title: Asymmetrical ligand binding by abscisic acid 8'-hydroxylase.
abstract: Abscisic acid (ABA), a plant stress hormone, has a chiral center (C1') in its molecule, yielding the enantiomers (1'S)-(+)-ABA and (1'R)-(-)-ABA during chemical synthesis. ABA 8'-hydroxylase (CYP707A), which is the major and key P450 enzyme in ABA catabolism in plants, catalyzes naturally occurring (1'S)-(+)-enantiomer, whereas it does not recognize naturally not occurring (1'R)-(-)-enantiomer as either a substrate or an inhibitor. Here we report a structural ABA analogue (AHI1), whose both enantiomers bind to recombinant Arabidopsis CYP707A3, in spite of stereo-structural similarity to ABA. The difference of AHI1 from ABA is the absence of the side-chain methyl group (C6) and lack of the alpha,beta-unsaturated carbonyl (C2'C3'-C4'O) in the six-membered ring. To explore which moiety is responsible for asymmetrical binding by CYP707A3, we synthesized and tested ABA analogues that lacked each moiety. Competitive inhibition was observed for the (1'R) enantiomers of these analogues in the potency order of (1'R,2'R)-(-)-2',3'-dihydro-4'-deoxo-ABA (K(I)=0.45 microM)>(1'R)-(-)-4'-oxo-ABA (K(I)=27 microM)>(1'R)-(-)-6-nor-ABA and (1'R,2'R)-(-)-2',3'-dihydro-ABA (no inhibition). In contrast to the (1'R)-enantiomers, the inhibition potency of the (1'S)-analogues declined with the saturation of the C2',C3'-double bond or with the elimination of the C4'-oxo moiety. These findings suggest that the C4'-oxo moiety coupled with the C2',C3'-double bond is the significant key functional group by which ABA 8'-hydroxylase distinguishes (1'S)-(+)-ABA from (1'R)-(-)-ABA.
authors.affiliation: The United Graduate School of Agricultural Science, Gifu University, Gifu 501-1193, Japan.
authors.forename: Kotomi
authors.initials: K
authors.lastname: Ueno
authors.forename: Hidetaka
authors.initials: H
authors.lastname: Yoneyama
authors.forename: Masaharu
authors.initials: M
authors.lastname: Mizutani
authors.forename: Nobuhiro
authors.initials: N
authors.lastname: Hirai
authors.forename: Yasushi
authors.initials: Y
authors.lastname: Todoroki
mesh_terms: D000040:Abscisic Acid / Q000737:chemistry / Q000494:pharmacology*; D017360:Arabidopsis / Q000201:enzymology*; D029681:Arabidopsis Proteins / Q000378:metabolism*; D001665:Binding Sites; D001667:Binding, Competitive; D003577:Cytochrome P-450 Enzyme System / Q000378:metabolism*; D004791:Enzyme Inhibitors / Q000138:chemical synthesis / Q000737:chemistry / Q000494:pharmacology*; D006900:Hydroxylation; D008024:Ligands; D008861:Microsomes / Q000201:enzymology; D006899:Mixed Function Oxygenases / Q000378:metabolism*; D008968:Molecular Conformation; D010937:Plant Growth Regulators / Q000737:chemistry / Q000494:pharmacology*; D010940:Plant Proteins; D011485:Protein Binding; D013237:Stereoisomerism; D013329:Structure-Activity Relationship; D013379:Substrate Specificity
chemical_list: D029681:Arabidopsis Proteins; D004791:Enzyme Inhibitors; D008024:Ligands; D010937:Plant Growth Regulators; D010940:Plant Proteins; D000040:Abscisic Acid; D003577:Cytochrome P-450 Enzyme System; D006899:Mixed Function Oxygenases; C107203:abscisic acid 8'-hydroxylase