ABA and Its Derivatives: Chemistry and Physiological Functions - Abscisic Acid: Metabolism, Transport and Signaling - page 16

(a) (b)

S - 1 R - 1

helix I

Fig. 1.13 Molecular surfaces in the active site of the 3D structural model of CYP707A3. a The
S -ABA bound active site, and b R -ABA superimposed and overlaid onto S -ABA (omitted for
simplicity)

the C-2′ side of the ring must be close to helix I (SRS 4) to keep C-8′ in close
proximity to the heme iron. If R -ABA binds in the same manner as S -ABA, C-8′
should contact helix I (Fig. 1.13 ). Determination of the exact mechanism requires
the crystal structure of CYP707A, which has yet to be obtained. Another large
difference is that the side chain methyl (C-6) and C-1′ hydroxy groups which
are required for the high ABA activity is dispensable for binding to CYP707A
enzymes (Ueno et al. 2005 ) (Fig. 1.14 ). Focusing on this difference, Ueno et al.
( 2005 ) developed AHI1 ( 50 ), an ABA analogue which functions as a competitive
inhibitor of CYP707A without causing an ABA response.

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Fig. 1.14 Differences in structural requirements for CYP707A3 inhibition and for ABA activity,
and AHI1, a CYP707A3 inhibitor lacking ABA activity