39 環状モノテルペノイド類の生合成 : p-メンタン骨格の形成における立体特異性と機構(口頭発表の部)  [in Japanese] 39 BIOSYNTHESIS OF CYCLIC MONOTERPENOIDS : THE CHIRAL INTERMEDIATE AND THE MECHANISM OF THE CHIRAL GENERATION  [in Japanese]

Limonene synthase responsible for the cyclization of geranyl diphosphate (GPP) to limonene was partially purified from the leaves of Mentha spicata and Citrus unshiu. Divalent metal cations, especially Mg^<2+>, binds to the diphosphate moiety of GPP in a mole ratio of 1:1 in the initial stages of the biosynthesis of cyclic monoterpenoids. The chelation weakens the C-O bond of these diphosphates to facilitate the elimination of the diphosphate group, and this results in the formation of allylic cation as an intermediate in the cyclization of GPP. Incubation of geranyl diphosphate (GPP) with the limonene synthase gave (S)-(-)-limonene in case of the enzyme isolated from M. spicata and (R)-(+)-limonene in case of the enzyme isolated from C. unshiu. On the other hand, incubation of enantiomers of linalyl diphosphate (LPP) with the limonene synthase resulted in the conversion of (S)-LPP into (S)-limonene in the case of the enzyme from M. spicata and (R)-LPP into (R)-limonene in the case of the enzyme from C. unshiu. These facts clearly indicate that allylic cations with the configuration similar to (S)- and (R)-LPP are involved as intermediates in the cyclization of GPP to (S)- and (R)-limonene, respectively. Thus, the process for the cyclization of the C_<10>-prenyl chain is probably as follows: GPP bound to the active sites of limonene synthase is transformed into the stereochemically specified allylic cation [(a) or (e) in Fig. 3] under the steric control characteristic of the enzyme in the respective plants and then the allylic cation is led to the formation of the stereospecifically featured cyclic monoterpenoids.