G protein–coupled receptors (GPCRs) are a large class of cell-surface receptors with a seven-transmembrane architecture and constitute one of the largest membrane receptor families known to date. In the human genome, genes encoding GPCRs account for roughly 4% of all protein-coding genes; among approved drugs, more than one-third act directly on GPCRs as their primary targets. Thus, understanding the basic concepts of GPCR biology is critically important for studying life sciences, engaging in drug discovery, and making informed choices when selecting related reagents.
I. What Are GPCRs? — A “Seven-Transmembrane” Superfamily of Receptors
1.1 Basic Concept
G protein–coupled receptors are intrinsic membrane protein receptors that span the plasma membrane seven times. They recognize various signaling molecules on the extracellular side and, on the intracellular side, initiate signal transduction by activating G proteins (guanine nucleotide–binding proteins).
Typical GPCRs share the following features:
1. They are found only in eukaryotes (in the classical sense, no homologous G protein–coupled GPCR system has yet been identified in bacteria);
2. They are mainly located on the plasma membrane and serve as key “sensors” that allow cells to perceive changes in the external environment;
3. They are tightly linked to numerous physiological processes, including vision, olfaction, taste, cardiovascular regulation, metabolic homeostasis, and immune responses.
1.2 Diverse Ligands
GPCRs recognize an exceptionally broad spectrum of ligands, including:
1. Small molecules: odorants, neurotransmitters (e.g., dopamine, serotonin), hormones (e.g., epinephrine, norepinephrine), lipid mediators, and others;
2. Peptides and proteins: peptide hormones, chemokines, and so on;
3. Special stimuli: for example, members of the rhodopsin family can be activated by light.
Note:
1. A “ligand” is a chemical or physical signal that binds to a GPCR;
2. These ligands themselves may be carbohydrates, lipids, peptides, or even proteins—they are the binding partners of GPCRs, not “receptors” in their own right.
2. Structural Features and Classification: Seven Transmembranes and Multiple Families
2.1 Structural Overview
Typical GPCRs are intrinsic membrane proteins composed of seven transmembrane α-helices. From the N-terminus to the C-terminus, they feature:
1. An extracellular N-terminus (N-terminus)
2. Seven transmembrane α-helical domains (TM1–TM7)
3. Three extracellular loops (ECL1–3) and three intracellular loops (ICL1–3)
4. An intracellular C-terminus (C-terminus)
Among these:
a. The third intracellular loop (ICL3) and the C-terminal tail are usually key regions for interaction with G proteins and other regulatory proteins (such as β-arrestins);
b. The extracellular N-terminus and extracellular loops often contain glycosylation sites and conserved cysteine residues, which form disulfide bonds that help stabilize the receptor structure;
c. Some proteins (such as channelrhodopsins) also possess a seven-transmembrane architecture but additionally form ion channels and therefore are not considered typical GPCRs;
d. Although adiponectin receptors (ADIPOR1/2) also contain seven transmembrane domains, their membrane topology is opposite to that of GPCRs (N-terminus facing the cytosol and C-terminus facing the extracellular space), and they do not couple to G proteins. Consequently, they are generally not classified as members of the GPCR family.
2.2 Classification Systems
2.2.1 Traditional A–F Classes
Based on sequence characteristics and functional properties, GPCRs have traditionally been divided into six classes, A–F:
1. Class A (Rhodopsin-like)
a. The most numerous class, encompassing the vast majority of all GPCRs
b. Includes rhodopsin, dopamine receptors, serotonin receptors, adrenergic receptors, and others
2. Class B (Secretin family)
a. Includes multiple peptide hormone receptors, such as the glucagon receptor
3. Class C (Metabotropic glutamate receptor family)
a. Includes metabotropic glutamate receptors, GABA_B receptors, calcium-sensing receptors, as well as certain taste and chemosensory receptors
4. Classes D and E
a. Found mainly in fungi or amoebae (e.g., fungal mating pheromone receptors)
5. Class F (Frizzled/Smoothened family)
a. Participates in developmental signaling pathways such as Wnt and Hedgehog
Class A accounts for more than 80% of all GPCRs and is the type most frequently encountered in basic research and drug development.
2.2.2 GRAFS Classification System
A later system, based on genomic and sequence analyses, is the GRAFS classification:
1. Glutamate
2. Rhodopsin
3. Adhesion
4. Frizzled/Taste2
5. Secretin
This system essentially corresponds to the traditional A–F classes but adopts a more mnemonic naming scheme and is widely used in genomics and bioinformatics studies.
