Second Messanger Systems Flashcards
What occurs when epinephrine binds to beta-adrenergic receptor?
- The occupied receptor causes replacements causes replacement of the GDP bound to Gs by GTP, activating Gs
- Gs (a subunit) moves to Adenylyl Cyclase and activates it
- Adenylyl cyclase catalyzes the formation of cAMP (and 5’-AMP)
- PKA is activated by cAMP
- Phosphorylation of cellular proteins by PKA causes the cellular response to epinephrine
- cAMP is degraded, reversing the activation of PKA
What are the changes in G-protein on formation of hormone-receptor complex?
- Hormone binds to receptor
- Exchange of GDP for GTP on Ga. dissociation of B y-subunits
- Activation of Adenylate cyclase by Gsa-GTP
- Ga has intrinsic GTPase and dissociates from adenylate cyclase
- Gas-GDP reassociates with By(beta gamma)-subunits
Gsa- GTP is active
Ga subunit has intrinsic GTPase activity
Gsa-GDP is inactive
How is protein kinase A?
Epinephrine or Norepinephrine bind to B-adrenergic receptor
Glucagon binds to glucagon receptor
Gsaos activated which activates adenylate cyclase, adenylate cyclase converts ATP to increase cAMP
Activates Protein kinase A
What are the steps in adenylate cyclase activation?
- Hormone binds to G-protein coupled receptor
- The inactivation G protein is bound to GDP
- Activated receptor interacts with G-protein causing Ga-subunit to release GDP and bind GTP ; Ga subunit-GTP bound and By(beta gamma ) dimer dissociate
- GTP bound Ga-subunit activates adenylate cyclase
- Adenylate cyclase catalyzes the conversion of ATP to cAMP (second messenger)
- cAMP binds to regulatory subunits of protein Kinase A; catalytic kinase subunits are active
- Catalytic kinase subunits transfer phosphate from ATP to Ser or Thr residues of protein substrates (enzymes) - Covalent modification of enzymes
- Phosphorylation of enzymes can activate or inhibit enzymes —> Regulation metabolic pathways
What is the termination of signal adenylate cyclase
- Intrinsic GTP —> GDP+Pi GTPase activity of G-a subunit (intrinsic timer)
- Phosohodiesterase enzyme hydrolyzes cAMP to AMP
- Hormone dissociates from receptor
Caffeine and theophylline inhibit cyclic nucleotide phosphidiesterase and prolong effects of cAMP
What are the enzyme regulatory mechasnisms ?
- Availability of substrates (minutes)
- Allosteric activation/inhibition of enzymes (minutes)
- Covalent modification of enzymes (minutes to hours ) -mediated via Protein kinase A
- Induction (up-regulation) or repression (down-regulation) of enzyme synthesis (hours to days)
What are the cAMP effects on PKA?
cAMP Exerts effects via activation PKA:Rapid effect (sounds to minutes)-covalent modification of pre-existing enzymes
cAMP/PKA also exerts effects via gene regulation through CREB: response relatively slow -hours to days
How does cAMP have an effect on gene expression?
CREB- cAMP response element binding protein
CRE- cAMP Response element
Gs activates Adenylate cyclase leads to increase cAMP. cAMP leads to protein kinase A. CREB and PKA leads to CREB-P
CREB-P interacts with CRE in DNA to affect gene expression
Describe a-2 adrenergic receptors
- Epinephrine binds to a2-adrenergic receptors activates Gia
- Inhibits Adenylate cyclase
- Reduces intracellular cAMP levels
- Compare a2 and B
Describe the affect of bacterial toxins on Ga subunits by covalent modifications
-ADP-ribosylation (addition of ADP-ribose)—> inactivation or proteins
- Cholera toxin ADP-ribosylates Gas—> Continuously active Gas
- Intrinsic GTPase activity of Gas destroyed, Gas locked in GTP-bound state
- Increased cAMP in intestinal mucosal cells
- Opening of CFTR—> loss of electrolytes and water —> Severe watery diarrhea
- Pertussis toxin ADP-ribosylates Gai —> Continuously inactive Gai
- Increased cAMP in respiratory-tract cells
- Respiratory distress, and whooping cough
Diphtheria toxin ADP-ribosylates eEF-2
Some pathogenic E. coli toxin also ADP ribossylates Gsa
How does a-1 adrenergic receptors work?
Epinephrine/norepinephrine binds to a-1 receptor
Activates Gqa
Activates Phospholipase C
Forms second messengers
- IP3
- Ca2+
- DAG
The activates PKC
What does Phospholipase C do?
Cleaves PIP2 to generate IP3 and DAG
Explain in detail the phosohoinositide system
Second messengers: IP3, DAG, Ca2+
- Epinephrine binds to a-1 receptor
- Activates Gqa which binds to GTP (in exchange for GDP)
- Gqa-GTP activates Phospholipase C
- PLC cleaves PIP2 to IP3 and DAG
- IP3 causes Ca2+ release from endoplasmic reticulum
- DAG and Ca2+ activate Protein kinase C fir cellular responses
- PKC requires DAG, Ca2+ and phospholipids (membrane)
Cholinergic muscuranic receptors activate Phospholipase C isoforms to form IP3,DAG and Ca2+
What does cGMP do?
Second messenger in smooth muscle relaxation, platelet aggregation and in visual system
Describe guanylate cyclase
Two forms
- Membrane bound guanylate cyclase (binds to signals outside cell and prod7ce cGMP inside cell)
- Activated by atrial natriuretic peptide)
- Cytosolic guanylate cyclase (activated by NO)
G-protein isn’t involved
