Computational Studies on Biosynthetic Carbocation Rearrangements Leading to Quiannulatene: Initial Conformation Regulates Biosynthetic Route, Stereochemistry, and Skeleton Type

Abstract

The results of quantum chemical calculations on the mechanism of the carbocation cascade of reactions in the biosynthetic pathways leading to the pentacyclic sesterterpenes quiannulatene and sesterfisherol provide reasonable answers to several persistent mechanistic questions in sesterterpene biosynthesis, including: 1) the reaction pathways of the multicyclic ring system construction and skeletal rearrangements, 2) the mechanism of triquinane skeleton formation, which requires more complicated rearrangements than previously proposed, 3) the stereochemistry of the final carbocation intermediate, and 4) the determining factor of biosynthetic selection for either 5/6/4/6/5 or 5/6/5/5/5 pentacyclic skeleton formation. This in-depth mechanistic study on sesterterpene biosynthesis revealed that the shape of the final product and the type of triquinane skeleton formed are regulated by the stereochemistry and conformation of the common starting material, geranylfarnesyl diphosphate (GFPP).

Inherent atomic mobility changes in carbocation intermediates during the sesterterpene cyclization cascade

“…Based on this analysis, it appears that the destination of the cyclization cascade is determined by the conformations of the carbocation intermediates, but not by enzymatic constraints. This insight is consistent with our previous findings [1].…”

“…Two methyl groups, i.e., C20 and C23, remain static during the first half of the cyclization cascade, indicating that they could be determinants of the affinity for the enzyme cavity of sesterterpene synthase. Interestingly, inherent mobility shows the same trend in phase I (5/12/5 tricycle formation) and phase III (ring rearrangement), but not in phase II (conformational changes and hydrogen shifts), of quiannulatene and sesterfisherol biosynthesis, indicating that phase II is the key process for the structural diversification, in accordance with our previous study [1]. Moreover, C20 becomes flexible in phase III, which could result in decreased affinity for the enzyme, and this might be relevant to substrate release.…”

“…All calculations were carried out using GRRM11 [21–25] with Gaussian 09 [26]. All TS structures were obtained from the literature [1,3]. Intrinsic reaction coordinate (IRC) calculations [27–30] were carried out with B3LYP/6-31+G(d,p) for quiannulatene and with M06-2X/6-31G(d,p) for sesterfisherol.…”

“…Terpene synthases are thought to have four main roles: (i) triggering the biosynthetic cyclization cascade by the elimination of pyrophosphate or by protonation; (ii) preorganization of the substrate to generate the reactive conformation; (iii) protection of reactive intermediates from water; and (iv) termination of the reaction by deprotonation or hydration. We previously established the importance of conformation in the carbocation cyclization cascade [1], focusing on two sesterterpenes, i.e., quiannulatene [1–2] and sesterfisherol [3–5], as representative examples. These two sesterterpenes are synthesized via a 5/12/5 tricyclic intermediate, which leads to a 5/6/8/5 tetracyclic intermediate.…”

Acceleration of Mechanistic Investigation of Plant Secondary Metabolism Based on Computational Chemistry

“…To investigate the origin of the terpenoid structural diversity, Sato et al carried out a detailed comparative study of two structurally similar sesterterpenes: sesterfisherol and quiannulatene, which were synthesized by NfSS and EvQS, respectively (Sato et al, 2018a). Based on their calculations, these sesterterpenoids are formed via the 5/12/5 tricyclic intermediate unlike for astellatene (Figure 2, path II).…”