3. How Are GPCRs Activated? — G Proteins and Four Major Signaling Pathways
3.1 From Ligand Binding to G Protein Activation
GPCRs transmit signals by coupling to heterotrimeric G proteins composed of three subunits: Gα, Gβ, and Gγ.
In the resting state:
· Gα is bound to GDP (guanosine diphosphate) and forms a heterotrimer with Gβγ on the inner side of the plasma membrane, in proximity to the receptor.
When a ligand binds to a GPCR:
1. The receptor undergoes a conformational change, exposing key interfaces for G protein binding;
2. The receptor functions as a guanine nucleotide exchange factor (GEF), promoting the exchange of GDP for GTP on Gα;
3. Gα–GTP dissociates from Gβγ, and both Gα–GTP and Gβγ go on to activate their respective downstream effectors;
4. Gα possesses intrinsic GTPase activity and hydrolyzes GTP to GDP, then reassociates with Gβγ, returning the system to the resting state.
3.2 The Four Major Gα Families and Their Canonical Pathways
Based on sequence homology and functional properties, Gα subunits can be broadly grouped into four families:
3.2.1 Gαs and Gαi/o: The cAMP / PKA Pathway
1. Target enzyme: Adenylate cyclase (AC)
2. Functions:
a. Gαs: activates AC and promotes the conversion of ATP to cAMP;
b. Gαi/o: inhibits AC and reduces cAMP production.
3. Downstream effects:
a. cAMP acts as a second messenger to activate protein kinase A (PKA) and certain ion channels;
b. PKA further phosphorylates multiple target proteins, regulating metabolism, gene transcription, and membrane excitability.
Therefore:
i. GPCRs coupled to Gαs (such as β-adrenergic receptors) typically increase intracellular cAMP levels;
ii. GPCRs coupled to Gαi (such as some dopamine receptors) typically decrease cAMP levels.
3.2.2 Gαq/11: The PIP₂ / PLCβ / IP₃ / DAG / PKC Pathway
1. Target enzyme: Phosphatidylinositol-specific phospholipase C-β (PLCβ)
2. Reaction:
a. PLCβ hydrolyzes membrane PIP₂ (phosphatidylinositol 4,5-bisphosphate) into:
i. IP₃ (inositol 1,4,5-trisphosphate)
ii. DAG (diacylglycerol)
3. Downstream effects:
b. IP₃ binds to IP₃ receptors on the endoplasmic reticulum (ER), triggering massive release of Ca²⁺ from the ER lumen into the cytosol and thereby increasing cytosolic free Ca²⁺ concentrations;
c. DAG remains in the membrane and, together with Ca²⁺, activates protein kinase C (PKC);
d. PKC further regulates a wide range of substrate proteins.
This pathway is critically important in smooth muscle contraction, secretory activity, and many aspects of metabolic regulation.
3.2.3 Gα12/13: The Rho / Cytoskeleton Pathway
1. Gα12/13 activates specific RhoGEFs, promoting the conversion of Rho GTPases from the GDP-bound to the GTP-bound state;
2. Activated Rho regulates cytoskeletal remodeling, cell migration, changes in cell morphology, and related processes;
3. This pathway is of particular interest in research on tumor metastasis, angiogenesis, and related pathophysiological events.
Note: Intracellular Ca²⁺, calmodulin, and other factors can cross-regulate these pathways, but the classic and direct route for Rho activation remains Gα12/13 → RhoGEF → Rho.
3.2.4 Gβγ: Ion Channels and Selected AC Pathways
Although GPCR signaling is often categorized by Gα subtype, Gβγ subunits themselves also have important signaling roles, especially in receptors coupled to Gαi/o:
1. Direct modulation of multiple ion channels, including:
a. G protein–gated inwardly rectifying K⁺ channels (GIRKs);
b. Certain voltage-gated Ca²⁺ channels (such as P/Q-type and N-type).
2. Regulation of some adenylate cyclase isoforms, among other effectors.
Thus, when a GPCR is activated, signaling is not confined to a single “Gα pathway”; instead, Gα and Gβγ together form an integrated signaling network.
3.3 Biased Coupling and “Functional Selectivity”
Many GPCRs can couple to more than one Gα subtype, yet they often display a preference for one particular subtype. This preference is closely related to:
1. The intrinsic structure of the receptor;
2. The type of ligand bound (different ligands may stabilize different receptor conformations);
3. The receptor’s phosphorylation state and other post-translational modifications.
This phenomenon is known as functional selectivity or biased agonism. It has become a key concept in contemporary GPCR drug design: the goal is to design ligands that selectively activate beneficial signaling pathways while minimizing activation of pathways associated with adverse effects, thereby reducing side effects.
