Proposed Mechanism for the Reaction Catalyzed by a Diterpene Cyclase, Aphidicolan-16-ol Synthase: Experimental Result
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- 作者：Hideaki Oikawa ; Kensuke Nakamura ; Hiroaki Toshima ; Tomonobu Toyomasu ; and Takeshi Sassa
- 刊名：Journal of the American Chemical Society
- 出版年：2002
- 出版时间：August 7, 2002
- 年：2002
- 卷：124
- 期：31
- 页码：9145 - 9153
- 全文大小：141K
- 年卷期：v.124,no.31(August 7, 2002)
- ISSN：1520-5126
文摘
To examine the mechanism of the cyclization reaction catalyzed by aphidicolan-16
-ol synthase(ACS), which is a key enzyme in the biosynthesis of diterpene aphidicolin, a specific inhibitor of DNApolymerase 
, skeletal rearrangement of 2a and biomimetic cyclization of 4b were employed. The structuresof the reaction products, which reflect penultimate cation intermediates, allowed us to propose a detailedreaction pathway for the Lewis acid-catalyzed cyclizations and rearrangements. Isolation of these productsin an aphidicolin-producing fungus led us to speculate that the mechanism of the ACS-catalyzed cyclizationreaction is the same as that of a nonenzymatic reaction. Ab initio calculations of the acid-catalyzed reactionintermediates and the transition states indicate that the overall reaction catalyzed by ACS is an exothermicprocess though the reaction proceeds via an energetically disfavored secondary cation-like transition state.In conjunction with the solvent effect in the acid-catalyzed reactions, this indicates that the actual role ofACS is to provide a template which enforces conformations of the intermediate cations leading to theproductive cyclization although it has been believed that the cation-
interaction between cationintermediates and aromatic amino acid residues in the active site is important for the enzymatic catalysis.This study provided important information on the role of various cationic species, especially secondarycation-like structures, in both nonenzymatic and enzymatic reactions.
-ol synthase(ACS), which is a key enzyme in the biosynthesis of diterpene aphidicolin, a specific inhibitor of DNApolymerase , skeletal rearrangement of 2a and biomimetic cyclization of 4b were employed. The structuresof the reaction products, which reflect penultimate cation intermediates, allowed us to propose a detailedreaction pathway for the Lewis acid-catalyzed cyclizations and rearrangements. Isolation of these productsin an aphidicolin-producing fungus led us to speculate that the mechanism of the ACS-catalyzed cyclizationreaction is the same as that of a nonenzymatic reaction. Ab initio calculations of the acid-catalyzed reactionintermediates and the transition states indicate that the overall reaction catalyzed by ACS is an exothermicprocess though the reaction proceeds via an energetically disfavored secondary cation-like transition state.In conjunction with the solvent effect in the acid-catalyzed reactions, this indicates that the actual role ofACS is to provide a template which enforces conformations of the intermediate cations leading to theproductive cyclization although it has been believed that the cation- interaction between cationintermediates and aromatic amino acid residues in the active site is important for the enzymatic catalysis.This study provided important information on the role of various cationic species, especially secondarycation-like structures, in both nonenzymatic and enzymatic reactions.