GPCRs Flashcards
Describe basic structure of GPCRs
7 transmembrane domains
-arranged into a barrel like pore where the ligand binds
N terminus outside
C terminus inside
What portion of GPCR binds ligand?
The extracellular and/or transmembrane domain.
the T1 and T7 loop back on each other, forming a barrel-like structure
What portion of the GPCR confers G-protein binding specificity?
The cytoplasmic i-loops.
carboxyl terminal ends
Define GPCR agonist vs. antagonist
Agonist: binding triggers receptor activation
Antagonist: binding blocks receptor activation by antagonists (competitive)
How do GPCRs activate heterotrimeric G-proteins?
- Ligand binds to GPCR
- Galpha releases GDP, binds GTP. == ACTIVATION
- Alpha and beta-gamma subunits dissociate, become available to effector proteins
how do GPCR activate heterotrimeric G proteins?
G Proteins:
alpha subunit - binds GTP/GDP
beta/gamma subunits
-ligand binding to the GPCR gives the G proteins ability to exchange GDP for GTP
What is the rate limiting step of the GPCR cascade?
nucleotide exchange (GDP–> GTP)
activates cellular pathway
Steps of GTP hydrolysis cycle of G protein signaling
- ligand binds
- alpha subunit exchanges GDP for GTP
- alpha-GTP unit dissociates from beta-gamma
- both units act on effector proteins
- Intrinsic alpha GTPase activity hydrolyzes GTP–> GDP
What proteins can help in Catalyzing the exchange of:
GDP –> GTP?
GTP –> GDP?
GDP –> GTP? GEF
GTP –> GDP? GAPs
How does cholera toxin affect the G-protein molecular switch?
The CTX ribosylates the G-subunit, and locks it in the GTP-bound ACTIVE state. Cannot be deactivated so you get constitutively expressed cAMP
How does bordella pertussis affect the G-protein molecular switch?
The PTX binds to the C-terminal side and locks the G-protein in the heterotrimeric INACTIVATED state. The G-protein cannot be activated, leads to whooping cough.
Within the ANS, symp and parasymp have what kind of transmission via what signaling molecules?
Symp: Adrenergic via NE and epinephrine
Parasymp: Muscarinic cholinergic via ACh
function of second messengers in receptor signaling
Second messengers are molecules that relay signals from the cell surface to targets inside the cell. They
often are involved in amplification of the signal.
Example of how second messengers are activated in beta adrenergic receptors (heart)?
- NE binds the receptor, causing GDP/GTP exchange
- –> activation of alpha unit - alpha-GTP unit binds to adenylyl cyclase (AC) to activate it.
- AC makes cAMP, a second messenger
- which in turn activate protein kinase A (PKA).
- PKA phosphorylates Ca channels, leading to influx of Ca.
These steps lead to contraction of cardiac muscle.
What effect would metaprolol (beta1 selective antagonist) have on heart rate?
Decrease heart rate
- blocks activation of G protein with GTP and prevents increase in cAMP
Example of how second messengers are activated in alpha 1 adrenergic receptors (peripheral vasculature)
- NE binds, causes the exchange of GDP/GTP
- –> activation of alpha unit
- alpha-GTP interacts with the PLC molecule
- PLC cleaves membrane lipid PIP2.
- Results in 2 second messengers DAG and IP3
- Both DAG and IP3 stimulates Ca2+ (another second messenger)
These steps lead to contraction of smooth muscles.
What effect would prazosin (an alpha1 selective antagonist) have on blood pressure?
decrease blood pressure (block vasoconstriction response)
- prevents creation of DAG and IP3
Explain how receptor activation leads to signal termination through receptor desensitization
The signaling pathway that activates G proteins, also activates receptor desensitization. One of the
binding partners of the beta-gamma subunit after it dissociates from the alpha-GTP unit is a kinase
called GRK.
phosphorylation recruits beta arrestin, which further prevents Gproteins from associating with receptor
GRK
kinase that phosphorylates the cytoplasmic loops of the G protein coupled receptor. This results in its
inability to interact with G proteins. = desensitization
What effect would prazosin (an alpha1 selective antagonist) have on blood pressure?
decrease blood pressure (block vasoconstriction response)
Explain how receptor activation leads to signal termination through receptor desensitization
The signaling pathway that activates G proteins, also activates receptor desensitization. One of the
binding partners of the beta-gamma subunit after it dissociates from the alpha-GTP unit is a kinase
called GRK.
GRK
kinase that phosphorylates the cytoplasmic loops of the G protein coupled receptor. This results in its
inability to interact with G proteins.
Atropine
parasympathetic antagonist/ ach antagonist / muscarinic antagonist: that prevents activation of G protein with GTP. Blocks parasympathetic (which depresses HR) --> then muscle contractions increase = faster heart rate
PDE inhibitors
ie: Caffeine
target phosphodiesterases (PDE) of the message pathway to activate signaling. PDE inhibitors are able to extend signaling.
normally PDEs degrade cAMP by
cleaving it to AMP, which is not capable of activating PKA, and thus terminate signaling
- Would inhalation of ipratropium (muscarinic antagonist) relieve acute asthma symptoms?
Would inhalation of albuterol (beta2-selective agonist) add to or counter effects of ipratropium?
- Yes
block parasympathetic constriction (favor airway opening) - add to
block muscarinic pathway and activate adrenergic pathway
( antagonist of one pathway, and agonist of other pathway would add to)
