[PAST EVENT] Applied Science and Chemistry Seminar
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Title: Endogenous cannabinoid ligand 2-arachidonyl glycerol (2-AG) and its interaction with cannabinoid type II cannabinoid receptor, CB2
Abstract:The endogenous cannabinoid ligand, 2-arachidonoylglycerol (2-AG), is a lipid-derived signaling molecule that activates cannabinoid receptors in brain and peripheral tissue. It is produced enzymatically from phospholipids in cell membranes. The 2-AG has amphipathic properties and readily partitions into lipid bilayers. In membranes at 36oC, 2-AG converts to 1-AG via acyl chain migration in less than one hour. Therefore, structural and functional studies were conducted at 4oC where conversion to 1-AG takes several days for completion. Neutron diffraction data using 2-AG with a selectively deuterated glycerol group or deuterated arachidonyl double bonds as well as 1H MAS NOESY cross-relaxation rates between the protons of 2-AG and nearby phospholipids revealed that the glycerol group of 2-AG resides in the lipid/water interface of bilayers, while the double bonds of the arachidonyl chain have a wide distribution in the hydrophobic core of the bilayer. 2H, and 1H-13C NMR order parameters as well as 13C NMR relaxation times indicate that the arachidonoyl chain retains a high level of flexibility in bilayers. Binding of 2-AG to purified, recombinant CB2 receptor functionally reconstituted into liposomes was probed by 2H magic-angle-spinning NMR on glycerol-deuterated 2-AG. In the presence of CB2, the 2H resonances of 2-AG-d5 shifted upfield and broadened. The upfield shift was absent when the binding pocket was blocked with a high affinity agonist like CP-55,940, demonstrating specificity of 2-AG interaction with CB2. The data suggest a rapid exchange of 2-AG between a membrane location and bound to CB2, in difference to synthetic cannabinoid ligands like CP-55,940 that have very low off rates from the receptor. Molecular simulations of 2-AG in a lipid bilayer and bound to CB2 are in good agreement with experimental parameters and suggest a mechanism of 2-AG binding to CB2 that results in receptor activation. The preferred location of 2-AG in lipid bilayers and the conformational flexibility of 2-AG appear to be critical for an efficient interaction with the ligand binding pocket of the receptor.