Comp Chem Research Developments

The Mechanism of Carbocation Cyclization by Squalene Cyclase

Jiali Gao

The remarkable regio and setereoselectivity of the enzymatic cyclizations of squalene (1) and 2,3-oxidosqualene (2) to form polycyclic triterpenes have fascinated chemists and biochemists for over half a century. These enzymatic reactions yield a variety of triterpenoids, including hopene and diplopterol, tetrahymanol, and lanosterol, among other natural products from plants. The prokaryotic squalene-hopene cyclase (SHC) converts squalene into the pentacyclic hopane skeleton accompanied by the creation of nine stereocenters, all in one-step, which are precursors of a wide variety of hopanoids that condense bacterial membranes. The eukaryotic oxidosqualene cyclases (OSC) catalyze the transformation of 2,3-oxidosqualene into the tetracyclic species, lanosterol, which is further converted into cholesterol. In a paper to be published in the Journal of the American Chemical Society, graduate student Ramkumar Rajamani in Gao’s laboratory describes a molecular dynamics study using a combined quantum mechanical and molecular mechanical (QM/MM) potential to investigate the squalene-to-hopene carbocation cyclization mechanism in SHC. This study is based on free energy simulations by constructing the free energy surface for the cyclization steps along the reaction pathway. The picture that emerges for the carbocation cyclization cascade is a delicate balance of thermodynamic and kinetic control that ultimately favors the formation of the final hopanoids carbon skeleton. The researchers found that the 5 to 6-membered ring expansion process is not a viable reaction pathway for either C-ring or D-ring formation in the cyclization reaction. The only significant intermediate is the A/B-bicyclic cyclohexyl cation (III), from which two asynchronous concerted reaction pathways lead to, respectively, the 6,6,6,5-tetracyclic carbon skeleton, and the 6,6,6,6,5-pentacyclic hopanoids. The findings explain the experimental observation that these two products were isolated in 1% and 99% yields, respectively, in the wild-type enzyme. Thus, the product distribution in the wild-type enzyme is dictated by kinetic control of these two reaction pathways.