title: Structural and mechanistic studies on anthocyanidin synthase catalysed oxidation of flavanone substrates: the effect of C-2 stereochemistry on product selectivity and mechanism.
abstract: During the biosynthesis of the tricyclic flavonoid natural products in plants, oxidative modifications to the central C-ring are catalysed by Fe(ii) and 2-oxoglutarate dependent oxygenases. The reactions catalysed by three of these enzymes; flavone synthase I, flavonol synthase and anthocyanidin synthase (ANS), are formally desaturations. In comparison, flavanone 3beta-hydroxylase catalyses hydroxylation at the C-3 pro-R position of 2S-naringenin. Incubation of ANS with the unnatural substrate (+/-)-naringenin results in predominantly C-3 hydroxylation to give cis-dihydrokaempferol as the major product; trans-dihydrokaempferol and the desaturation product, apigenin are also observed. Labelling studies have demonstrated that some of the formal desaturation reactions catalysed by ANS proceed via initial C-3 hydroxylation followed by dehydration at the active site. We describe analyses of the reaction of ANS with 2S- and 2R-naringenin substrates, including the anaerobic crystal structure of an ANS-Fe-2-oxoglutarate-naringenin complex. Together the results reveal that for the 'natural' C-2 stereochemistry of 2S-naringenin, C-3 hydroxylation predominates (>9 : 1) over desaturation, probably due to the inaccessibility of the C-2 hydrogen to the iron centre. For the 2R-naringenin substrate, desaturation is significantly increased relative to C-3 hydroxylation (ca. 1 : 1); this is probably a result of both the C-3 pro-S and C-2 hydrogen atoms being accessible to the reactive oxidising intermediate in this substrate. In contrast to the hydroxylation-elimination desaturation mechanism for some ANS substrates, the results imply that the ANS catalysed desaturation of 2R-naringenin to form apigenin proceeds with a syn-arrangement of eliminated hydrogen atoms and not via an oxygenated (gem-diol) flavonoid intermediate. Thus, by utilising flavonoid substrates with different C-2 stereochemistries, the balance between C-3 hydroxylation or C-2, C-3 desaturation mechanisms can be altered.
authors.affiliation: Chemical Research Laboratory, Department of Chemistry and Oxford Centre for Molecular Sciences, Mansfield Road, Oxford OX1 3TA, UK.
authors.forename: Richard W D
authors.initials: RW
authors.lastname: Welford
authors.forename: Ian J
authors.initials: IJ
authors.lastname: Clifton
authors.forename: Jonathan J
authors.initials: JJ
authors.lastname: Turnbull
authors.forename: Stuart C
authors.initials: SC
authors.lastname: Wilson
authors.forename: Christopher J
authors.initials: CJ
authors.lastname: Schofield
mesh_terms: D002384:Catalysis; D018360:Crystallography, X-Ray; D044950:Flavanones / Q000737:chemistry* / Q000378:metabolism; D008958:Models, Molecular; D015394:Molecular Structure; D010084:Oxidation-Reduction; D010105:Oxygenases / Q000737:chemistry*; D011487:Protein Conformation; D017434:Protein Structure, Tertiary; D013237:Stereoisomerism; D013379:Substrate Specificity
chemical_list: D044950:Flavanones; D010105:Oxygenases; C118159:anthocyanidin synthase; C005273:naringenin; C007619:eriodictyol; C028610:flavanone