60,61]
The flavonoids share a structure comprised of two six carbon aromatic rings and a heterocyclic three carbon ring containing one oxygen atom, whereas the chalcones lack the heterocyclic ring
Summary
The flavonoids represent a large group of plant secondary metabolites. To date, some 10,000 different flavonoids have been characterized, compared with about 12,000 alkaloids and about 30,000 terpenoids [1,2,3]. Plant 2-ODD oxygenases constitute a class of non-heme, iron-containing cytosolic enzymes which generally (but not exclusively) require as coreductant 2-oxoglutarate. The enzymes have been implicated in a variety of plant metabolic pathways, including the synthesis of amino acids, hormones, signaling molecules and a variety of secondary metabolites [20]. The 6-hydroxylation of the flavonoid molecule catalyzed by a 2-ODD oxygenase has been reported in Chrysosplenium americanum [36] and a Cytochrome P450 from soybean [37] These four common plant 2-ODDs are divided two distinct subgroups, one containing FNS I and FHT and another containing FLS and ANS. FNS I and FHT appear to possess a narrower substrate selectivity than either ANS or FLS
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