Docking in the <i>Abies grandis</i> abietaenol synthase to investigate the mechanism underlying conifer resin acid diterpene synthase evolution

The pine family (Pinaceae) produces oleoresins in which the diterpenoid resin acids contain tricyclic backbones of either the (iso)pimarane and/or rearranged abietane type. These are produced by closely related (class I) diterpene synthases, whose functional divergence is of significant interest, not least for mechanistic insights into the catalyzed complex carbocation-cascade reactions. The abietaenol synthase from Abies grandis (AgAS) has long served as a model for this enzymatic subfamily and was used here to examine key residues potentially involved in transitions from such ancestral activity to production of (iso)pimaradienes, which seems to have occurred independently at least three times in the Pinaceae. While the equivalent substitutions in AgAS led to production of some amount of isopimaradiene(s), based on the transition observed in the most closely related enzymes (i.e., those also from the Abies genus), the threonine substitution for an alanine was most impactful, with the resulting A723T variant producing almost entirely isopimara-7,15-diene. This prompted application of the TerDockin computational approach to investigate the underlying mechanism. Through iterative application, requiring placement and constraint of the reactant water from the native reaction (i.e., that added to yield the 13α-hydroxyl of the primary abietaenol epimer), it was found that the introduced threonine most likely acts directly as a catalytic base to short-circuit the native reaction by deprotonating the initially formed isopimara-13E-en-8-yl carbocation intermediate. These results not only provide mechanistic insight but also have some implications for such modeling of the complex reactions catalyzed by terpene synthases more generally, as discussed herein.