Ligand-Dependent Modulation of the Dynamics of Intracellular Loops Dictates Functional Selectivity of 5-HT2AR

NMR Structure of Retinal Guanylate Cyclase Activating Protein 5 (GCAP5) with R22A Mutation That Abolishes Dimerization and Enhances Cyclase Activation

Guanylate cyclase activating protein-5 (GCAP5) in zebrafish photoreceptors promotes the activation of membrane receptor retinal guanylate cyclase (GC-E). Previously, we showed the R22A mutation in GCAP5 (GCAP5R22A) abolishes dimerization of GCAP5 and activates GC-E by more than 3-fold compared to that of wild-type GCAP5 (GCAP5WT). Here, we present ITC, NMR, and functional analysis of GCAP5R22A to understand how R22A causes a decreased dimerization affinity and increased cyclase activation. ITC experiments reveal GCAP5R22A binds a total of 3 Ca2+, including two sites in the nanomolar range followed by a single micromolar site. The two nanomolar sites in GCAP5WT were not detected by ITC, suggesting that R22A may affect the binding of Ca2+ to these sites. The NMR-derived structure of GCAP5R22A is overall similar to that of GCAP5WT (RMSD = 2.3 Å), except for local differences near R22A (Q19, W20, Y21, and K23) and an altered orientation of the C-terminal helix near the N-terminal myristate. GCAP5R22A lacks an intermolecular salt bridge between R22 and D71 that may explain the weakened dimerization. We present a structural model of GCAP5 bound to GC-E in which the R22 side-chain contacts exposed hydrophobic residues in GC-E. Cyclase assays suggest that GC-E binds to GCAP5R22A with ∼25% higher affinity compared to GCAP5WT, consistent with more favorable hydrophobic contact by R22A that may help explain the increased cyclase activation.

Biased Signaling in Mutated Variants of β2-Adrenergic Receptor: Insights from Molecular Dynamics Simulations

The molecular basis of receptor bias in G protein-coupled receptors (GPCRs) caused by mutations that preferentially activate specific intracellular transducers over others remains poorly understood. Two experimentally identified biased variants of β2-adrenergic receptors (β2AR), a prototypical GPCR, are a triple mutant (T68F, Y132A, and Y219A) and a single mutant (Y219A); the former bias the receptor toward the β-arrestin pathway by disfavoring G protein engagement, while the latter induces G protein signaling explicitly due to selection against GPCR kinases (GRKs) that phosphorylate the receptor as a prerequisite of β-arrestin binding. Though rigorous characterizations have revealed functional implications of these mutations, the atomistic origin of the observed transducer selectivity is not clear. In this study, we investigated the allosteric mechanism of receptor bias in β2AR using microseconds of all-atom Gaussian accelerated molecular dynamics (GaMD) simulations. Our observations reveal distinct rearrangements in transmembrane helices, intracellular loop 3, and critical residues R1313.50 and Y3267.53 in the conserved motifs D(E)RY and NPxxY for the mutant receptors, leading to their specific transducer interactions. Moreover, partial dissociation of G protein from the receptor core is observed in the simulations of the triple mutant in contrast to the single mutant and wild-type receptor. The reorganization of allosteric communications from the extracellular agonist BI-167107 to the intracellular receptor-transducer interfaces drives the conformational rearrangements responsible for receptor bias in the single and triple mutants. The molecular insights into receptor bias of β2AR presented here could improve the understanding of biased signaling in GPCRs, potentially opening new avenues for designing novel therapeutics with fewer side-effects and superior efficacy.

Collective and Coordinated Conformational Changes Determine Agonism or Antagonism at the Human Trace Amine-Associated Receptor 1

The human trace amine-associated receptor (hTAAR1), a G protein-coupled receptor, has been postulated as a new target in the treatment of neuropsychiatric conditions. The mechanism associated with activation or inactivation by agonists or antagonists in hTAAR1 and other GPCRs has not yet been fully elucidated. In this study, we combined computational methods including homology modeling, docking, and molecular dynamic simulations to reveal novel conformational changes associated with agonist and antagonist interactions in hTAAR1. Our findings suggest a differential cascade of coordinated movements based on the presence of either an agonist or antagonist and primarily involving the second extracellular loop, transmembrane domain 5, and the third intracellular domains of hTAAR1. Our study provides an opportunity to predict the effects on new ligands with agonistic or antagonistic activity at hTAAR1 based on the reported conformational changes.

Molecular Insights into the Heterotropic Allosteric Mechanism in Cytochrome P450 3A4-Mediated Midazolam Metabolism

Cytochrome P450 3A4 (CYP3A4) is the main P450 enzyme for drug metabolism and drug-drug interactions (DDIs), as it is involved in the metabolic process of approximately 50% of drugs. A detailed mechanistic elucidation of DDIs mediated by CYP3A4 is commonly believed to be critical for drug optimization and rational use. Here, two typical probes, midazolam (MDZ, substrate) and testosterone (TST, allosteric effector), are used to investigate the molecular mechanism of CYP3A4-mediated heterotropic allosteric interactions, through conventional molecular dynamics (cMD) and well-tempered metadynamics (WT-MTD) simulations. Distance monitoring shows that TST can stably bind in two potential peripheral sites (Site 1 and Site 2) of CYP3A4. The binding of TST at these two sites can induce conformational changes in CYP3A4 flexible loops on the basis of conformational analysis, thereby promoting the transition of the MDZ binding mode and affecting the ratio of MDZ metabolites. According to the results of the residue interaction network, multiple allosteric communication pathways are identified that can provide vivid and applicable insights into the heterotropic allostery of TST on MDZ metabolism. Comparing the regulatory effects and the communication pathways, the allosteric effect caused by TST binding in Site 2 seems to be more pronounced than in Site 1. Our findings could provide a deeper understanding of CYP3A4-mediated heterotropic allostery at the atomic level and would be helpful for rational drug use as well as the design of new allosteric modulators.

In Silico Study of Allosteric Communication Networks in GPCR Signaling Bias

Signaling bias is a promising characteristic of G protein-coupled receptors (GPCRs) as it provides the opportunity to develop more efficacious and safer drugs. This is because biased ligands can avoid the activation of pathways linked to side effects whilst still producing the desired therapeutic effect. In this respect, a deeper understanding of receptor dynamics and implicated allosteric communication networks in signaling bias can accelerate the research on novel biased drug candidates. In this review, we aim to provide an overview of computational methods and techniques for studying allosteric communication and signaling bias in GPCRs. This includes (i) the detection of allosteric communication networks and (ii) the application of network theory for extracting relevant information pipelines and highly communicated sites in GPCRs. We focus on the most recent research and highlight structural insights obtained based on the framework of allosteric communication networks and network theory for GPCR signaling bias.