Spencer Thomas A, Ditchfield Robert
Department of Chemistry, Burke Laboratory, Dartmouth College, Hanover, New Hampshire 03755, United States.
ACS Omega. 2023 Jul 12;8(29):26497-26507. doi: 10.1021/acsomega.3c03259. eCollection 2023 Jul 25.
Understanding how the highly unstable carbocation intermediates in terpenoid biosynthesis are stabilized and protected during their transient existence in enzyme active sites is an intriguing challenge which has to be addressed computationally. Our efforts have focused on evaluating the stabilization afforded via carbocation-π complexation between a biochemical carbocation and an aromatic amino acid residue. This has involved making measurements on an X-ray structure of an enzyme active site that shows a π donor proximate to a putative carbocation site and using these to build models which are analyzed computationally to provide an estimated stabilization energy (SE). Previously, we reported estimated SEs for several such carbocation-π complexes involving phenylalanine. Herein, we report the first such estimate involving tryptophan as the π donor. Because there was almost no published information about indole as a π-complexation donor, we first located computationally equilibrium π and σ complexes of 3-ethylindole with the -butyl cation as relevant background information. Then, measurements on the X-ray structure of the enzyme CotB2 complexed with geranylgeranyl thiodiphosphate (GGSPP), specifically on the geometric relationship of the putative carbocation at C15 of GGSPP to W186, were used to build a model that afforded a computed SE of -15.3 kcal/mol.
了解在萜类生物合成中高度不稳定的碳正离子中间体在酶活性位点短暂存在期间是如何被稳定和保护的，是一项必须通过计算解决的有趣挑战。我们的工作重点是评估生物化学碳正离子与芳香族氨基酸残基之间通过碳正离子-π络合所提供的稳定性。这涉及对酶活性位点的X射线结构进行测量，该结构显示一个π供体靠近一个假定的碳正离子位点，并利用这些测量结果构建模型，通过计算分析这些模型以提供估计的稳定能（SE）。此前，我们报道了几种涉及苯丙氨酸的此类碳正离子-π络合物的估计SE值。在此，我们报道了首例以色氨酸作为π供体的此类估计。由于几乎没有关于吲哚作为π络合供体的公开信息，我们首先通过计算确定了3-乙基吲哚与叔丁基阳离子的平衡π和σ络合物，作为相关背景信息。然后，对与香叶基香叶基硫代二磷酸（GGSPP）复合的CotB2酶的X射线结构进行测量，特别是对GGSPP的C15处假定的碳正离子与W186的几何关系进行测量，以此构建了一个模型，其计算得到的SE为-15.3千卡/摩尔。
