Modulation of inherent dynamical tendencies of the bisabolyl cation via preorganization in epi-isozizaene synthase

Abstract

The relative importance of preorganization, selective transition state stabilization and inherent reactivity are assessed through quantum chemical and docking calculations for a sesquiterpene synthase (epi-isozizaene synthase, EIZS). Inherent reactivity of the bisabolyl cation, both static and dynamic, appears to determine the pathway to product, although preorganization and selective binding of the final transition state structure in the multi-step carbocation cascade that forms epi-isozizaene appear to play important roles.

Fig. 1. Proposed steps involved in epi-isozizane formation: (I) pyrophosphate removal, (II) pyrophosphate addition, (III) conformational change, (IV) pyrophosphate removal, (V) π-cation cyclization, (VI) 1,2-hydride shift, (VII) π-cation cyclization, (VIII) π-cation cyclization, (IX) asynchronous double-alkyl shift, (X) 1,2-methyl shift, (XI) deprotonation. OPP = pyrophosphate.

Fig. 2. Docked structures (bottom) of 1 → 2 TSS conformations (top) with the best docking scores (TS1a–d) and the lowest energy 1 → 2 TSS conformation (TS1e). Relative energies and docking scores are shown in Table 1. The carbons between which a bond will form in the 2 → 3 reaction (C6 and C10) are labeled in each TSS. In docked structures, the substrate is blue, protein sidechains are grey, the diphosphate group is red/orange and Mg ions are purple.

Fig. 3. Natural products produced by EIZS.

Fig. 4. Carbocations produced during dynamics trajectories.

Fig. 5. Computed mPW1PW91/6-31+G(d,p) free energy profile for formation of 6 via TS1d (here, labeled TS1-3) without conformational changes of intermediates (energies are relative to the global minimum of 1; note that for this series of conformations, 2 is not a minimum), along with productive docking poses (substrate is blue, protein sidechains are grey, the diphosphate group is red/orange, Mg ions are purple, groups involved in bond making/breaking are highlighted in transition state structures). Dotted lines and arrows indicate the qualitative effects of binding to EIZS, estimated based on computed docking scores for productive poses.