Functional studies cast light on receptor states

doi:10.1016/j.tips.2015.05.008

Trends in Pharmacological Sciences

Volume 36, Issue 9, September 2015, Pages 596-604

https://doi.org/10.1016/j.tips.2015.05.008 Get rights and content

Highlights

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Ligand affinity and efficacy are determined by GPCR state constants.
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Relative active state affinity constants can be estimated from concentration–response curves.
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Analysis of allosteric interactions can provide estimates of all receptor state parameters.
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Receptor state parameters are useful for measuring biased signaling and receptor selectivity.
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Estimates of receptor state parameters are unperturbed by receptor coupling proteins.

Contemporary analysis of the functional responses of G-protein-coupled receptors (GPCRs) usually addresses drug–receptor interactions from the perspective of the average behavior of the receptor population. This behavior is characterized in terms of observed affinity and efficacy. Efficacy is a measure of how well a drug activates the receptor population and observed affinity a measure of how potently a drug occupies the receptor population. The latter is quantified in terms of the dissociation constant of the ligand–receptor complex. At a deeper level of analysis, drug–receptor interactions are described in terms of ligand affinity constants for active and inactive receptor states. Unlike observed affinity and efficacy, estimates of receptor state affinity constants are unperturbed by G proteins, guanine nucleotides, or other signaling proteins that interact with the receptor. Recent advances in the analysis of the functional responses of GPCRs have enabled the estimation of receptor state affinity constants. These constants provide a more fundamental measure of drug–receptor interactions and are useful in analyzing structure–activity relationships and in quantifying allosterism, biased signaling, and receptor-subtype selectivity.

Section snippets

A single-receptor view of drug action

Drug–receptor interactions are often illuminated when viewed from the perspective of single receptors. Single receptors isomerize between active and inactive states depending on the nature of the ligand bound to them (Figure 1A) 1, 2, 3, 4. When unbound, most receptors remain inactive except for occasional fleeting activations (constitutive activity). These activations have greater frequency and longer duration when the receptor is bound with an agonist. Agonists bind to both receptor states,

A model for GPCR activation

The simulation depicted in Figure 1 adequately portrays activation of the soluble ligand-binding domain of the dimeric metabotropic glutamate receptor 4 [4]. An analogous model with two cooperatively linked orthosteric sites would resemble the behavior of many ligand-gated ion channels of the Cys-loop and glutamate families [11]. However, how does the simulation relate to a receptor coupled to G proteins?

The interactions among orthosteric ligand (D), receptor states (R and R*), G protein, and

Relationship between population parameters and receptor state affinity constants

When a ligand induces a protein to assume a different conformation, some of the intrinsic binding energy associated with the induced state is used to cause the conformational change [19]. Hence, the observed affinity constant of a ligand for the receptor population can be much less than its affinity for the state that it induces. The amount of agonist-induced activation of a GPCR can be expressed as a ratio (activation ratio, R_act) equivalent to the fractional amount of ligand–receptor

A relative estimate of the active state affinity constant

An easy state parameter to estimate in functional studies is a relative value of the active state affinity constant. For the case of two full agonists A and B, relative affinity for the active state (K_act-B/K_act-A) is equivalent to the corresponding ratio of potencies (EC_50-A/EC_50-B) 25, 26. For full and partial agonists, the ratio of equiactive agonist concentrations approaches a constant limiting value at low concentrations of the agonists (EAMR) [26]. EAMR was later termed RA_i and defined as

Analysis of allosterism yields all of the receptor state parameters

Allosterism is defined by a subcommittee of the International Union of Basic and Clinical Pharmacology as a modification of the properties of a ligand caused by the binding of a second ligand at a distinct site [33]. This mechanism can account for reciprocal modulation in ligand binding. It can also account for effects on ligand efficacy that are unrelated to a change in the conformation of the receptor. For example, an allosteric inhibitor could bind to the open state of a ligand-gated ion

Implications for drug discovery

With estimates of an agonist's receptor state affinity constants in hand, an investigator has a means of comparing the activity of an agonist at different receptor subtypes and determining its ability to persuade a given receptor to signal through different pathways. Different receptor coupling proteins provide a window for estimating agonist affinity for effector-selective states of the receptor [20]. These estimates depend only on the active and inactive states of the receptor involved in

Concluding remarks

The past few years have witnessed a surge in our understanding of receptor structure, which will surely continue as more active and inactive receptor structures are solved. The population analysis that has driven pharmacology over the past few decades is insufficient for advancing analysis of receptor function in the present era. A scientist interested in designing a more potent analog of a drug, for example, might dock the parent drug onto the active and inactive receptor structures in silico

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Trends in Pharmacological Sciences

ReviewFunctional studies cast light on receptor states

Highlights

Section snippets

A single-receptor view of drug action

A model for GPCR activation

Relationship between population parameters and receptor state affinity constants

A relative estimate of the active state affinity constant

Analysis of allosterism yields all of the receptor state parameters

Implications for drug discovery

Concluding remarks

J. Mol. Biol.

J. Pharmacol. Toxicol. Methods

Cell

Biophys. J.

J. Biol. Chem.

J. Biol. Chem.

Cell

The gating isomerization of neuromuscular acetylcholine receptors

J. Physiol.

Crystal structure of the human β2 adrenergic G-protein-coupled receptor

Nature

Crystal structure of the β2 adrenergic receptor–Gs protein complex

Nature

Fine tuning of sub-millisecond conformational dynamics controls metabotropic glutamate receptors agonist efficacy

Nat. Commun.

Binding, gating, affinity and efficacy: the interpretation of structure–activity relationships for agonists and of the effects of mutating receptors

Br. J. Pharmacol.

Operational models of pharmacological agonism

Proc. Biol. Sci.

Comparison of dissociation constants and of relative efficacies of selected agonists acting on parasympathetic receptors

Ann. N. Y. Acad. Sci.

A modification of receptor theory

Br. J. Pharmacol.

Agonist binding, agonist affinity and agonist efficacy at G protein-coupled receptors

Br. J. Pharmacol.

On the stochastic properties of bursts of single ion channel openings and of clusters of bursts

Philos. Trans. R. Soc. Lond. B: Biol. Sci.

Signaling through the muscarinic receptor–adenylate cyclase system of the heart is buffered against GTP over a range of concentrations

Mol. Pharmacol.

A kinetic model of GPCRs: analysis of G protein activity, occupancy, coupling and receptor-state affinity constants

J. Recept. Signal. Transduct. Res.

Structures of active conformations of Gi alpha 1 and the mechanism of GTP hydrolysis

Science

The use of β-haloalkylamines in the differentiation of receptors and in the determination of dissociation constants of receptor–agonist complexes

Adv. Drug Res.

Analysis of functional responses at G protein coupled receptors: estimation of relative affinity constants for the inactive receptor state

J. Pharmacol. Exp. Ther.

Binding energy, specificity, and enzymic catalysis: the Circe effect

Adv. Enzymol. Relat. Areas Mol. Biol.

Review
Functional studies cast light on receptor states

Crystal structure of the human β₂ adrenergic G-protein-coupled receptor

Crystal structure of the β₂ adrenergic receptor–G_s protein complex

Structures of active conformations of G_i alpha 1 and the mechanism of GTP hydrolysis