Functional Analyses of Tyr420 and Leu607 of <i>Alicyclobacillus acidocaldarius</i> Squalene-Hopene Cyclase. Neoachillapentaene, a Novel Triterpene with the 1,5,6-Trimethylcyclohexene Moiety…
作者:Tsutomu SATO、Shigehiro SASAHARA、Toshiyuki YAMAKAMI、Tsutomu HOSHINO
DOI:10.1271/bbb.66.1660
日期:2002.1.1
The functions of Tyr420 and Leu607 were analyzed by constructing various site-directed mutants. The mutation at position 420 into Ala and Gly gave bicyclic α- and γ-polypodatetraene in significant amounts, but with a trace amount of tricyclic malabaricatriene. The kinetic data for and the product distribution of the Y420F mutant indicate that the major function of Tyr420 is to stabilize the 6/6-fused bicyclic cation. Mutation experiments on Leu607 demonstrate that the appropriate steric bulk size at position 607 is required to strongly bind with the product-like conformation formed during the polycyclization process. Introduction of the bulkiest Trp residue into 420 or 607 led to the production of a novel monocyclic triterpene having the (5R,6R)-1,5,6-trimethylcyclohexene ring, named neoachillapentaene, indicating that the enzymatic cyclization proceeded via a constrained boat structure. Folding of the squalene molecule into a boat conformation by squalene cyclase has not been reported before.
通过构建各种定点突变体,分析了 Tyr420 和 Leu607 的功能。将 420 位突变为 Ala 和 Gly 会产生大量的双环 α- 和 γ-聚十二碳四烯,但会产生微量的三环马拉巴里三烯。Y420F 突变体的动力学数据和产物分布表明,Tyr420 的主要功能是稳定 6/6 融合双环阳离子。对 Leu607 的突变实验表明,在多环化过程中形成的类似产物构象需要在 607 位有适当的立体体积才能与之紧密结合。将体积最大的 Trp 残基引入 420 位或 607 位后,产生了一种具有 (5R,6R)-1,5,6-三甲基环己烯环的新型单环三萜,命名为新水苏烯,这表明酶促环化是通过受限的舟状结构进行的。角鲨烯分子通过角鲨烯环化酶折叠成舟形构象的情况以前从未报道过。