Mechanism monoterpene conversion

Loganin (6) is a key intermediate in the biosynthetic pathway leading to other iridoid monoterpenes as well as complex indole alkaloids. Many of these additional compounds are derived by conversion of this bicyclic iridoid monoter-pene into secologanin (18). The mechanism by which ring cleavage occurs to yield secologanin is not well understood, but apparently the cleavage happens after oxidation of the... 

Preliminary characterization of the partially purified cineole synthetase indicated that the enzyme, like other monoterpene synthetases, exhibited a pH optimum on the acid side of neutrality, was inhibited by thiol-directed reagents, and required a divalent cation for catalysis (Mn + was preferred) (Croteau and Karp, 1977a). A single protein appeared to be involved in conversion of the acyclic precursor to 1,8-cineole, and no free intermediates could be discerned in the reaction. A mechanism for the formation of 1,8-cineole was proposed that involved the hydration of a double bond of an enzyme-bound monocyclic intermediate, followed by displacement of the enzyme by the hydroxyl group to form the heterocyclic ring and simultaneously release the product (Croteau and Karp, 1977a). 

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