Molecular interactions of agonist and inverse agonist ligands at serotonin 5-HT2CG protein-coupled receptors: computational ligand docking and molecular dynamics studies validated by experimental mutagenesis results

Aminergic GPCR-Ligand Interactions: A Chemical and Structural Map of Receptor Mutation Data

The aminergic family of G protein-coupled receptors (GPCRs) plays an important role in various diseases and represents a major drug discovery target class. Structure determination of all major aminergic subfamilies has enabled structure-based ligand design for these receptors. Site-directed mutagenesis data provides an invaluable complementary source of information for elucidating the structural determinants of binding of different ligand chemotypes. The current study provides a comparative analysis of 6692 mutation data points on 34 aminergic GPCR subtypes, covering the chemical space of 540 unique ligands from mutagenesis experiments and information from experimentally determined structures of 52 distinct aminergic receptor-ligand complexes. The integrated analysis enables detailed investigation of structural receptor-ligand interactions and assessment of the transferability of combined binding mode and mutation data across ligand chemotypes and receptor subtypes. An overview is provided of the possibilities and limitations of using mutation data to guide the design of novel aminergic receptor ligands.

Mutagenesis Analysis Reveals Distinct Amino Acids of the Human Serotonin 5-HT2C Receptor Underlying the Pharmacology of Distinct Ligands

While exploring the structure-activity relationship of 4-phenyl-2-dimethylaminotetralins (PATs) at serotonin 5-HT_2C receptors, we discovered that relatively minor modification of PAT chemistry impacts function at 5-HT_2C receptors. In HEK293 cells expressing human 5-HT_2C-INI receptors, for example, (-)-trans-3'-Br-PAT and (-)-trans-3'-Cl-PAT are agonists regarding Gα_q-inositol phosphate signaling, whereas (-)-trans-3'-CF₃-PAT is an inverse agonist. To investigate the ligand-receptor interactions that govern this change in function, we performed site-directed mutagenesis of 14 amino acids of the 5-HT_2C receptor based on molecular modeling and reported G protein-coupled receptor crystal structures, followed by molecular pharmacology studies. We found that S3.36, T3.37, and F5.47 in the orthosteric binding pocket are critical for affinity (K_i) of all PATs tested, we also found that F6.44, M6.47, C7.45, and S7.46 are primarily involved in regulating EC/IC₅₀ functional potencies of PATs. We discovered that when residue S5.43, N6.55, or both are mutated to alanine, (-)-trans-3'-CF₃-PAT switches from inverse agonist to agonist function, and when N6.55 is mutated to leucine, (-)-trans-3'-Br-PAT switches from agonist to inverse agonist function. Notably, most point-mutations that affected PAT pharmacology did not significantly alter affinity (K_D) of the antagonist radioligand [³H]mesulergine, but every mutation tested negatively impacted serotonin binding. Also, amino acid mutations differentially affected the pharmacology of other commercially available 5-HT_2C ligands tested. Collectively, the data show that functional outcomes shared by different ligands are mediated by different amino acids and that some 5-HT_2C receptor residues important for pharmacology of one ligand are not necessarily important for another ligand.

α-Mangostin Extraction from the Native Mangosteen (Garcinia mangostana L.) and the Binding Mechanisms of α-Mangostin to HSA or TRF

In order to obtain the biological active compound, α-mangostin, from the traditional native mangosteen (Garcinia mangostana L.), an extraction method for industrial application was explored. A high yield of α-mangostin (5.2%) was obtained by extraction from dried mangosteen pericarps with subsequent purification on macroporous resin HPD-400. The chemical structure of α-mangostin was verified mass spectrometry (MS), nuclear magnetic resonance (¹H NMR and ¹³C NMR), infrared spectroscopy (IR) and UV-Vis spectroscopy. The purity of the obtained α-mangostin was 95.6% as determined by HPLC analysis. The binding of native α-mangostin to human serum albumin (HSA) or transferrin (TRF) was explored by combining spectral experiments with molecular modeling. The results showed that α-mangostin binds to HSA or TRF as static complexes but the binding affinities were different in different systems. The binding constants and thermodynamic parameters were measured by fluorescence spectroscopy and absorbance spectra. The association constant of HSA or TRF binding to α-mangostin is 6.4832×10⁵ L/mol and 1.4652×10⁵ L/mol at 298 K and 7.8619×10⁵ L/mol and 1.1582×10⁵ L/mol at 310 K, respectively. The binding distance, the energy transfer efficiency between α-mangostin and HSA or TRF were also obtained by virtue of the Förster theory of non-radiation energy transfer. The effect of α-mangostin on the HSA or TRF conformation was analyzed by synchronous spectrometry and fluorescence polarization studies. Molecular docking results reveal that the main interaction between α-mangostin and HSA is hydrophobic interactions, while the main interaction between α-mangostin and TRF is hydrogen bonding and Van der Waals forces. These results are consistent with spectral results.