4. What Physiological Functions Do GPCRs Participate In? — From Sensation to Homeostasis
GPCRs are involved in virtually all major physiological processes. A few representative examples are outlined below.
4.1 Sensory Systems
1. Vision
Rhodopsin and related rhodopsin-like receptors in the retina convert light stimuli into chemical signals and are core molecules of the visual system. Light exposure induces the isomerization of retinal from the 11-cis to the all-trans configuration, triggering conformational changes in opsin, activating G proteins, and initiating downstream signaling cascades.
2. Olfaction
Olfactory receptors (a large group of GPCRs) in the olfactory epithelium and vomeronasal organ recognize a wide variety of volatile odorants and pheromones, enabling the organism to detect and respond to changes in ambient odors.
3. Taste (subset)
Some receptors for sweet, bitter, and umami tastes are GPCRs, which convert chemical stimuli into neural signals via G protein–mediated signaling pathways.
4.2 Central Nervous System: Behavior and Emotion
Many key neurotransmitter receptors in the brain are GPCRs, such as:
1. Dopamine receptors
2. Most subtypes of serotonin (5-HT) receptors
3. Metabotropic glutamate receptors (mGluRs)
4. GABA_B receptors, among others
These receptors participate in the regulation of:
1. Mood and affect (e.g., depression, anxiety)
2. Reward and addictive behaviors
3. Cognition, learning, and memory
4. Motor control (including pathways relevant to Parkinson’s disease)
Consequently, many antidepressants, antipsychotic drugs, and antiparkinsonian medications target these GPCRs.
4.3 Autonomic Nervous System: Cardiovascular and Visceral Functions
Among autonomic neurotransmitters, norepinephrine primarily acts on α- and β-adrenergic receptors (all GPCRs). Acetylcholine acts on both M-type muscarinic receptors (GPCRs) and N-type nicotinic receptors (ligand-gated ion channels). In autonomic effector organs such as the heart and smooth muscle, these GPCRs often play a dominant role in regulating:
1. Heart rate and myocardial contractility
2. Vascular constriction and dilation → blood pressure regulation
3. Bronchial constriction and dilation
4. Gastrointestinal motility and secretion
Common drugs such as β-blockers (for hypertension and heart failure) and certain anti-asthmatic agents exert their effects by acting directly on these GPCRs.
4.4 Endocrine System and Homeostasis: Water Balance, Metabolism, and Beyond
Many hormone receptors that regulate water and salt balance, blood glucose, and energy metabolism are GPCRs, for example:
1. Vasopressin V2 receptor (regulates water reabsorption in renal tubules)
2. Various peptide hormone receptors (such as the GLP-1 receptor)
By modulating the activity of these receptors, the body maintains internal environmental homeostasis.
4.5 Immune Responses and Inflammation
A large number of chemokine receptors in the immune system (such as members of the CCR and CXCR families) are GPCRs and guide the migration and homing of immune cells. In addition, histamine receptors (such as H1 and H2), which are also GPCRs, play important roles in allergic reactions and inflammatory responses.
It is important to note that many classical interleukin receptors belong to the cytokine receptor family rather than to GPCRs. Only a subset of chemokine receptors whose historical nomenclature overlaps with “IL receptors” are in fact GPCRs. For example, the “IL-8 receptor” described in early literature is now usually referred to as CXCR1/2; these chemokine receptors are genuine GPCRs, whereas most receptors designated “IL-xR” are not.
5. Structural Biology and the Nobel Prize Story (Extended Reading)
5.1 From Rhodopsin to the β₂-Adrenergic Receptor
Early structural studies of GPCRs were largely guided by analogy to bacterial rhodopsin-like proteins.
Around the year 2000, researchers solved the high-resolution crystal structure of bovine rhodopsin, the first true GPCR structure ever determined.
In 2007, the crystal structure of the human β₂-adrenergic receptor was resolved, marking the first mammalian GPCR structure other than rhodopsin. Because this receptor spans the membrane seven times and must remain conformationally stable in a membrane-like environment, the work was extremely challenging at the time. Researchers successfully obtained the structure by employing strategies such as T4 lysozyme fusion and stabilization with antibody fragments.
