Human serotonin 5-HT_2C G protein-coupled receptor homology model from the β₂ adrenoceptor structure: Ligand docking and mutagenesis studies

Activation of the serotonin (5-hydroxytryptamine, 5-HT) 5HT_2C G protein-coupled receptor (GPCR) is proposed as novel pharmacotherapy for obesity and neuropsychiatric disorders. In contrast, activation of the 5-HT _2A and 5-HT_2B GPCRs is associated with untoward hallucinogenic and cardiopulmonary effects, respectively. There is no crystal structure available to guide design of 5-HT_2C receptor-specific ligands. For this reason, a homology model of the 5-HT_2C receptor was built based on the crystal structure of the human β₂ adrenoceptor GPCR to delineate molecular determinants of ligand-receptor interactions for drug design purposes. Computational and experimental studies were carried out to validate the model. Binding of N(CH₃) ₂-PAT [(1R, 3S)-(-)-trans-1-phenyl-3-N,N-dimethylamino-1,2,3,4- tetrahydronaphthalene], a novel 5-HT_2C agonist/5-HT_2A/2B inverse agonist, and its secondary [NH(CH₃)-PAT] and primary (NH ₂-PAT) amine analogs were studied at the 5-HT_2C wild type (WT) and D3.32A, S3.36A, and Y7.43A 5-HT_2C point-mutated receptors. Reference ligands included the tertiary amines lisuride and mesulergine and the primary amine 5-HT. Modeling results indicated that 5-HT_2C residues D3.32, S3.36, and Y7.43 play a role in ligand binding. Experimental ligand binding results with WT and point-mutated receptors confirmed the impact of D3.32, S3.36, and Y7.43 on ligand affinity.