Homology modeling and docking studies on oxidosqualene cyclases associated with primary and secondary metabolism of Centella asiatica

Centella asiatica is a well-known medicinal plant, produces large amount of triterpenoid saponins, collectively known as centelloids, with a wide-spectrum of pharmacological applications. Various strategies have been developed for the production of plant secondary metabolites in cell and tissue cultures; one of these is modular metabolic engineering, in which one of the competitive metabolic pathways is selectively suppressed to channelize precursor molecules for the production of desired molecules by another route. In plants the precursor 2,3-oxidosqualene is shared in between two competitive pathways involved with two isoforms of oxidosqualene cyclases. One is primary metabolic route for the synthesis of phytosterol like cycloartenol by cycloartenol synthase; another is secondary metabolic route for the synthesis of triterpenoid like beta-amyrin by beta-amyrin synthase. The present work is envisaged to evaluate specific negative modulators for cycloartenol synthase, to channelize the precursor molecule for the production of triterpenoids. As there are no experimentally determined structures for these enzymes reported in the literature, we have modeled the protein structures and were docked with a panel of ligands. Of the various modulators tested, ketoconazole has been evaluated as the negative modulator of primary metabolism that inhibits cycloartenol synthase specifically, while showing no interaction with beta-amyrin synthase. Amino acid substitution studies confirmed that, ketoconazole is specific modulator for cycloartenol synthase, LYS728 is the key amino acid for the interaction. Our present study is a novel approach for identifying a suitable specific positive modulator for the over production of desired triterpenoid secondary metabolites in the cell cultures of plants.