Differences between the structural requirements for ABA 8′-hydroxylase inhibition and for ABA activity

A major catabolic enzyme of the plant hormone abscisic acid (ABA) is the cytochrome P450 monooxygenase ABA 8′-hydroxylase. For designing a specific inhibitor of this enzyme, the substrate specificity and inhibition of CYP707A3, an ABA 8′-hydroxylase from Arabidopsis thaliana, was investigated using 45 structural analogues of ABA and compared to the structural requirements for ABA activity. Substrate recognition by the enzyme strictly required the 6′-methyl groups (C-8′ and C-9′), which were unnecessary for ABA activity, whereas elimination of the 3-methyl (C-6) and 1′-hydroxyl groups, which significantly affected ABA activity, had little effect on the ability of analogues to competitively inhibit the enzyme. Fluorination at C-8′ and C-9′ resulted in resistance to 8′-hydroxylation and competitive inhibition of the enzyme. In particular, 8′,8′-difluoro-ABA and 9′,9′-difluoro-ABA yielded no enzyme reaction products and strongly inhibited the enzyme (KI = 0.16 and 0.25 μM, respectively).

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