Subsequently, the structure of the activated β₂-adrenergic receptor in complex with a G protein was also determined, providing a direct, visual snapshot of the conformational changes associated with GPCR activation. For example:
1. Outward movement of the fifth and sixth transmembrane helices to create a binding pocket for Gα;
2. How receptor conformational changes promote GDP/GTP exchange on Gα, among other mechanistic details.
5.2 The 2012 Nobel Prize in Chemistry
In recognition of their outstanding contributions to the discovery, functional characterization, and structural elucidation of GPCRs,
· Robert J. Lefkowitz
· Brian K. Kobilka
were jointly awarded the 2012 Nobel Prize in Chemistry. Structural studies on human GPCRs performed by Kobilka’s team—especially the β₂-adrenergic receptor and its complex with G protein—are widely regarded as landmark achievements in the GPCR field.
6. Summary: Why Is It Worth Truly Understanding GPCRs?
For students, researchers, and customers who use related reagents or drugs, a solid understanding of GPCRs brings at least three practical benefits:
1. Building a coherent framework for signal transduction
The GPCR–G protein–second messenger (cAMP, IP₃/DAG, etc.)–protein kinase/ion channel axis is one of the most classical core pathways in cellular signaling.
2. Understanding the mechanisms of commonly used drugs
Many widely used clinical drugs—such as antihypertensives, antidepressants, antihistamines, and anti-asthmatic agents—are GPCR ligands (agonists or antagonists). Understanding GPCRs helps you truly interpret pharmacology and drug labels, rather than merely memorizing drug names.
3. Providing theoretical support for experimental design and reagent selection
a. When designing experiments on signaling pathways or cellular responses, you need to know which receptor is involved, which Gα family it couples to, and which pathway it activates;
b. When selecting reagents such as antibodies, ligands, agonists, or antagonists, understanding receptor coupling and downstream effects helps you interpret experimental results more accurately and avoid inappropriate use.
7. Overview of Typical Chemicals Related to GPCR Structure and Signaling Pathway Studies
Product name | CAS No. | Aladdin Cat. No. | Grade and purity | Role or features in GPCR-related research |
Dopamine | 51-61-6 | Moligand™ ≥98% | A classical monoamine neurotransmitter that acts on dopamine GPCRs (D1–D5 receptor family). Involved in reward, motor control, emotion, etc. Widely used in studies of neural pathways and as an agonist/control compound in signaling pathway research. | |
Serotonin (5-Hydroxytryptamine, 5-HT) | 50-67-9 | Moligand™ ≥98% (HPLC) | A monoamine neurotransmitter and endogenous agonist for 5-HT receptors (most of which are GPCRs). Broadly involved in mood, sleep, gastrointestinal motility, and more. Commonly used as a standard compound in 5-HT signaling pathway studies. | |
γ-Aminobutyric acid (GABA) | 56-12-2 | Moligand™ ≥99% | The major inhibitory neurotransmitter. Some of its receptors (GABA_B receptors) are GPCRs. Can be used as a standard or substrate for studying inhibitory signaling pathways. | |
L-Glutamic acid (L-Glutamate) | 56-86-0 | Animal-free; USP, JP, European Pharmacopoeia (Ph.Eur); for cell culture ≥98.5% | The major excitatory neurotransmitter. Endogenous ligand for metabotropic glutamate receptors (mGluRs, GPCRs). Used in studies of excitatory neurotransmission and synaptic plasticity. | |
Epinephrine (Adrenaline) | 51-43-4 | Moligand™ ≥98% | A sympathetic neurotransmitter and hormone, endogenous ligand for α/β-adrenergic receptors (all GPCRs). Commonly used in studies of sympathetic nervous system and cardiovascular signaling pathways. | |
Norepinephrine (Noradrenaline) | 51-41-2 | Moligand™ ≥98% | A sympathetic neurotransmitter that selectively activates multiple adrenergic receptors. Often used in in vivo and in vitro experiments on GPCR pathways involving β₁ and α receptors. | |
Histamine | 51-45-6 | Moligand™ ≥96% | An endogenous amine mediator involved in immunity and allergic responses. H1–H4 histamine receptors are all GPCRs. Suitable as a standard compound for experiments on “GPCR signaling in immunity/allergy/inflammation.” | |
Arginine Vasopressin (AVP) | 113-79-1 | ≥98% | A classical peptide hormone acting on V1a, V1b, and V2 receptors (GPCRs), involved in water–salt balance and vascular tone. Can be used as a peptide GPCR ligand product. | |
