Beta(1)-selective agonist (-)-1-(3,4-dimethoxyphenetylamino)-3-(3,4-dihydroxy)-2-propanol [(-)-RO363] differentially interacts with key amino acids responsible for beta(1)-selective binding in resting and active states

Adrenoceptors: Receptors, Ligands and Their Clinical Uses, Molecular Pharmacology and Assays

The nine G protein-coupled adrenoceptor subtypes are where the endogenous catecholamines adrenaline and noradrenaline interact with cells. Since they are important therapeutic targets, over a century of effort has been put into developing drugs that modify their activity. This chapter provides an outline of how we have arrived at current knowledge of the receptors, their physiological roles and the methods used to develop ligands. Initial studies in vivo and in vitro with isolated organs and tissues progressed to cell-based techniques and the use of cloned adrenoceptor subtypes together with high-throughput assays that allow close examination of receptors and their signalling pathways. The crystal structures of many of the adrenoceptor subtypes have now been determined opening up new possibilities for drug development.

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.

Differences in allosteric communication pipelines in the inactive and active states of a GPCR

G-protein-coupled receptors (GPCRs) are membrane proteins that allosterically transduce the signal of ligand binding in the extracellular (EC) domain to couple to proteins in the intracellular (IC) domain. However, the complete pathway of allosteric communication from the EC to the IC domain, including the role of individual amino acids in the pathway is not known. Using the correlation in torsion angle movements calculated from microseconds-long molecular-dynamics simulations, we elucidated the allosteric pathways in three different conformational states of β2-adrenergic receptor (β2AR): 1), the inverse-agonist-bound inactive state; 2), the agonist-bound intermediate state; and (3), the agonist- and G-protein-bound fully active state. The inactive state is less dynamic compared with the intermediate and active states, showing dense clusters of allosteric pathways (allosteric pipelines) connecting the EC with the IC domain. The allosteric pipelines from the EC domain to the IC domain are weakened in the intermediate state, thus decoupling the EC domain from the IC domain and making the receptor more dynamic compared with the other states. Also, the orthosteric ligand-binding site becomes the initiator region for allosteric communication in the intermediate state. This finding agrees with the paradigm that the nature of the agonist governs the specific signaling state of the receptor. These results provide an understanding of the mechanism of allosteric communication in class A GPCRs. In addition, our analysis shows that mutations that affect the ligand efficacy, but not the binding affinity, are located in the allosteric pipelines. This clarifies the role of such mutations, which has hitherto been unexplained.

Comparison of dynamics of extracellular accesses to the β(1) and β(2) adrenoceptors binding sites uncovers the potential of kinetic basis of antagonist selectivity

From the molecular mechanism of antagonist unbinding in the β(1) and β(2) adrenoceptors investigated by steered molecular dynamics, we attempt to provide further possibilities of ligand subtype and subspecies selectivity. We have simulated unbinding of β(1)-selective Esmolol and β(2)-selective ICI-118551 from both receptors to the extracellular environment and found distinct molecular features of unbinding. By calculating work profiles, we show different preference in antagonist unbinding pathways between the receptors, in particular, perpendicular to the membrane pathway is favourable in the β(1) adrenoceptor, whereas the lateral pathway involving helices 5, 6 and 7 is preferable in the β(2) adrenoceptor. The estimated free energy change of unbinding based on the preferable pathway correlates with the experimental ligand selectivity. We then show that the non-conserved K347 (6.58) appears to facilitate in guiding Esmolol to the extracellular surface via hydrogen bonds in the β(1) adrenoceptor. In contrast, hydrophobic and aromatic interactions dominate in driving ICI-118551 through the easiest pathway in the β(2) adrenoceptor. We show how our study can stimulate design of selective antagonists and discuss other possible molecular reasons of ligand selectivity, involving sequential binding of agonists and glycosylation of the receptor extracellular surface.

