  4.2.3.132 | metabolism |
abietadiene synthase catalyzes the committed step in resin acid biosynthesis by catalyzing the cyclization and rearrangement of (E,E,E)-geranylgeranyl diphosphate to a mixture of abietadiene double-bond isomers. The enzyme is bifunctional: Protonation across the terminal 14-15 double bond of (E,E,E)-geranylgeranyl diphosphate followed by bicyclization and deprotonation, produces the stable intermediate (+)-copalyl diphosphate (CPP). Then the enzyme ionizes the CPP to promote cyclization to the tricyclic perhydrophenanthrene backbone. However, this cyclization is further coupled to a 1,2-methyl migration, by means of intramolecular proton transfer within a pimarenyl intermediate, to generate the C13 isopropyl group characteristic of the abietane skeleton. Finally, deprotonation of the resulting abietenyl carbocation at one of three alternative positions (C7, C12, or C15) leads to the three principal olefin products abieta-7(8),13(14)-diene (abietadiene), abieta-8(14),12(13)-diene (levopimaradiene), and abieta-8(14)-13(15)-diene (neoabietadiene), respectively |
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