Receptor Signaling and GPCRs Overview

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Questions and Answers

G-protein coupled receptors pass through the membrane 5 times.

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G-proteins are inactive when GDP is bound.

True

There are 3 subunits in a G-protein: α, β, and ζ.

False

Agonist-induced activation leads to a conformational change in G-protein coupled receptors.

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The main purpose of Gα proteins is to regulate amplifier or effector protein activity.

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GTP replaces GDP only when the G-protein is in its inactive state.

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There are 500 different types of G-protein coupled receptors.

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G-proteins are linked to the receptor in their inactive state.

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Gαs subunits activate adenylyl cyclase.

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Pertussis toxin activates adenylyl cyclase.

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CAMP activates protein kinases A (PKAs).

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E. coli toxin causes reduced cAMP levels in the colonic epithelium.

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Loperamide acts as a μ-opioid receptor antagonist.

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Second messengers are produced by GTP-Gα and/or Gβγ activating the effector.

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The GTPase activity increases the G-protein's activity.

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Guanine-nucleotide exchange factors (GEF) inhibit signaling by preventing GDP release.

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β-arrestin is involved in the internalization of receptors during GPCR desensitization.

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CAMP is generated by the activation of phospholipase C.

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GTPase-accelerating proteins (GAPs) stimulate GTPase activity to turn off signaling.

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Continued stimulation of a receptor leads to its trafficking to the plasma membrane.

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Gαs activates adenylyl cyclase, whereas Gαi inhibits it.

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Diacylglycerol (DAG) produces second messengers.

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Inositol 1,4,5-trisphosphate (IP3) is generated by adenylate cyclase.

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The diversity of G-protein-coupled receptors (GPCRs) includes receptors for dopamine.

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Calmodulin is activated by cyclic AMP.

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Histamine receptors are a type of G-protein-coupled receptor.

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G-proteins are active when GDP is bound.

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G-proteins consist of two subunits: α and β.

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Ligand binding to GPCRs induces a conformational change that activates the receptor.

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There are 4 families of Gα proteins classified by their structural similarities.

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Diacylglycerol (DAG) is involved in producing second messengers.

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The βγ subunit of G-proteins cannot exert any signaling activity.

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Adenylate cyclase is regulated by G-proteins to generate inositol 1,4,5-trisphosphate (IP3).

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G-proteins only hydrolyze GDP when activated.

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Calmodulin interacts with calcium ions and plays a role in cellular signaling.

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In G-protein signaling, activation of kinases leads to the phosphorylation of cellular proteins.

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G-proteins dissociate from the receptor when GTP is hydrolyzed to GDP.

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Cyclic AMP functions as a second messenger in GPCR signaling.

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G-proteins only affect adenylate cyclase and have no influence on phospholipase C.

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Cholera toxin inhibits adenylyl cyclase by modifying Gαs.

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Cyclic adenosine monophosphate (cAMP) serves as a second messenger that activates protein kinases A (PKAs).

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Gαq/11 subunits are responsible for the activation of adenylyl cyclase.

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Elevated levels of cAMP in colonic epithelium due to E.coli toxin lead to severe diarrhea.

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Loperamide acts as a μ-opioid receptor agonist, offering a treatment for disruption caused by E.coli toxin.

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The GTPase activity returns the G-protein to its active state.

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Phosphorylation of a receptor can lead to lower affinity for agonists.

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Guanine nucleotide dissociation inhibitor (GDI) stimulates the release of GDP.

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β-arrestin is responsible for the downregulation of GPCRs.

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The primary role of GTPase-accelerating proteins (GAPs) is to stimulate adenylyl cyclase.

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CAMP is generated by the activation of phospholipase D.

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Ligand-bound GPCRs act as guanine-nucleotide exchange factors (GEF).

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Sustained stimulation of a receptor leads to its recycling to the cell membrane.

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G-proteins are inactive when GTP is bound.

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The βγ subunit of G-proteins can exert signaling activity.

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Guanine-nucleotide exchange factors (GEF) stimulate GDP release to promote signaling.

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Activation of G-proteins involves the direct binding with specific ligands.

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Cyclic adenosine monophosphate (cAMP) enhances the activity of protein kinases A (PKAs).

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GTPase-accelerating proteins (GAPs) activate G-proteins by replacing GDP with GTP.

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Loperamide functions as a μ-opioid receptor agonist.

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Gαi subunits specifically activate phospholipase C to produce second messengers.

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Diacylglycerol (DAG) acts solely through cyclic AMP to produce second messengers.

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G-protein-coupled receptors have a uniform mechanism of signaling across all types.

