Flavone synthase II

Akashi, T. et al.. Molecular cloning and biochemical characterization of a novel cytochrome P450, flavone synthase II, that catalyzes direct conversion of flavanones to flavones. Plant Cell Physiol, 40, 1182, 1999. 

Akashi, T., Aoki, T., and Ayabe, S., Identification of a cytochrome P450 cDNA encoding (25)-flavanone 2-hydroxylase of licorice Glycyrrhiza echinata L. Fabaceae) which represents licodione synthase and flavone synthase II. FEES Lett., 431, 287, 1998. 

Ueyama, Y. et al., Molecular and biochemical characterization of torenia flavonoid 3 -hydroxylase and flavone synthase II and modification of flower color by modulating the expression of these... 

Martens S, Forkmann G. 1999. Cloning and expression of flavones synthase II from Gerbera hybrids. Plant J 20 611-618. 

MARTENS, S., FORKMANN, G., Genetic control of flavone synthase II activity in flowers of Gerbera hybrids. Phytochemistry, 1998,49,1953-1958. 

Tetrahydroxychalcone (18) was converted to naringenin (1) by chalcone iso-merase (CHI), and then to flavones (29) by flavone synthase I (FSI) and flavone synthase II (FSII), respectively (Fig. 6). 

However, the flavanone naringenin (10) [but not dihydro-kaempferol (13)] is the substrate for flavone formation in snapdragons. Antirrhinum majus (Scrophulariaceae) (flavone synthase II) (Fig. 11.10). In this plant, flavones arise from dehydrogenation of flavanones and not from dehydration of dihydroflavonols (Britsch et al., 1981). A similar enzyme system converts dihydroflavonols to flavonols (Britsch et al., 1981). In other work, the enzyme responsible for oxidation of flavanones to flavones in snapdragon Antirrhinum majus) was isolated from a microsomal fraction and shown to require NADPH and molecular oxygen (Britsch et al., 1981 Dewick, 1989 Forkmann and Stotz, 1981). The system appears to be a cytochrome P-450-dependent monooxygenase. This system also is known from Glycine max... 

Figure 5.4. Abbreviated scheme for biosynthesis of major flavonoid subclasses, showing the primary enzymes and substrates leading to different subclasses. Bold-faced, uppercase abbreviations refer to enzyme names, whereas substrate names are presented in lowercase letters. PAL, phenylalanine ammonia lyase C4H, cinnamate 4-hydroxylase 4CL, 4-coumarate CoA ligase CHS, chalcone synthase CHI, chalcone isomerase CHR, chalcone reductase IPS, isoflavone synthase F3H, flavonone 3-hydroxylase F3 H, flavonoid 3 -hydroxylase F3 5 H, flavonoid 3 5 -hydroxylase FNSI/II, flavone synthase DFR, dihydroflavonol 4-reductase FLS, flavonol synthase ANS, anthocyanidin synthase LAR, leucoanthocyanidin reductase ANR, anthocyanidin reductase UFGT, UDP-glucose flavonoid 3-O-glucosyltransferase. R3 = H or OH. R5 = H or OH. Glc = glucose. Please refer to text for more information.

Flavone synthase (FNS EC 1.14.11.22) introduces a double bond between C2 and C3 of a flavanone to produce the corresponding flavone. This activity was initially identified in parsley cell suspension cultures and subsequently shown to be encoded by a 2-oxoglutarate-dependent dioxygenase [67, 78, 79], This enzyme, now known as FNS-I, appears to have very limited distribution. To date, it has only been identified in the Apiaceae family (Umbellifers). The more widely occurring FNS-II (CYP93B) was initially identified from snapdragon (Antirrhinum majus) flowers [80] and was subsequently shown to be a P450 enzyme. FNS-I, FNS-II, and the various roles flavones play in plant species have recently been reviewed by Martens and Mithofer [81], Subsequent to this review, Yu et al. [82] demonstrated that the characteristic lack of natural accumulation of flavones in Brassicaceae could not be overcome in A. thaliana even by overexpression of recombinant parsley FNS-I. 

An enzyme from the flowers of Sinningia cardinalis Reichsteinia cardinalis, Gesneriaceae) has hydrolase activity associated with microsomal fractions and requires NADPH as an essential cofactor (hydrolyase activity II). This enzyme converts naringenin (10) and apigenin (5) to eriodictyol (17) and luteolin (4), respectively (Dewick, 1989). The flavone synthase activity of this enzyme was abolished completely by treatment with the cytochrome P-450 inhibitor ancymidol, but the flavonoid 3 -monooxygen-ase activity was not altered. 

Fig. 6.8 Common polyketide pathway leading to flavonoids (CHS = chalcone synthase) and to the stilbene resveratiol (STS = stilbene synthase). CHI = chalcone isomerase FNS I = flavone synthase 1 FNS II = flavone synthase 11 FHT = flavanone 3-hydroxylase FLS = flavonol synthase DFR = dihydroflavonol 4-reductase ANS = anthocyanidin synthase (abbreviations of enzymes according to McAidle et al. 2006)...

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