2BRT

ANTHOCYANIDIN SYNTHASE FROM ARABIDOPSIS THALIANA COMPLEXED with naringenin

Summary for 2BRT

• Entry DOI: 10.2210/pdb2brt/pdb
• Related: 1GP4 1GP5 1GP6
• Descriptor: LEUCOANTHOCYANIDIN DIOXYGENASE, FE (II) ION, 2-OXOGLUTARIC ACID, ... (5 entities in total)
• Functional Keywords: oxidoreductase, dioxygenase, flavonoid biosynthesis, iron, metal-binding, vitamin c
• Biological source: ARABIDOPSIS THALIANA (MOUSE-EAR CRESS)
• Total number of polymer chains: 1
• Total formula weight: 40794.36

Authors

Turnbull, J.J.,Clifton, I.J.,Welford, R.W.D.,Schofield, C.J. (deposition date: 2005-05-11, release date: 2006-08-29, Last modification date: 2023-12-13)

Primary citation

Welford, R.W.D.,Clifton, I.J.,Turnbull, J.J.,Wilson, S.C.,Schofield, C.J.
Structural and Mechanistic Studies on Anthocyanidin Synthase Catalysed Oxidation of Flavanone Substrates: The Effect of C-2 Stereochemistry on Product Selectivity and Mechanism
Org.Biomol.Chem., 3:3117-, 2005

PubMed 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.

PubMed: 16106293
DOI: 10.1039/B507153D

Experimental method

X-RAY DIFFRACTION (2.2 Å)