Week 25 pharmacology - drug targets GPCR Flashcards
G-protein coupled receptor (GPCR) - abundance and importance
-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
GPCR - structure and mechanism
-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)
GCPR - receptor signalling
GPCR signalling ensures specificity (targeting precise pathways), sensitivity (responding to minute ligand concentrations), and fidelity
(accurate signal transmission)
Clinical relevance
GPCRs are targets for drugs treating a variety of conditions, including hypertension, asthma, and mental health disorders
Key features of GPCR structure
->7-Transmembrane (TM) domains: GPCRs are characterised by seven alpha-helical domains that span the cell membrane
->N- and C-Termini:
-N-Terminus: Located extracellularly, often involved in ligand binding or interaction
-C-Terminus: Intracellular, typically interacts with G-proteins or other signalling molecules
Key features of GPCR structure (2)
->Extracellular and intracellular loops:
-ECLs (Extracellular Loops): Three loops (ECL-1, ECL-2, ECL-3) between the TM domains, often contributing to ligand specificity.
-ICLs (Intracellular Loops): Three loops (ICL-1, ICL-2, ICL-3) between the TM domains, critical for G-protein interaction and signal transduction
->Helical Bundle
-The TM helices are organised into a bundle embedded within the cell membrane
Functions
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
GPCR - structure
Structural Similarity:
->Most GPCRs share a conserved 7-transmembrane helix structure, a hallmark of this
receptor family
->The general architecture ensures broad functionality while maintaining structural
integrity
Distinct Amino Acid Sequences:
->Variability in amino acid sequences, especially in the transmembrane domains and
extracellular loops, underpins the specificity for ligand binding
->This allows GPCRs to recognize a diverse range of ligands, from small molecules like adrenaline to larger peptide hormones
FSH and its receptor
->Ligand: FSH is a large protein hormone involved in reproductive processes
->Receptor: The FSH receptor (FSHR), a member of the GPCR family, has an extracellular binding domain that specifically recognises and binds FSH
->This interaction triggers intracellular signalling pathways crucial for ovarian follicle development in females and spermatogenesis in males
-Binding Dynamics:
->The extracellular binding site of GPCRs is particularly suited for large molecules like FSH, demonstrating their structural adaptability
->This highlights the GPCR family’s role in mediating diverse physiological responses, from neurotransmitter signalling to hormone regulation
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
Structure and Function:
-Gα Subunit: Binds to guanosine diphosphate (GDP) or guanosine triphosphate (GTP)
-The switch between GDP and GTP is key to its activation state.
Gβ and Gγ Subunits:
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
Mechanism of activation -> G proteins
->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
Biological implications -> G proteins
G-proteins are pivotal in translating extracellular signals into intracellular actions -> the diversity of Gα, Gβ, and Gγ subtypes allows for specificity and diversity in signalling pathways
Key steps in the G-protein cycle -> 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
Key steps in the G-protein cycle -> 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
Key steps in the G-protein cycle -> 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
Key steps in the G-protein cycle -> 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
Key steps in the G-protein cycle -> 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
Repeatability
The steps (1–5) constitute the G-protein activation-deactivation cycle, which can occur multiple times in response to ligand binding and subsequent receptor activation
GPCR- effectors
Receptor-Selective G Protein Activation:
-Specific GPCRs are linked to particular classes of Gα subunits
-Upon ligand binding, GPCRs activate the associated G protein to generate a targeted response
Types of responses
Stimulatory (green): Mediated by Gα subunits like Gαs, which activate downstream effectors
->Leads to an enhancement of cellular processes
Inhibitory (red): Mediated by Gα subunits like Gαi, which inhibit specific effectors, suppresses or moderates cellular activity
Effector activation
Effector proteins (e.g., enzymes, ion channels) are activated or inhibited by Gα subunits or Gβγ dimers
The cellular response is tailored to the specific signalling pathway initiated by the GPCR
Types of Gα subunits -> Gαs (stimulatory):
Gαs (stimulatory):
-Effector: adenylyl cyclase
-2nd Messenger: increases cyclic AMP (cAMP) levels
-Function: stimulatory.
-Enhances neurotransmitter release
-Promotes smooth muscle relaxation
Types of Gα subunits -> Gαi (inhibitory)
Gαi (Inhibitory):
-Effector: Adenylyl cyclase
-2nd Messenger: decreases cAMP levels
-Function: inhibitory
-Reduces neurotransmitter release
-Induces smooth muscle contraction
Types of Gα subunits -> Gαq (stimulatory)
Gαq
Effector: Phospholipase C.
-2nd Messenger: increases intracellular calcium via the inositol trisphosphate (IP3) pathway
-Function: stimulatory
-Promotes neurotransmitter release
-Induces smooth muscle contraction
Types of Gα subunits -> Gβγ subunits
Gβγ subunits: Play a regulatory role by interacting with ion channels or other signalling pathways
Adrenaline / noradrenaline
Activates adrenergic receptors:
Gαs-coupled:
-Receptors: β1, β2, β3-adrenoceptors
-Function: Stimulates adenylyl cyclase, increasing cAMP levels
Gαq-coupled:
-Receptor: α1-adrenoceptors
-Function: Activates phospholipase C, leading to increased intracellular calcium
Gαi-coupled:
-Receptor: α2-adrenoceptors
-Function: Inhibits adenylyl cyclase, reducing cAMP levels
Acetylcholine
Activates muscarinic receptors:
Gαi-coupled ->
-Receptors: M2, M4 muscarinic receptors
-Function: Inhibits adenylyl cyclase, decreasing cAMP levels and modulating smooth muscle contraction and neuronal activity
Gαq-coupled ->
-Receptors: M1, M3, M5 muscarinic receptors
-Function: Activates phospholipase C, leading to calcium release and smooth muscle contraction
GPCR (Gsα) effector- 2nd messenger signalling - cAMP
