Tetrahydroberberrubine (TU)， an active tetrahydroprotoberberines (THPBs)， is gaining increasing popularity as a potential candidate for treatment of anxiety and depression. One of its two enantiomers， l-TU， has been reported to be an antagonist of both D1 and D2 receptors， but the functional activity of the other enantiomer， d-TU， is still unknown. In this study， experiments were combined with in silico molecular simulations to (1) confirm and discover the functional activities of l-TU and d-TU， and (2) systematically evaluate the molecular mechanisms beyond the experimental observations. l-TU proved to be an antagonist of both D1 and D2 receptors (IC = 385 nM and 985 nM， respectively)... More
Tetrahydroberberrubine (TU)， an active tetrahydroprotoberberines (THPBs)， is gaining increasing popularity as a potential candidate for treatment of anxiety and depression. One of its two enantiomers， l-TU， has been reported to be an antagonist of both D1 and D2 receptors， but the functional activity of the other enantiomer， d-TU， is still unknown. In this study， experiments were combined with in silico molecular simulations to (1) confirm and discover the functional activities of l-TU and d-TU， and (2) systematically evaluate the molecular mechanisms beyond the experimental observations. l-TU proved to be an antagonist of both D1 and D2 receptors (IC = 385 nM and 985 nM， respectively)， while d-TU shows no affinity against either D1 or D2 receptor， based on the cAMP assay (D1 receptor) and calcium flux assay (D2 receptor). Results from both flexible-ligand docking studies and molecular dynamic (MD) simulations provided insights at the atomic level. The l-TU-bound structures predicted by MD (1) undergo an outward rotation of the extracellular helical bundles; (2) have an enlarged orthosteric binding pocket; and (3) have a central toggle switch that is prevented from rotating freely. These features are unique to the l-TU enantiomer and provide an explanation for its antagonistic behavior toward both D1 and D2 receptors. The present study provides new sight on the structural and functional relationships of l-TU and d-TU binding to dopamine receptors， and provides guidance to the rational design of novel molecules targeting these two dopamine receptors in the future.
