This view shows enzymes only for those organisms listed below, in the list of taxa known to possess the pathway. If an enzyme name is shown in bold, there is experimental evidence for this enzymatic activity.
|Superclasses:||Biosynthesis → Secondary Metabolites Biosynthesis → Phenylpropanoid Derivatives Biosynthesis → Flavonoids Biosynthesis → Flavones Biosynthesis|
Expected Taxonomic Range: Magnoliophyta
Flavones such as apigenin and luteolin represent one of the most abundant and important subclasses of the 9000 flavonoids described so far [Martens99]. Flavones belong to the most widespread compounds in plants and have been found in all major land-plant lineages.
The biosynthesis of flavones has been studied intensively because of their contribution to unique physiological and developmental processes in plants. Luteolin has a defined role as a signalling molecule involved in the induction and expression of nodulation genes [Peters86]. Flavones in general have been demonstrated to be significantly involved in the evolvement of the preferred attractant for bee pollinators, i.e. blue flower colours which are restricted to the higher developed angiosperm plant families [Harborne00]. In addition, flavones possess antioxidative activity [Lukacin01] frequently employed in the prevention of cancer and coronary heart disease [Martens05].
About This Pathway
The oxidation of flavanones to flavones is catalyzed by flavone synthases introducing a double bound between the C atoms 2 and 3 of the C-ring. Interestingly, the biosynthesis of flavone is carried out by two completely different flavone synthases, i.e. flavone synthase I (EC 18.104.22.168) and flavone synthase II.
While flavone synthase II (FNS II), a membrane bound cytochrome P450 dependent monooxygenase is found in many plant families the soluble dioxygenase flavone synthase I (FNS I) has only been isolated from members of the Apiacea family to date [Britsch90]. FNS I belongs to the family of 2-oxoglutarate-dependent dioxygenases which are involved in many different metabolic processes of plants, bacteria, fungi and animals [Lukacin01]. Among the five 2-oxoglutarate-dependent enzymes involved in flavonoid biosynthesis (compare flavonol biosynthesis) FNS I is considered highly pathway specific because of its distinct substrate specificity [Martens03].
The flavone synthases I and II catalyze the 2,3-desaturation of the flavanone in one step, i.e. the formation of apigenin from naringnin and luteollin from eriodictyol, respectively (this pathway). Although it had been suggested that FSN I may catalyze a sequential C3 hydroxylation and dehydration a vicinal desaturation without the release of a free intermediate was experimentally verified [Martens03]. The formation of the dihydroxylated B-ring compound luteolin requires an additional hydroxylation step carried out by the cytochrome P450 dependent monooxygenase flavonoid 3'(β)-hydroxylase [Kitada01] [Ueyama02]. The two possible routes leading to luteolin (via eridictyol or apigenin) are displayed in this pathway.
Unification Links: AraCyc:PWY-5060
Kitada01: Kitada C, Gong Z, Tanaka Y, Yamazaki M, Saito K (2001). "Differential expression of two cytochrome P450s involved in the biosynthesis of flavones and anthocyanins in chemo-varietal forms of Perilla frutescens." Plant Cell Physiol 42(12);1338-44. PMID: 11773526
Lukacin01: Lukacin R, Matern U, Junghanns KT, Heskamp ML, Britsch L, Forkmann G, Martens S (2001). "Purification and antigenicity of flavone synthase I from irradiated parsley cells." Arch Biochem Biophys 393(1);177-83. PMID: 11516175
Martens03: Martens S, Forkmann G, Britsch L, Wellmann F, Matern U, Lukacin R (2003). "Divergent evolution of flavonoid 2-oxoglutarate-dependent dioxygenases in parsley." FEBS Lett 544(1-3);93-8. PMID: 12782296
Ueyama02: Ueyama Y, Fukuchi-Mizutani M, Fukui Y, Miyazaki K, Ohkawa H, Kusumi T, Tanaka Y (2002). "Molecular and biochemical characterization of torenia flavonoid 3'-hydroxylase and flavone synthase II and modification of flower color by modulating the expression of these genes." Plant Science, 163, 253-263.
Bredebach11: Bredebach M, Matern U, Martens S (2011). "Three 2-oxoglutarate-dependent dioxygenase activities of Equisetum arvense L. forming flavone and flavonol from (2S)-naringenin." Phytochemistry 72(7);557-63. PMID: 21353683
Schoenbohm00: Schoenbohm C, Martens S, Eder C, Forkmann G, Weisshaar B (2000). "Identification of the Arabidopsis thaliana flavonoid 3'-hydroxylase gene and functional expression of the encoded P450 enzyme." Biol Chem 2000;381(8);749-53. PMID: 11030432
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