Molecular basis of ligand dissociation in β-adrenergic receptors

The important and diverse biological functions of β-adrenergic receptors (βARs) have promoted the search for compounds to stimulate or inhibit their activity. In this regard, unraveling the molecular basis of ligand binding/unbinding events is essential to understand the pharmacological properties of these G protein-coupled receptors. In this study, we use the steered molecular dynamics simulation method to describe, in atomic detail, the unbinding process of two inverse agonists, which have been recently co-crystallized with β₁ and β₂ARs subtypes, along four different channels. Our results indicate that this type of compounds likely accesses the orthosteric binding site of βARs from the extracellular water environment. Importantly, reconstruction of forces and energies from the simulations of the dissociation process suggests, for the first time, the presence of secondary binding sites located in the extracellular loops 2 and 3 and transmembrane helix 7, where ligands are transiently retained by electrostatic and Van der Waals interactions. Comparison of the residues that form these new transient allosteric binding sites in both βARs subtypes reveals the importance of non-conserved electrostatic interactions as well as conserved aromatic contacts in the early steps of the binding process.

Synthesis, 3D-QSAR, and structural modeling of benzolactam derivatives with binding affinity for the D(2) and D(3) receptors

A series of 37 benzolactam derivatives were synthesized, and their respective affinities for the dopamine D(2) and D(3) receptors evaluated. The relationships between structures and binding affinities were investigated using both ligand-based (3D-QSAR) and receptor-based methods. The results revealed the importance of diverse structural features in explaining the differences in the observed affinities, such as the location of the benzolactam carbonyl oxygen, or the overall length of the compounds. The optimal values for such ligand properties are slightly different for the D(2) and D(3) receptors, even though the binding sites present a very high degree of homology. We explain these differences by the presence of a hydrogen bond network in the D(2) receptor which is absent in the D(3) receptor and limits the dimensions of the binding pocket, causing residues in helix 7 to become less accessible. The implications of these results for the design of more potent and selective benzolactam derivatives are presented and discussed.

The structure and function of G-protein-coupled receptors

G-protein-coupled receptors (GPCRs) mediate most of our physiological responses to hormones, neurotransmitters and environmental stimulants, and so have great potential as therapeutic targets for a broad spectrum of diseases. They are also fascinating molecules from the perspective of membrane-protein structure and biology. Great progress has been made over the past three decades in understanding diverse GPCRs, from pharmacology to functional characterization in vivo. Recent high-resolution structural studies have provided insights into the molecular mechanisms of GPCR activation and constitutive activity.

Structure of a beta1-adrenergic G-protein-coupled receptor

G-protein-coupled receptors have a major role in transmembrane signalling in most eukaryotes and many are important drug targets. Here we report the 2.7 A resolution crystal structure of a beta(1)-adrenergic receptor in complex with the high-affinity antagonist cyanopindolol. The modified turkey (Meleagris gallopavo) receptor was selected to be in its antagonist conformation and its thermostability improved by earlier limited mutagenesis. The ligand-binding pocket comprises 15 side chains from amino acid residues in 4 transmembrane alpha-helices and extracellular loop 2. This loop defines the entrance of the ligand-binding pocket and is stabilized by two disulphide bonds and a sodium ion. Binding of cyanopindolol to the beta(1)-adrenergic receptor and binding of carazolol to the beta(2)-adrenergic receptor involve similar interactions. A short well-defined helix in cytoplasmic loop 2, not observed in either rhodopsin or the beta(2)-adrenergic receptor, directly interacts by means of a tyrosine with the highly conserved DRY motif at the end of helix 3 that is essential for receptor activation.

GPCR engineering yields high-resolution structural insights into beta2-adrenergic receptor function

The beta2-adrenergic receptor (beta2AR) is a well-studied prototype for heterotrimeric guanine nucleotide-binding protein (G protein)-coupled receptors (GPCRs) that respond to diffusible hormones and neurotransmitters. To overcome the structural flexibility of the beta2AR and to facilitate its crystallization, we engineered a beta2AR fusion protein in which T4 lysozyme (T4L) replaces most of the third intracellular loop of the GPCR ("beta2AR-T4L") and showed that this protein retains near-native pharmacologic properties. Analysis of adrenergic receptor ligand-binding mutants within the context of the reported high-resolution structure of beta2AR-T4L provides insights into inverse-agonist binding and the structural changes required to accommodate catecholamine agonists. Amino acids known to regulate receptor function are linked through packing interactions and a network of hydrogen bonds, suggesting a conformational pathway from the ligand-binding pocket to regions that interact with G proteins.

