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

(-)-Δ 9 - trans -tetrahydrocannabinol (THC), which is the principal psychoactive constituent of Cannabis , mediates its action by binding to two members of the G-protein-coupled receptor (GPCR) family: the cannabinoid CB 1 (CB 1 R) and CB 2 (CB 2 R) receptors. Molecular dynamics simulations showed that the pentyl chain of THC could adopts an I-shape conformation, filling an intracellular cavity between Phe 3.36 and Trp 6.48 for initial agonist-induced receptor activation, in CB 1 R but not in CB 2 R. This cavity opens to the five-carbon chain of THC by the conformational change of the γ-branched, flexible, Leu 6.51 side chain of CB 1 R, which is not feasible by the β-branched, mode rigid, Val 6.51 side chain of CB 2 R. In agreement with our computational results, THC could not decrease the forskolin-induced cAMP levels in cells expressing mutant CB 1 R L6.51V receptor but could activate the mutant CB 2 R V6.51L receptor as efficiently as wild-type CB 1 R. Additionally, JWH-133, a full CB 2 R agonist, contains a branched dimethyl moiety in the ligand chain that bridges Phe 3.36 and Val 6.51 for receptor activation. In this case, the substitution of Val 6.51 to Leu in CB 2 R makes JWH-133 unable to activate CB 2 R V6.51L . 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.