Mechanism and stereochemistry of enzymatic cyclization of 24,30-bisnor-2,3-oxidosqualene by recombinant β-amyrin synthase
Mechanism and stereochemistry of enzymatic cyclization of 24,30-bisnor-2,3-oxidosqualene by recombinant β-amyrin synthase
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DOI：
10.1021/ja0490368
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发表时间：
2004-06-09
影响因子：
15
通讯作者：
Ebizuka, Y
中科院分区：
文献类型：
--
作者：
Abe, I;Sakano, Y;Ebizuka, Y
来自豌豆的重组β-香树脂素合酶将24，30-双去甲-2，3-氧化角鲨烯转化为29，30-双去甲-β-香树脂素、29，30-双去甲锗醇和29，30-双去甲-δ-香树脂素的3：1：0.2混合物。此外，与[23- 13 C]-和[23，23 - 2 H]-标记的同位素异构体的酶反应表明，环化并不通过形成具有五元E-环的羽扇豆基伯阳离子进行，而是四环C-18阳离子亲电加成到末端双键上直接产生了具有较小张力的六元E-环的有利的五环仲阳离子。有趣的是，三个区域异构体的形成表明，末端甲基基团的情况下，导致在酶的活性位点的油烷基阳离子的E-环的折叠构象的结构扰动。
Recombinant β-amyrin synthase fromPisum sativumconverted 24,30-bisnor-2,3-oxidosqualene into a 3:1:0.2 mixture of 29,30-bisnor-β-amyrin, 29,30-bisnorgermanicol, and 29,30-bisnor-δ-amyrin. Further, enzyme reactions with [23-13C]- and [23,23-2H]-labeled isotopomers demonstrated that the cyclization did not proceed through formation of a lupanylprimarycation 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 pentacyclicsecondarycation 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.