Signaling: receptors Flashcards
Describe the basic structure of G-protein coupled receptors (a family of over 200 membrane receptors)
- 7 trans membrane domains
- N terminus outside
- C terminus inside
- trans mem. 1 by trans. mem 7 creating a barrel like structure
- Bind ligands in “small pocket” of alpha helicies just below surface of cell membrane
What is the conformation of intracellular G-proteins when the GPCR is not bound to an agonist?
3 subunits:
Alpha
Beta & Gamma (never found alone)
Bound to GDP
What happens when an agonist binds to the GPCR?
Conformational change!
GDP released from G-protein subunits -> GTP bound by G-protein subunits (at alpha)
=> G-protein is ACTIVATED
What is the next step once the G-protein is activated?
Activation reduces affinity of Alpha and Beta-gamma subunits -> dissociate and diffuse along plasma membrane (ligand bound)
Alpha and beta-gamma each interact with unique effectors
What is another function of the alpha G-protein subunit? (beyond interacting with effectors)
Acts as GTPase
- “built in timer”
- Once it hydrolyzes GTP -> GDP => inactive
- Inactive form has increased affinity for beta-gamma subunit -> will re-bind to G-protein Coupled receptor
(if agonist is still present cycle can restart)
How does the pertussis toxin affect the G-protein?
Binds/ inserts ADP-ribose into Alph-subunit => locks inactive confirmation
(prevents receptor coupling)
How does cholera toxin affect the G-protein?
Inserts ADP-ribose near hydrolyzing site of alpha-subunit => prevents hydrolysis of GTP -> keeps Alpha in active form -> over expressed
What is the beta-adrenergic GCPR pathway in response to an agonist? (in the heart)
Agonist (NE, epinephrine, isoproterenol) -> Activate alpha Gs -> Gs binds Adenylate cyclase (AC) -> Increase levels of cAMP [SECOND MESSENGER] -> cAMP activates Protein kinase A (PKA) -> PKA phosphorylates (opens) Ca+ channels
=> [Ca2+] increases & HR & BP increase
What are some antagonists of the Beta-adrenergic GCPR? What do they do?
Beta-blockers => decrease HR and BP
propanolol
metoprolol
What is the alpha-adrenergic GCPR pathway in response to an agonist? (in peripheral vasculature)
Agonist (NE, epinephrine, phenylephrine) -> activate alpha Gq -> Gq binds phospholipase c (PLC) -> PLC cleaves PIP2 -> IP3 & DAG [SECOND MESSENGERS]
IP3 -> binds receptor on ER -> releases Ca2+
DAG -> activates PKC -> PKC stimulates L channel (L-ch) -> allows Ca2+ in
=> [Ca2+} increases - smooth muscle contract - decreased blood flow to periphery - Increase BP
What are some antagonists of the Alpha-adrenergic GCPR pathway? What do they do?
Alpha- blockers => decrease BP
prazosin
How does the m2-muscarinic cholinergic receptor inhibit Gs in the heart?
Agonist (acetylcholine, muscarine) bind M2AchR -> activate Gi -> Gi agonizes/ inhibits Gs-> dominant inhibition is possible ->
cAMP degraded by phosphodiestreases (PDEs) -> AMP (does not activate PKA) -> no signaling to increase Ca2+
What types of substances can inhibit phosphodiesterases- PDEs? (maintain cAMP levels)
Caffeine/ theophyline
Millinirone (PDE3)
Rollpram (PDE4)
Viagra
How does the M2AchR affect the beta-gamma pathway?
agonist (Ach) activates Beta-Gamma -> Beta-gamma binds and activates K+ channel -> K+ exits cell, decreases Ca2+ influx, hyperpolarizes cell
=> decreases HR & contraction
How does the B2 GPCR in the lung differ from the B1 GPCR in the heart and Gs GPCR in the periphery?
Same initial pathway:
agonist -> B1 alpha activation -> cAMP hydrolysis -> PKA activation
=> INHIBITS SMOOTH MUSCLE CONTRACTION - BROCODILATION
- same in blood vessels to lung, heart and muscle
What is the parasympathetic response of the m3 muscarinic cholinergic receptor to the sypathetic Gs GCPR in the lung?
agonist (Ach) binds -> activates Gq -> activates PLC -> cleaves PIP3 -> IP3 and DAG
=> increase Ca2+ - smooth muscle contraction - BRONCHOCONSTRICTION
What happens in the desensitization pathway?
G-protein coupled receptor kinase (GRK) binds active GPCR (sometimes at beta-gamma)-> phosphorylates -> Beta-arrestin binds -> induces endocytosis & prevents alpha protein recoupling
What are the two possible outcomes one a GPCR has been endocytosed?
Degradation at lysosome or endosome
Returned to cell membrane
(or de phosphorylated and arrestin acts as scaffold for new receptor complex)
What can induce the desensitization pathway?
long term opiate/ analgestic use
