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Endocannabinoids are naturally occurring lipid-like molecules that bind to cannabinoid receptors (CB1and CB2) and regulate many of human bodily functions via the endocannabinoid system. There is a tremendous interest in developing selective drugs that target the CB receptors. However, the biophysical mechanisms responsible for the subtype selectivity for endocannbinoids have not been established. Recent experimental structures of CB receptors show that endocannbinoids potentially bind via membrane using the lipid access channel in the transmembrane region of the receptors. Furthermore, the N-terminus of the receptor could move in and out of the binding pocket thereby modulating both the pocket volume and its residue composition. On the basis of these observations, we propose two hypothesis to explain the selectivity of the endocannabinoid, anandamide for CB1receptor. First, the selectivity arises from distinct enthalpic ligand-protein interactions along the ligand binding pathway formed due to the movement of N-terminus and subsequent shifts in the binding pocket composition. Second, selectivity arises from the volumetric differences in the binding pocket allowing for differences in ligand conformational entropy. To quantitatively test these hypothesis, we perform extensive molecular dynamics simulations (∼0.9 milliseconds) along with Markov state modeling and deep learning-based VAMPnets to provide an interpretable characterization of the anandamide binding process to cannabinoid receptors and explain its selectivity for CB1. Our findings reveal that the distinct N-terminus positions along lipid access channels between TM1 and TM7 lead to different binding mechanisms and interactions between anandamide and the binding pocket residues. To validate the critical stabilizing interactions along the binding pathway, relative free energy calculations of anandamide analogs are used. Moreover, the larger CB2pocket volume increases the entropic effects of ligand binding by allowing higher ligand fluctuations but reduced stable interactions. Therefore, the opposing enthalpy and entropy effects between the receptors shape the endocannabinoid selectivity. Overall, the CB1selectivity of anandamide is explained by the dominant enthalpy contributions due to ligand-protein interactions in stable binding poses. This study shed lights on potential selectivity mechanisms for endocannabinoids that would aid in the discovery of CB selective drugs
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Structural insights into melatonin receptors
The long‐anticipated high‐resolution structures of the human melatonin G protein‐coupled receptors MT1and MT2, involved in establishing and maintaining circadian rhythm, were obtained in complex with two melatonin analogs and two approved anti‐insomnia and antidepression drugs using X‐ray free‐electron laser serial femtosecond crystallography. The structures shed light on the overall conformation and unusual structural features of melatonin receptors, as well as their ligand binding sites and the melatonergic pharmacophore, thereby providing insights into receptor subtype selectivity. The structures revealed an occluded orthosteric ligand binding site with a membrane‐buried channel for ligand entry in both receptors, and an additional putative ligand entry path in MT2from the extracellular side. This unexpected ligand entry mode contributes to facilitating the high specificity with which melatonin receptors bind their cognate ligand and exclude structurally similar molecules such as serotonin, the biosynthetic precursor of melatonin. Finally, the MT2structure allowed accurate mapping of type 2 diabetes‐related single‐nucleotide polymorphisms, where a clustering of residues in helices I and II on the protein–membrane interface was observed which could potentially influence receptor oligomerization. The role of receptor oligomerization is further discussed in light of the differential interaction of MT1and MT2with GPR50, a regulatory melatonin coreceptor. The melatonin receptor structures will facilitate design of selective tool compounds to further dissect the specific physiological function of each receptor subtype as well as provide a structural basis for next‐generation sleeping aids and other drugs targeting these receptors with higher specificity and fewer side effects.
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- Award ID(s):
- 1231306
- PAR ID:
- 10457444
- Publisher / Repository:
- Wiley-Blackwell
- Date Published:
- Journal Name:
- The FEBS Journal
- Volume:
- 287
- Issue:
- 8
- ISSN:
- 1742-464X
- Page Range / eLocation ID:
- p. 1496-1510
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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