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The activation of kinases in G-protein signaling leads to phosphorylation of cellular proteins, causing changes in cell function.

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Inositol 1,4,5-trisphosphate (IP3) is primarily generated through the activation of phospholipase D.

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G-proteins are able to affect multiple effector enzymes including adenylyl cyclase and phospholipase C.

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Guanine-nucleotide exchange factors (GEF) assist in accelerating signaling by promoting the dissociation of GDP.

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The βγ subunit of G-proteins plays a crucial role in enhancing receptor sensitivity by promoting the release of GDP.

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Cyclic adenosine monophosphate (cAMP) is produced by the activation of adenylyl cyclase, which in turn is inhibited by Gαi.

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Sustained activation of GPCRs leads to their downregulation via trafficking to the lysosomes.

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GTPase-accelerating proteins (GAPs) are responsible for promoting GTP binding to G-proteins.

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Internalization of receptors during GPCR desensitization is mediated by β-arrestin binding.

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GTP replaces GDP only when the G-protein is in its active state, which occurs after receptor activation.

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Adenylyl cyclase and phospholipase C are both directly activated by the same G-protein subunits.

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Cholera toxin inhibits the activation of adenylyl cyclase by modifying Gαs.

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Gαi proteins play a role in inhibiting adenylyl cyclase, contributing to decreased cAMP levels.

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Cyclic adenosine monophosphate (cAMP) is metabolized to produce inositol 1,4,5-trisphosphate (IP3).

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Loperamide acts as an antagonist to μ-opioid receptors in the gastrointestinal tract by coupling to Gαi.

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E. coli toxin causes lower cAMP levels in the colonic epithelium leading to fluid retention.

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Study Notes

Receptor Signaling

Receptors are cellular proteins that bind to signaling molecules (ligands) and trigger a response.
Four types of cell receptors are covered: Ligand-gated ion channels, enzyme-linked receptors, G-protein-coupled receptors, and nuclear receptors.
G-protein-coupled receptors (GPCRs) are a large family of transmembrane receptors involved in a variety of cellular processes.
GPCRs are monomeric proteins with a molecular weight between 35K and 70K.
These receptors pass through the cell membrane seven times.
There are over 500 different GPCRs, and they are involved in diverse signaling pathways like light, taste, and smell.

GPCR Structure and Function

GPCRs function through a cascade of events involving the association and dissociation of protein subunits.
G-proteins are a key component of GPCR signaling cascades.
They are heterotrimeric, composed of three subunits: α, β, and γ.
The α subunit of the G-protein binds and hydrolyzes GTP to GDP, regulating the activity of the G-protein.
The βγ subunit can also exert signaling activity.

GPCR Activation and Signaling

Ligand binding to a GPCR induces a conformational change that activates the G-protein.
The activated G-protein, with GTP bound to the α subunit, dissociates from the receptor and the βγ subunit.
The activated α-subunit and/or the free βγ subunit then bind to and activate effector proteins.
Effector proteins generate secondary messengers that amplify and propagate the signal within the cell.
The signal is terminated by GTP hydrolysis on the α-subunit, which returns the G-protein to its inactive state.
The system resets, enabling the cycle to start again when a new ligand binds.

GPCR Effector Proteins

GPCRs regulate a variety of effector proteins.
Two significant examples are adenylate cyclase (AC) and phospholipase C (PLC)

Adenylate Cyclase (AC)

AC generates cyclic AMP (cAMP), an important second messenger involved in diverse cellular functions.
AC activity can be regulated by G-proteins:
- Gαs activates AC.
- Gαi inhibits AC.
Cholera toxin covalently modifies Gαs, preventing GTP hydrolysis, leading to elevated cAMP levels and excessive water and ion efflux in the colon, which causes severe diarrhea and dehydration.
Loperamide or Imodium, a μ-opioid receptor agonist used to treat diarrhea, acts by activating the Gαi pathway which inhibits AC activity.

Phospholipase C (PLC)

PLC hydrolyzes phosphatidylinositol 4,5-bisphosphate (PIP2) to diacylglycerol (DAG) and inositol 1,4,5-trisphosphate (IP3).
IP3 increases intracellular calcium levels, while DAG activates protein kinase C (PKC).
PLC activation leads to a complex cascade of intracellular events through calcium, calmodulin, and kinases, resulting in changes in cellular function.

