Flavonoid hydroxylation

The recombinant product is able to catalyze methylation at a variety of flavonoid hydroxyl positions. 

In contrast to the ubiquitous distribution of flavonoids hydroxylated at C-3 or C-4 of their heterocyclic rings, the unsubstituted flavans (2-phenylchromans) are more rarely found. The flavans co-occur with chalcones, flavanones, flavan-3,4-diols, flavonols, and 1,3-diphenyl-propanes. 

E. coli/7-OMT from Streptomyces avermitilis (synthesis of cofactor malonyl-coenzyme A, polyketide synthesis, flavonoid hydroxylation)... 

As strong antioxidants and scavengers of superoxide, hydroxyl and peroxyl radicals, tea flavonoids can suppress radical chain reactions and terminate lipid peroxidation (Kumamoto and Sonda, 1998, Yang and Wang, 1993). 

HARDWICK W F, KALYANARAMAN B, PRITSOS C A and PARDINI R S (1988) The production of hydroxyl and semiquinone free radicals during the autoxidation of redox active flavonoids, in Simic MG, Taylor K A, Ward J F and von Sonntag C Oxygen Radicals in Biology and Medicine, Plenum Press, New York, 149-52. 

The best cofactors are typically flavonoid derivatives that contain many hydroxyl groups, the most favorable at position 3 of the flavones. The strongest cofactors have electron-rich systems that associate with electron-poor compounds such as the flavylium cation. 

The apparent necessity of a second signal for haustorial initiation provides another possibility for chemical control. Riopel and coworkers (2,12-14) observed a few years ago that gum tragacanth, a foliar extract, was a very potent haustorial initiator. Two active compounds were isolated (Xenognosin A shown below, and a flavonoid with similar structure). The structural features required for haustorial formation include a meta relationship of hydroxyl and methoxyl groups and an alkyl branching ortho to the methoxy substituent. 

Huen MSY, Kwok-Min H, Leung JWC, Sigel E, Baur R, Wong JTF. Naturally occurring 2 -hydroxyl-substituted flavonoids as high-affinity benzodiazepine site ligands. Biochem Pharmacol 2003 66 2397-2407. 

Flavonols and flavones have a double bond between C2 and C3 in the flavonoid structure and an oxygen atom at the C4 position. Furthermore, flavonols also have a hydroxyl group at the C3 position. Dihydroflavonols have the same structure as flavonols without the double bond between C2 and C3. 

Flavanonols Flavanonols can be considered as flavanones with a hydroxyl group on position 3. They are sometimes referred to as dihydroflavonols. Similar to the situation of flavanones, flavanonols are no longer a minor subgroup of flavonoids, and they are a structurally highly diverse and multisubstituted subgroup (Grayer and Veitch 2006). A well-known flavanonol is taxifolin from citrus fruits (Kawaii and others 1999) (see Table 5.1 and Fig. 5.2). 

These structurally diverse compounds exhibit a range of biological activities in vitro that may explain their potential health-promoting properties, including antioxidant and anti-inflammatory effects and the induction of apoptosis (Hooper and others 2008). Most of the recent interest in flavonoids as health-promoting compounds is related to their powerful antioxidant properties. The criteria to establish the antioxidant capacity of these compounds is based on several structural characteristics that include (a) the presence of o-dihydroxyl substituents in the B-ring (b) a double bond between positions 2 and 3 and (c) hydroxyl groups in positions 3 and 5. 

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