The Leu/Val 6.51 Side Chain of Cannabinoid Receptors Regulates the Binding Mode of the Alkyl Chain of Δ 9 -Tetrahydrocannabinol.

(-)-Δ--tetrahydrocannabinol (THC), which is the principal psychoactive constituent of , mediates its action by binding to two members of the G-protein-coupled receptor (GPCR) family: the cannabinoid CB (CBR) and CB (CBR) receptors. Molecular dynamics simulations showed that the pentyl chain of THC could adopts an I-shape conformation, filling an intracellular cavity between Phe and Trp for initial agonist-induced receptor activation, in CBR but not in CBR. This cavity opens to the five-carbon chain of THC by the conformational change of the γ-branched, flexible, Leu side chain of CBR, which is not feasible by the β-branched, mode rigid, Val side chain of CBR. In agreement with our computational results, THC could not decrease the forskolin-induced cAMP levels in cells expressing mutant CBR receptor but could activate the mutant CBR receptor as efficiently as wild-type CBR. Additionally, JWH-133, a full CBR agonist, contains a branched dimethyl moiety in the ligand chain that bridges Phe and Val for receptor activation. In this case, the substitution of Val to Leu in CBR makes JWH-133 unable to activate CBR. In conclusion, our combined computational and experimental results have shown that the amino acid at position 6.51 is a key additional player in the initial mechanism of activation of GPCRs that recognize signaling molecules derived from lipid species.