11-cis-Retinal | 564-87-4 | – | – | The chromophore (photosensitive ligand) of rhodopsin. Upon light exposure, 11-cis isomerizes to the all-trans form, triggering activation of rhodopsin (a GPCR). A classic substrate in visual signaling research. |
9-cis-Retinal | 514-85-2 | ≥95% (HPLC) | A natural retinal isomer related to 11-cis, commonly used in studies of the visual cycle and retinal differentiation. | |
All-trans-Retinal | 116-31-4 | Moligand™ ≥98% | The photoisomerized product of 11-cis-retinal and an important intermediate in vitamin A metabolism. | |
Adenosine 5′-triphosphate disodium salt (ATP disodium) | 987-65-5 | ≥98% | The substrate used by AC for cAMP production; ATP itself can also regulate signaling via P2 receptors. Often used together with AC/PKA systems in “GPCR–cAMP pathway” experiments. | |
Adenosine 3′,5′-cyclic monophosphate (cAMP) | 60-92-4 | Moligand™ ≥99% | A classical second messenger whose production is regulated by Gαs/Gαi modulation of AC. Commonly used as a pathway activator or standard in analyses of PKA/CREB and related signaling. | |
Inositol 1,4,5-trisphosphate (IP₃) and its salts | Depending on the salt form, e.g., hexapotassium salt 103476-24-0; sodium salt 141611-10-1, etc. | – | – | A key second messenger generated in the Gαq/11–PLCβ pathway. It induces Ca²⁺ release from the ER. Widely used in Ca²⁺ signaling and PLC/PKC pathway studies. Aladdin offers multiple salt forms (potassium, sodium, ammonium) to meet different experimental needs. |
D-myo-Inositol 1,4,5-trisphosphate hexapotassium salt | 103476-24-0 | ≥98% | D-myo stereochemistry, matching the endogenous configuration. A classical IP₃ potassium salt with high water solubility, suitable for Ca²⁺ release and PLC pathway functional assays. | |
D-myo-Inositol 1,4,5-trisphosphate sodium salt | 141611-10-1 | Moligand™ ≥98% | Structurally identical to IP₃ produced by intracellular PLC; the sodium salt form is convenient for aqueous solution preparation and is often used for IP₃R and Ca²⁺ imaging experiments. | |
D-myo-Inositol 1,4,5-trisphosphate (ammonium salt) | 345958-55-6 | ≥99% | D-myo IP₃ ammonium salt with high purity, suitable as an IP₃ standard or positive control in in vitro signaling pathway systems. | |
DL-myo-Inositol 1,4,5-trisphosphate hexammonium salt (DL racemate) | 112571-68-3 | – | Racemic DL mixture, suitable for biochemical/enzymatic systems where stereoselectivity is not critical. When matching the endogenous configuration is important, D-type salts are recommended as first choice. | |
Carvedilol | 72956-09-3 | ≥98% (HPLC) | A representative nonselective β-blocker with additional α₁-blocking activity, used for heart failure and hypertension. A classic example of a small-molecule antagonist of adrenergic GPCRs, suitable as a “GPCR drug” model compound. | |
Losartan potassium | 124750-99-8 | ≥99% | An antagonist of the angiotensin II AT₁ receptor (a GPCR), and a classic antihypertensive drug. Can be highlighted in modules on “classic GPCR-targeted drugs / receptor antagonists.” | |
Goserelin acetate | 145781-92-6 | ≥98% | A GnRH (LHRH) analog that modulates sex hormone secretion by acting on the GnRH receptor (a GPCR). Commonly used in the treatment of prostate and breast cancers and suitable as an example of a “peptide GPCR drug ligand.” |
8. References
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[4] Fredriksson R, Lagerström MC, Lundin LG, Schiöth HB. The G-protein-coupled receptors in the human genome form five main families. Mol Pharmacol. 2003;63(6):1256–1272.
[5] Palczewski K, et al. Crystal structure of rhodopsin: A G protein-coupled receptor. Science. 2000;289(5480):739–745.
[6] Cherezov V, Rosenbaum DM, Hanson MA, et al. High-resolution crystal structure of an engineered human β2-adrenergic G protein-coupled receptor. Science. 2007;318(5854):1258–1265.
[7] Rasmussen SGF, DeVree BT, Zou Y, et al. Crystal structure of the β2 adrenergic receptor–Gs protein complex. Nature. 2011;477(7366):549–555.
[8] Lefkowitz RJ. A Brief History of G Protein-Coupled Receptors. Nobel Lecture, 8 December 2012. NobelPrize.org.
[9] The Royal Swedish Academy of Sciences. Studies of G-protein–coupled receptors. Scientific Background on the Nobel Prize in Chemistry 2012.
[10] Zhang WW. They solved the mystery of G protein-coupled receptors—on the 2012 Nobel Prize laureates in Chemistry. Science and Technology Daily. 2012-10-11.
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