Role of Tyr356(7.43) and Ser190(4.57) in Antagonist Binding in the Rat β1-Adrenergic Receptor

Site-directed mutagenesis and photoaffinity labeling experiments suggest the existence of at least two distinct binding orientations for aryloxypropanolamine competitive antagonists in the beta-adrenergic receptor (beta-AR), one where the aryloxy moiety is located near transmembrane alpha-helix 7 (tm 7) and another where it is near tm 5. To explore a hydrophobic pocket involving tms 1, 2, 3, and 7 for potential aryloxy interaction sites, we selected Tyr(356(7.43)) and Trp(134(3.28)) in the rat beta(1)-AR for site-directed mutagenesis studies. Ser(190(4.57)) was also investigated, as the equivalent residues are known antagonist interaction sites in the muscarinic M(1) and the dopamine D(2) receptors. Binding affinities (pK(i)) of a series of structurally diverse aryloxypropanolamine competitive antagonists were determined for wild type and Y356A, Y356F, W134A, and S190A mutant rat beta(1)-ARs stably expressed in Chinese hamster ovary cells. To visualize possible antagonist/receptor interactions, the compounds were docked into a three-dimensional model of the wild-type rat beta(1)-AR. The results indicate that Tyr(356(7.43)) is an important aromatic interaction site for five of the eight competitive antagonists studied, whereas none of the compounds appeared to interact directly with Trp(134(3.28)). Only two of the competitive antagonists interacted with Ser(190(4.57)) on tm 4. Overall, the results extend our understanding of how beta(1)-AR competitive antagonists bind to the hydrophobic pocket involving tms 1, 2, 3, and 7; highlight the importance of Tyr(356(7.43)) in this binding pocket; and demonstrate the involvement of tm 4 in competitive antagonist binding.

Agonist binding and activation of the rat beta(1)-adrenergic receptor: role of Trp(134(3.28)), Ser(190(4.57)) and Tyr(356(7.43))

We investigated the role of Trp(134(3.28)), Ser(190(4.57)) and Tyr(356(7.43)) in agonist binding to, and activation of, the rat beta(1)-adrenergic receptor by comparing pK(i)s and functional responses of W134A, S190A and Y356F mutant receptors to wild type, all stably expressed in CHO cells. All three mutations significantly (P < 0.05) reduced adenylyl cyclase intrinsic activity (IA) compared to wild type in response to stimulation with both (-)-isoprenaline (53-88%) and (-)-RO363 (46-61%), and there was no significant correlation either between IA or pD(2) and pK(i) (P > 0.4), suggesting that changes in pK(i) were not sufficient to explain the fall in adenylyl cyclase activity. The most pronounced reduction in affinity (126-fold, P < 0.01) was displayed by xamoterol for the Y356F mutation, suggesting that xamoterol is able to directly interact with Tyr(356(7.43)). For the other agonists, the change in pK(i) values for the mutant receptors ranged from a 20-fold decrease to a 2-fold increase compared to the wild type. In a three-dimensional model of the rat beta(1)-adrenergic receptor, Trp(134(3.28)) and Tyr(356(7.43)) form part of a hydrophobic binding pocket involving residues in transmembrane helices 1, 2, 3 and 7. Our results suggest that Trp(134(3.28)) and Tyr(356(7.43)), together with Trp(353(7.40)), are able to interact via pi-pi interactions to stabilize the extracellular ends of transmembrane helices 3 and 7. Ser(190(4.57)) appears to be involved in a hydrogen bonding network, which maintains the spatial relationship between transmembrane helices 3 and 4. These interhelical interactions suggest that the three mutated residues stabilize the active receptor state by maintaining the proper packing of their respective transmembrane helix within the helix bundle, facilitating the appropriate movement and rotation of the transmembrane regions during the activation process.

Probing receptor structure/function with chimeric G-protein-coupled receptors

Owing its name to an image borrowed from Greek mythology, a chimera is seen to represent a new entity created as a composite from existing creatures or, in this case, molecules. Making use of various combinations of three basic domains of the receptors (i.e., exofacial, transmembrane, and cytoplasmic segments) that couple agonist binding into activation of effectors through heterotrimeric G-proteins, molecular pharmacology has probed the basic organization, structure/function relationships of this superfamily of heptahelical receptors. Chimeric G-protein-coupled receptors obviate the need for a particular agonist ligand when the ligand is resistant to purification or, in the case of orphan receptors, is not known. Chimeric receptors created from distant members of the heptahelical receptors enable new strategies in understanding how these receptors transduce agonist binding into receptor activation and may be able to offer insights into the evolution of G-protein-coupled receptors from yeast to humans.