Pharmacology - Drug Targets - GPCR Flashcards
What is the abundance and importance of G-protein coupled receptors?
- The human genome encodes ~800 GPCR proteins, making them the most
prevalent receptor type. - They mediate the actions of neurotransmitters, hormones, and other
signalling molecules. - Approximately 30% of all commercial drugs act on GPCRs, emphasising
their critical role as therapeutic targets.
What is the structure and mechanisms of GPCR?
- Receptor: Embedded in the cell membrane, it binds the extracellular
ligand (e.g., neurotransmitter, hormone). - G Protein: Consists of α, β, and γ subunits. The inactive state is bound to
GDP. - Effector: The activated G protein regulates downstream signalling
pathways via effectors (e.g., enzymes, ion channels).
What is the clinical relevance of GPCRs?
*GPCRs are targets for drugs treating a variety of conditions, including hypertension,
asthma, and mental health disorders.
*Examples:
* β-adrenergic receptors: Targeted by beta-blockers for cardiovascular
diseases.
* Dopamine receptors: Targeted in treatments for Parkinson’s disease and schizophrenia.
What are the key features of the GPCR structure?
*7-Transmembrane (TM) Domains
*N- and C-Termini
*Extracellular and Intracellular Loops
*Helical bundle
Draw the GPCR structure (to memorise things)
*look in folder
Where does ligand binding and signal transmission occur?
*Ligand Binding:
* Occurs at the extracellular N-terminus or within the
transmembrane helices, depending on the receptor type.
*Signal Transmission:
* Interaction with intracellular G-proteins via the ICLs and C-
terminus transmits the extracellular signal into the cell.
FSH and its receptor, what does it do? + binding dynamics?
Receptor: The FSH receptor (FSHR), a member of the GPCR family, has an
extracellular binding domain that specifically recognizes and binds FSH.
3. This interaction triggers intracellular signaling pathways crucial for ovarian follicle development in females and spermatogenesis in males.
- The extracellular binding site of GPCRs is particularly suited for large
molecules like FSH, demonstrating their structural adaptability.
What is a G-protein?
- Guanine nucleotide-binding proteins that act as molecular switches in
cellular signalling. - They are heterotrimeric proteins, meaning they consist of three distinct
subunits: Gα, Gβ, and Gγ
What do the Ga subunits do + structure?
- Binds to guanosine diphosphate (GDP) or guanosine triphosphate
(GTP). - The switch between GDP and GTP is key to its activation state.
What do the GB and GY subunits do + structure?
- These subunits form a stable Gβγ dimer, which can also participate
in signalling. - The entire G-protein complex is located intracellularly, anchored to the cell membrane via lipid anchors for mobility.
What is the mechanism of activation a G-protein?
- When a ligand binds to a GPCR, the receptor undergoes a conformational
change. - This activates the G-protein by facilitating the exchange of GDP for GTP
on the Gα subunit. - Once activated, the Gα and Gβγ subunits dissociate and interact with
different intracellular effectors to propagate the signal.
What are the key steps in the G-protein cycle?
- inactive state
- activation
- dissociation
- hydrolysis of GTP
- reassociation
What happens in the inactive state?
- Gα subunit binds GDP and forms a stable complex with Gβγ.
- This trimeric G-protein associates with the intracellular side of the GPCR
in the resting state.
What happens during activation?
When a ligand binds to the GPCR, the receptor undergoes a
conformational change.
* This activates the GPCR, which then acts as a guanine nucleotide
exchange factor (GEF).
* The GPCR catalyses the exchange of GDP for GTP on the Gα subunit,
activating the G-protein
What happens during dissociation?
- The Gα-GTP subunit dissociates from the Gβγ dimer.
- Both Gα-GTP and Gβγ can now independently interact with and regulate downstream effector proteins.
What happens during the hydrolysis of GTP?
- The Gα subunit contains intrinsic GTPase activity.
- It hydrolyses GTP (guanosine triphosphate) back to GDP (guanosine diphosphate).
- This hydrolysis inactivates the Gα subunit, terminating its signalling
capability.
What happens during reassociation?
The inactive Gα-GDP re-associates with the Gβγ dimer, forming the
inactive heterotrimeric G-protein complex.
