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2brt
From Proteopedia
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ANTHOCYANIDIN SYNTHASE FROM ARABIDOPSIS THALIANA COMPLEXED WITH NARINGENIN
Overview
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.
About this Structure
2BRT is a Single protein structure of sequence from Arabidopsis thaliana with FE2, AKG and NAR as ligands. Active as Leucocyanidin oxygenase, with EC number 1.14.11.19 Structure known Active Site: AC1. Full crystallographic information is available from OCA.
Reference
Structural and mechanistic studies on anthocyanidin synthase catalysed oxidation of flavanone substrates: the effect of C-2 stereochemistry on product selectivity and mechanism., Welford RW, Clifton IJ, Turnbull JJ, Wilson SC, Schofield CJ, Org Biomol Chem. 2005 Sep 7;3(17):3117-26. Epub 2005 Aug 1. PMID:16106293
Page seeded by OCA on Mon Nov 5 17:47:11 2007