Regulatory Control of GPCRs

GPCR signaling is tightly regulated, and there are several mechanisms involved:
- Guanine-nucleotide exchange factors (GEFs): Ligand-bound receptors can act as GEFs, accelerating signal transduction.
- Guanine nucleotide dissociation inhibitors (GDIs): The βγ subunit of the G-protein can act as a GDI, preventing GDP release and stopping signaling.
- GTPase-accelerating proteins (GAPs): GAPs stimulate the GTPase activity of the α-subunit, turning off signaling.

Desensitization

GPCRs can undergo desensitization upon sustained stimulation.
This process ensures appropriate signal termination and prevents overstimulation.
Three main mechanisms contribute to desensitization:
- Uncoupling of the receptor from the G-protein through phosphorylation.
- Internalization of the receptor through β-arrestin binding.
- Downregulation of the receptor through degradation in lysosomes.

Diversity of GPCRs and Signaling Pathways

GPCRs exhibit significant diversity, with various families and subgroups associated with different signaling pathways:
- Gs family: activates AC, increasing cAMP levels.
- Gi family: inhibits AC, decreasing cAMP levels.
- Gq/11 family: activates PLC, leading to increased calcium and DAG levels.
The diverse signaling pathways mediated by GPCRs are essential for a broad range of physiological processes.

Receptor Signaling and G Protein-Coupled Receptors (GPCRs)

Receptor signaling is the process by which cells receive and respond to external stimuli through receptors.
GPCRs are a large family of transmembrane proteins that act as receptors for various signaling molecules.
GPCRs are comprised of monomeric proteins with molecular weights ranging from 35,000 to 70,000.
GPCRs traverse the membrane seven times and are responsible for sensing light, taste, and smell.
GPCRs are activated by ligands, which induce a conformational change, revealing a binding site for the G-protein α subunit.
G proteins are trimeric enzymes composed of α, β, and γ subunits.
G proteins bind to GTP and hydrolyze it to GDP, transitioning between inactive (GDP-bound) and active (GTP-bound) states.
GTP-bound α subunit dissociates from the βγ dimer, initiating signal transduction.
There are four families of Gα subunits: Gαs, Gαi, Gαq, and Gα12.
βγ subunits act as dimers and can also regulate signal transduction.

G-Protein Signaling Cascade

Activation of GPCRs triggers a series of events:
- Ligand binds to the GPCR, inducing a conformational change.
- G protein binds to the activated receptor.
- GDP on the α subunit is replaced by GTP.
- GTP-bound α subunit detaches from the βγ dimer and activates downstream effectors.

Key Effectors: Adenylate Cyclase and Phospholipase C

Adenylate cyclase (AC) is an enzyme responsible for converting ATP into cAMP, a second messenger.
Gαs activates AC, while Gαi inhibits it.
cAMP activates protein kinase A (PKA), which phosphorylates proteins, affecting various cellular processes.
Cholera toxin inhibits GTP hydrolysis by Gαs, leading to elevated cAMP levels and diarrhea.
Phospholipase C (PLC) hydrolyzes PIP2, producing diacylglycerol (DAG) and inositol 1,4,5-trisphosphate (IP3) as second messengers.
IP3 triggers calcium release from intracellular stores, while DAG activates protein kinase C (PKC).

GPCR Regulation and Desensitization

Guanine nucleotide exchange factors (GEFs) promote GDP-GTP exchange on G proteins.
Guanine nucleotide dissociation inhibitors (GDIs) inhibit GDP release from G proteins.
GTPase-accelerating proteins (GAPs) stimulate GTP hydrolysis, terminating signaling.
GPCR desensitization mechanisms include uncoupling, internalization, and downregulation.
Uncoupling involves phosphorylation of the receptor, reducing its affinity for the ligand.
Internalization involves β-arrestin binding leading to receptor internalization.
Downregulation involves receptor degradation in lysosomes.

Diversity of GPCR Signaling Pathways

Different GPCRs couple to distinct G protein subtypes, leading to diverse signaling pathways.
Example of GPCRs coupled to Gαs: 5-HT4, 5-HT7, Adrenergic β1, β2, β3.
Example of GPCRs coupled to Gαi: 5-HT1, 5-HT5, ACh M2, M4, Adenosine A1, A3, Adrenergic α2.
Example of GPCRs coupled to Gαq/11: 5-HT2, ACh M1, M3, M5, Adrenergic α1, Glutamate mGlu1, 5, Histamine H1, Vasopressin V1.

Receptor Signalling

Receptor signalling is a process by which cells communicate with their environment using specific receptors.

G-protein Coupled Receptors (GPCRs)

GPCRs are a large family of transmembrane receptors that play a critical role in mediating cellular responses to a wide variety of stimuli.
They are Monomeric proteins (MW 35K-70K) that pass through the membrane 7 times.
There are at least 500 different GPCRs including those for light, taste and smell.

