30 環状モノテルペノイドの生合成 : リモネン生合成における立体制御(口頭発表の部)

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(4S)-(-)- and (4R)-(+)-Limonenes (2a and 2b) are biosynthesized from geranyl diphosphate (GPP: 1a) with limonene synthase in Mentha spicata and Citrus unshiu, respectively. We have investigated the stereocontrol in the biosynthesis of the limonenes, and now describe evidence involving the role of the divalent metal ion, the mechanistic interpretation of the biospecific generation of the chirality at C-4 from non-chiral 1a and the regioselective formation of the double bond at C-8. Limonene synthase was activated only in the presence of divalent metal ions. The metal ion, as studied with Mg^<2+>, was found to form a 1 to 1 chelate at an equivalent distance from both P_α and P_β of 1a and to facilitate the elimination of the diphosphate moiety by weakening the C-OP bond, and the subsequent formation of a cationic intermediate that can readily cycling into limonene. Studies using (1S)-[1-^2H] and (1R)-[1-^2H]GPPs and (3S)-(+)-, (3R)-(-)-, and (3RS)-LPPs as substrate indicated that the cationic intermediate is generated from 1a by the limonene synthase of M. spicata by elimination of the diphosphate moiety at si-si face of the C-2 double bond and is in the anti-endo spatial arrangement leading to the formation of 2a. In the case of C. unshiu, in contrast, the elimination of the diphosphate moiety took place at the re-re face of the C-2 double bond and to give the anti-endo cationic intermediate which is enantiomeric to the intermediate in the case of M. spicata. The chiral generation at C-4 in the biosynthesis of limonene was found to be determined by the facial direction of elimination of the diphosphate moiety from 1a. The C-8 double bond of both 2a and 2b was proved to be formed by regioselective elimination of the hydrogen atom from the C-10 methyl group of 1a, on the basis studies using [8,8,8-^2H3] and [10,10,10-^2H3]GPP as substrate.
天然有機化合物討論会の論文
1992-09-10

30 環状モノテルペノイドの生合成 : リモネン生合成における立体制御(口頭発表の部)

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