* This complex is now ready to interact with the receptor again for another cycle
What are the different types of responses?
- stimulatory (green)
- inhibitory (red)
- effector activation
What is the stimulatory response mediated by + what does it do?
- Mediated by Gα subunits like Gαs, which activate downstream effectors
(e.g., adenylyl cyclase to increase cAMP). - Leads to an enhancement of cellular processes
What is the inhibitory response mediated by + what does it do?
- Mediated by Gα subunits like Gαi, which inhibit specific effectors (e.g.,
adenylyl cyclase to decrease cAMP). - Suppresses or moderates cellular activity.
What is the effector activation response activated/inhibited by?
- Ga subunits or Gy dimers
What are the different types of Ga subunits?
- G⍺s/gsa
- G⍺i./gia
*G⍺q/gqa
What is the second messenger and function of the Gs⍺ subunit?
- Effector: Adenylyl cyclase.
- 2nd Messenger: Increases cyclic AMP (cAMP) levels.
- Function: Stimulatory.
- Enhances neurotransmitter release.
- Promotes smooth muscle relaxation
What is the second messenger and function of the Gi⍺ subunit?
- Effector: Adenylyl cyclase.
- 2nd Messenger: Decreases cAMP levels.
- Function: Inhibitory.
- Reduces neurotransmitter release.
- Induces smooth muscle contraction
What is the second messenger and function of the Gq⍺ subunit?
- Effector: Phospholipase C.
- 2nd Messenger: Increases intracellular calcium via the inositol
trisphosphate (IP3) pathway. - Function: Stimulatory.
- Promotes neurotransmitter release.
- Induces smooth muscle contraction.
What do Gβy subunits do?
Play a regulatory role by interacting with ion channels (e.g., potassium
channels) or other signalling pathways
Adrenaline/ noradrenaline: activation of Gs⍺ coupled receptors?
- Receptors: β1, β2, β3-adrenoceptors.
- Function: Stimulates adenylyl cyclase, increasing cAMP levels.
Adrenaline/ noradrenaline: activation of Gi⍺ coupled receptors?
- Receptor: α2-adrenoceptors.
- Function: Inhibits adenylyl cyclase, reducing cAMP levels
Adrenaline/ noradrenaline: activation of Gq⍺ coupled receptors?
Receptor: α1-adrenoceptors.
* Function: Activates phospholipase C, leading to increased
intracellular calcium
Acetylcholine: activation of Gi⍺ coupled receptors?
- Receptors: M2, M4 muscarinic receptors.
- Function: Inhibits adenylyl cyclase, decreasing cAMP levels and
modulating smooth muscle contraction and neuronal activity.
Acetylcholine: activation of Gq⍺ coupled receptors?
- Receptors: M1, M3, M5 muscarinic receptors.
- Function: Activates phospholipase C, leading to calcium release
and smooth muscle contraction
second messenger: cAMP homeostasis: role of phosphodiester?
*Phosphodiesterase (PDE) enzymes hydrolyse cAMP into 5’-AMP, effectively terminating the cAMP-mediated signal
second messenger: cAMP homeostasis: recycling of 5’ AMP?
*5’-AMP is subsequently converted back to ATP by cellular metabolic processes,
preparing the system for another cycle of signalling
second messenger: cAMP homeostasis: importance in signal regulation?
*This cycle ensures precise control of signalling, preventing prolonged activation of
pathways and maintaining cellular homeostasis
second messenger: cAMP homeostasis: target for drug action?
*PDE inhibitors, such as sildenafil, modulate cAMP levels by preventing its breakdown, thereby prolonging its effects
What is calcium’s role in pathways?
- A highly potent molecule influencing diverse processes:
- Muscle contraction (smooth, cardiac, and striated muscle).
- Neurotransmitter and hormone secretion (exocytosis).
- Activation of protein kinases through phosphorylation cascades.
- Cross-talk with other signalling pathways, such as adenylyl cyclase
and nitric oxide synthase
What does calcium dysregulation do?
- Both hypercalcemia (excess calcium) and hypocalcaemia (low calcium)
can lead to severe consequences. - Example: Glutamate toxicity, a condition associated with excessive
calcium influx, can result in neuronal damage or death.