Overview of GPCR Signaling

GPCRs are activated by specific ligands, which bind to the receptor and induce a conformational change.
The activated receptor then binds to a G-protein, a heterotrimeric protein consisting of α, β, and γ subunits.

G-Protein

The G-protein is inactive with GDP bound to the α subunit.
Ligand binding causes the α subunit to exchange GDP for GTP, leading to the activation of the G-protein.
Activated G-protein dissociates into α and βγ subunits, both of which can interact with various effector proteins.
There are four families of Gα proteins (Gαs, Gαi, Gαq & Gα12), characterized by their structural similarities and their respective roles in regulating downstream signals.
While the α subunit is largely responsible for effector protein regulation, the βγ dimer can also participate in signaling.

Signal Transduction

Activated G-protein subunits interact with effector proteins, such as adenylyl cyclase and phospholipase C (PLC), resulting in the production of second messenger molecules.
Second messengers like cAMP, IP3 and diacylglycerol relay the signal downstream, triggering cellular responses.
The system resets as GTPase activity on the α subunit hydrolyzes GTP back to GDP, leading to reassociation of the G-protein subunits and terminating the signal.

Effectors

Adenylyl cyclase catalyzes the conversion of ATP to cAMP, an important second messenger involved in various cellular processes.
- Gαs activates adenylyl cyclase, while Gαi inhibits it.
- Cholera toxin locks Gαs in an active state, leading to excessive cAMP production.
Phospholipase C hydrolyzes PIP2 to produce inositol 1,4,5-trisphosphate (IP3) and diacylglycerol (DAG).
- IP3 mobilizes calcium from intracellular stores.
- DAG activates protein kinase C (PKC).

Regulation of GPCR Signaling

Guanine-nucleotide exchange factors (GEFs), like ligand-bound receptors, accelerate signaling by promoting GDP dissociation.
Guanine nucleotide dissociation inhibitors (GDIs), such as the βγ subunit of G-proteins, inhibit signaling by preventing GDP release.
GTPase-accelerating proteins (GAPs) stimulate GTPase activity on the α subunit, turning off signaling.

GPCR Desensitization

Uncoupling: phosphorylation by kinases reduces receptor affinity for agonists, stopping G-protein recruitment.
Internalization: sustained stimulation leads to β-arrestin binding and receptor internalization, potentially activating other signaling pathways.
Downregulation: continual stimulation causes receptor trafficking to lysosomes for degradation, resulting in reduced receptor levels.

Cyclic AMP (cAMP)

cAMP activates protein kinase A (PKA), which phosphorylates various proteins, including ion channels and metabolic enzymes.
cAMP is metabolized by phosphodiesterases.

E. coli Toxin

E.coli toxin, implicated in traveler's diarrhea, covalently modifies Gαs, preventing GTP hydrolysis and locking it in the active state.
This leads to elevated cAMP levels in the intestinal epithelium, causing water and ion efflux, resulting in severe diarrhea and dehydration.

Treatment for E. coli Toxin-Induced Diarrhea

Loperamide (Imodium) acts as a μ-opioid receptor agonist in the large intestine.
Opiate receptors are coupled to Gi, leading to inhibition of adenylyl cyclase and a reduction in cAMP levels.
This represents a functional antagonism, where one drug counteracts the effects of another by acting on a different receptor.

Diversity of GPCR Signaling Pathways

There are numerous GPCRs that couple to different G-proteins, leading to diverse cellular responses.
Examples include:
- 5-HT receptors, involved in serotonin signaling.
- ACh receptors, involved in acetylcholine signaling.
- Adrenergic receptors, involved in adrenaline signaling.
- Dopamine receptors, involved in dopamine signaling.
- Glutamate receptors, involved in glutamate signaling.
- Histamine receptors, involved in histamine signaling.
- Opioid receptors, involved in opioid signaling.
- Vasopressin receptors, involved in vasoconstriction.

Key Takeaways

GPCR signaling is a complex and highly regulated process crucial for cell communication.
G-proteins play a central role in relaying signals from receptors to downstream effectors.
Different GPCRs and G-proteins can generate diverse cellular responses.
Understanding GPCR signaling is essential for understanding how cells respond to external stimuli and for developing new therapeutic strategies.

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This quiz covers the fundamentals of receptor signaling, focusing on G-protein-coupled receptors (GPCRs) and their structure and function. Explore the different types of receptors and their roles in cellular signaling pathways. Test your knowledge on the intricate dynamics of GPCRs and G-proteins.