Lecture 10: G protein coupled receptors Flashcards
What are the different classes of receptors?
- Receptors with intrinsic enzyme activity.
- Receptors linked to enzymes/ protein kinases
- Receptors coupled to target proteins via a G protein
- Intracellular receptors
- Receptors that are ion channels
Give an overview of GPCRs
- G-protein-coupled receptors (GPCRs) are the largest class of cell-surface receptors that mediate a wide variety of physiological responses.
- They are involved in cell communication and are crucial for various physiological processes.
What are the largest class of cell-surface receptors?
GPCRs are the largest class of cell-surface receptors.
Is it true that GPCRs mediate a VERY wide variety of responses?
Yes, and they are involved in basically all physiological pathways.
What percentage of drugs target GPCR- mediated pathways?
Approximately 60%
What is the basic structure of a GPCR?
- Extracellular domain: N-terminal + loops E1-E4
- Transmembrane domain: Seven α-helical transmembrane (TM) regions
- Cytosolic domain: Loops C1-C3 + C-terminal tail (C4)
- C4 domain is lipid-anchored, securing it to the membrane.
What is the 3D structure of a GPCR?
- GPCRs form a barrel-like shape spanning the plasma membrane.
- The ligand-binding site is a hydrophobic cavity between TM domains.
- The extracellular loop (E2) often covers the ligand-binding site.
Describe step 1 of the activation of GPCRs
Step 1: Ligand Binding
- A ligand (hormone, neurotransmitter, or drug) binds to the receptor.
- Small ligands fit into the TM pocket, while large peptide ligands interact with the extracellular domain.
Describe step 2 of the activation of GPCRs
Step 2: Conformational Change
- Ligand binding twists the TM helices, rearranging the cytosolic domains.
- This reveals a binding site for G-proteins on the cytosolic side.
Describe step 3 of the activation of GPCRs
Step 3: G-Protein Activation
- GPCRs are coupled to a heterotrimeric G-protein, which consists of three subunits:
Gα (binds GDP/GTP)
Gβ
Gγ
- The inactive G-protein is bound to GDP.
- Upon GPCR activation, GDP is exchanged for GTP, activating the Gα subunit.
- The G-protein dissociates into Gα-GTP and Gβγ subunits, each regulating different intracellular pathways.
Describe step 4 of the activation of GPCRs
Step 4: Signal Propagation
- Gα-GTP and Gβγ subunits modulate target proteins, including:
- Adenylate cyclase (AC)
- Phospholipase C (PLC)
- Ion channels
Describe the regulation and desensitisation of GPCRs
- Turning Off the GPCR Signal
1) Intrinsic GTPase Activity of Gα:
- Gα hydrolyzes GTP to GDP, inactivating itself.
- Gα reassociates with Gβγ, reforming the inactive heterotrimer.
2) Desensitization by Phosphorylation:
- Activated GPCRs are phosphorylated at their C3 loop by GPCR kinases (GRKs).
- More phosphorylation reduces receptor activity (like a dimmer switch).
3) Arrestin Binding:
- Highly phosphorylated GPCRs bind arrestin, which prevents G-protein activation.
- Arrestin-bound GPCRs are internalized and recycled or degraded.
4) Receptor Recovery:
- Removing arrestin and dephosphorylating the receptor restores function.
- This explains why adapting to light/dark environments takes time.
What physiological functions are GPCRs involved in?
GPCRs are involved in many essential physiological functions, including fight-or-flight responses, metabolic regulation, and neurotransmission.
What is the fight or flight response mediated by ?
Adrenergic Receptors
What triggers the fight or flight response?
Triggered by epinephrine (adrenaline) and cortisol from adrenal glands.
What are the effects of the fight or flight response?
- Increased heart rate
- Dilated pupils
- Sweating
- Rapid energy release from glycogen and fat
- Suppressed immune function (long-term stress effect)
What are the 2 Adrenergic GPCRs?
α-adrenergic receptors
β-adrenergic receptors
What do α-adrenergic receptors do?
Inhibit insulin secretion, promote glycogen breakdown.
What do β-adrenergic receptors do?
Stimulate glucagon secretion, fat breakdown.
Describe the Epinephrine Signaling Pathway
1) Epinephrine binds β-adrenergic GPCR.
2) Gαs activates adenylate cyclase (AC), increasing cAMP.
3) cAMP activates Protein Kinase A (PKA).
4) PKA phosphorylates proteins → Induces fight-or-flight responses.
Describe the role of cAMP as a Second Messenger
- cAMP is a rapidly synthesized, diffusible signal.
- cAMP activates PKA, which modulates transcription factors, ion channels, and metabolic enzymes.
How does insulin lower blood sugar, through which pathway?
via PIP₃ pathway
How does glucagon raise blood sugar, through which messenger?
via cAMP second messenger.
Describe how the cholera toxin, which is produced by Vibrio cholerae, hijacks the GPCR pathways, leading to severe diarrheal disease
1) Cholera toxin binds GM1 ganglioside receptors on intestinal cells.
2) It is taken into cells via endocytosis and transported to the ER.
3) Cholera toxin’s A1 chain catalyzes ADP-ribosylation of Gαs.
4) Gαs is locked in the “ON” state → Permanent adenylate cyclase activation.
5) Massive cAMP increase → Overactivation of CFTR ion channels.
6) Excessive Cl⁻ and water loss → Severe diarrhea, dehydration, and death.
