Mechanism and Stereochemistry of Enzymatic Cyclization of 24,30-Bisnor-2,3-oxidosqualene by Recombinant -Amyrin Synth
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- 作者：Ikuro Abe ; Yuichi Sakano ; Megumi Sodeyama ; Hideya Tanaka ; Hiroshi Noguchi ; Masaaki Shibuya ; and Yutaka Ebizuka
- 刊名：Journal of the American Chemical Society
- 出版年：2004
- 出版时间：June 9, 2004
- 年：2004
- 卷：126
- 期：22
- 页码：6880 - 6881
- 全文大小：56K
- 年卷期：v.126,no.22(June 9, 2004)
- ISSN：1520-5126
文摘
Recombinant 
-amyrin synthase from Pisum sativum converted 24,30-bisnor-2,3-oxidosqualene into a 3:1:0.2 mixture of 29,30-bisnor--amyrin, 29,30-bisnorgermanicol, and 29,30-bisnor-
-amyrin. Further, enzyme reactions with [23-13C]- and [23,23-2H]-labeled isotopomers demonstrated that the cyclization did not proceed through formation of a lupanyl primary cation with a five-membered E-ring, but an electrophilic addition of the tetracyclic C-18 cation on to the terminal double bond directly generated a thermodynamically favored pentacyclic secondary cation with a less-strained six-membered E-ring. Interestingly, the formation of the three regioisomers suggested that the absence of the terminal methyl groups resulted in a structural perturbation in the folding conformation of the E-ring of the oleanyl cation at the active site of the enzyme.
-amyrin synthase from Pisum sativum converted 24,30-bisnor-2,3-oxidosqualene into a 3:1:0.2 mixture of 29,30-bisnor--amyrin, 29,30-bisnorgermanicol, and 29,30-bisnor--amyrin. Further, enzyme reactions with [23-13C]- and [23,23-2H]-labeled isotopomers demonstrated that the cyclization did not proceed through formation of a lupanyl primary cation with a five-membered E-ring, but an electrophilic addition of the tetracyclic C-18 cation on to the terminal double bond directly generated a thermodynamically favored pentacyclic secondary cation with a less-strained six-membered E-ring. Interestingly, the formation of the three regioisomers suggested that the absence of the terminal methyl groups resulted in a structural perturbation in the folding conformation of the E-ring of the oleanyl cation at the active site of the enzyme.