Distinct Binding Mechanisms for Allosteric Sodium Ion in Cannabinoid Receptors.

The therapeutic potential of cannabinoid receptors is not fully explored due to psychoactive side effects and lack of selectivity associated with orthosteric ligands. Allosteric modulators have the potential to become selective therapeutics for cannabinoid receptors. Biochemical experiments have shown the effects of the allosteric Na binding on cannabinoid receptor activity. However, the Na coordination site and binding pathway are still unknown. Here, we perform molecular dynamic simulations to explore Na binding in the cannabinoid receptors, CB and CB. Simulations reveal that Na binds to the primary binding site from different extracellular sites for CB and CB. A distinct secondary Na coordination site is identified in CB that is not present in CB. Furthermore, simulations also show that intracellular Na could bind to the Na binding site in CB. Constructed Markov state models show that the standard free energy of Na binding is similar to the previously calculated free energy for other class A GPCRs.