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Histamine H4 receptor (H4R) orthologues are G‐protein‐coupled receptors (GPCRs) that exhibit species‐dependent basal activity. In contrast to the basally inactive mouse H4R (mH4R), human H4R (hH4R) shows a high degree of basal activity. We have performed long‐timescale molecular dynamics simulations and rigidity analyses on wild‐type hH4R, the experimentally characterized hH4R variants S179M, F169V, F169V+S179M, F168A, and on mH4R to investigate the molecular nature of the differential basal activity. H4R variant‐dependent differences between essential motifs of GPCR activation and structural stabilities correlate with experimentally determined basal activities and provide a molecular explanation for the differences in basal activation. Strikingly, during the MD simulations, F16945.55 dips into the orthosteric binding pocket only in the case of hH4R, thus adopting the role of an agonist and contributing to the stabilization of the active state. The results shed new light on the molecular mechanism of basal H4R activation that are of importance for other GPCRs. [ABSTRACT FROM AUTHOR]

Free-energy profiles for the activation/deactivation of the β 2 -adrenergic receptor (ADRB2) with neutral antagonist and inverse agonist ligands have been determined with well-tempered multiple-walker (MW) metadynamics simulations. The inverse agonists carazolol and ICI118551 clearly favor single inactive conformational minima in both the binary and ternary ligand-receptor-G-protein complexes, in accord with the inverse-agonist activity of the ligands. The behavior of neutral antagonists is more complex, as they seem also to affect the recruitment of the G-protein. The results are analyzed in terms of the conformational states of the well-known microswitches that have been proposed as indicators of receptor activity.

We use enhanced-sampling simulations with an effective collective variable to study the activation of the β 2 -adrenergic receptor in the presence of ligands with different efficacy. The free-energy profiles are computed for the ligand-free ( apo ) receptor and binary ( apo -receptor + G-protein α-subunit and receptor + ligand) and ternary complexes. The results are not only compatible with available experiments but also allow unprecedented structural insight into the nature of GPCR conformations along the activation pathway and their role in the activation mechanism. In particular, the simulations reveal an unexpected mode of action of partial agonists such as salmeterol and salbutamol that arises already in the binary complex without the G-protein. Specific differences in the polar interactions with residues in TM5, which are required to stabilize an optimal TM6 conformation that facilitates G-protein binding and receptor activation, play a major role in differentiating them from full agonists.

Histamine H 4 receptor (H 4 R) orthologues are G-protein-coupled receptors (GPCRs) that exhibit species-dependent basal activity. In contrast to the basally inactive mouse H 4 R (mH 4 R), human H 4 R (hH 4 R) shows a high degree of basal activity. We have performed long-timescale molecular dynamics simulations and rigidity analyses on wild-type hH 4 R, the experimentally characterized hH 4 R variants S179M, F169V, F169V+S179M, F168A, and on mH 4 R to investigate the molecular nature of the differential basal activity. H 4 R variant-dependent differences between essential motifs of GPCR activation and structural stabilities correlate with experimentally determined basal activities and provide a molecular explanation for the differences in basal activation. Strikingly, during the MD simulations, F169 45.55 dips into the orthosteric binding pocket only in the case of hH 4 R, thus adopting the role of an agonist and contributing to the stabilization of the active state. The results shed new light on the molecular mechanism of basal H 4 R activation that are of importance for other GPCRs., (© 2019 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim.)

Protein nanobodies have been used successfully as surrogates for unstable G-proteins in order to crystallize G-protein-coupled receptors (GPCRs) in their active states. We used molecular dynamics (MD) simulations, including metadynamics enhanced sampling, to investigate the similarities and differences between GPCR-agonist ternary complexes with the α-subunits of the appropriate G-proteins and those with the protein nanobodies (intracellular binding partners, IBPs) used for crystallization. In two of the three receptors considered, the agonist-binding mode differs significantly between the two alternative ternary complexes. The ternary-complex model of GPCR activation entails enhancement of ligand binding by bound IBPs: Our results show that IBP-specific changes can alter the agonist binding modes and thus also the criteria for designing GPCR agonists., (© 2017 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim.)