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What is the net overall effect of activated PKA in glycogen metabolism?
What is the net overall effect of activated PKA in glycogen metabolism?
Which enzyme is phosphorylated by activated PKA to promote glycogen breakdown?
Which enzyme is phosphorylated by activated PKA to promote glycogen breakdown?
What role does cAMP play in the activation of PKA?
What role does cAMP play in the activation of PKA?
How does the β2-adrenergic receptor influence glycogen metabolism?
How does the β2-adrenergic receptor influence glycogen metabolism?
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In the process of glycogen breakdown, the conversion of phosphorylase b to phosphorylase a is mediated by which mechanism?
In the process of glycogen breakdown, the conversion of phosphorylase b to phosphorylase a is mediated by which mechanism?
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What distinguishes relay molecules from second messengers in signal transduction pathways?
What distinguishes relay molecules from second messengers in signal transduction pathways?
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Which of the following is NOT commonly recognized as a second messenger in human systems?
Which of the following is NOT commonly recognized as a second messenger in human systems?
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Which effector systems are directly involved in generating second messengers?
Which effector systems are directly involved in generating second messengers?
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What role do second messengers play in signal transduction?
What role do second messengers play in signal transduction?
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Which of the following statements about downstream signaling is true?
Which of the following statements about downstream signaling is true?
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Which enzyme is activated by Gαs in the AC-cAMP-PKA pathway?
Which enzyme is activated by Gαs in the AC-cAMP-PKA pathway?
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What is the primary second messenger produced by adenylyl cyclase?
What is the primary second messenger produced by adenylyl cyclase?
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Which of the following statements about phospholipase C is true?
Which of the following statements about phospholipase C is true?
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All isoforms of adenylyl cyclase are activated by forskolin except which one?
All isoforms of adenylyl cyclase are activated by forskolin except which one?
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Which adenylyl cyclase isoforms are inhibited by Gαi?
Which adenylyl cyclase isoforms are inhibited by Gαi?
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What is the primary function of protein kinase A?
What is the primary function of protein kinase A?
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Which subunits form the inactive tetramer of protein kinase A?
Which subunits form the inactive tetramer of protein kinase A?
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Where do the RI regulatory subunits primarily originate from?
Where do the RI regulatory subunits primarily originate from?
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How does cAMP activate protein kinase A?
How does cAMP activate protein kinase A?
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What is the homology percentage between the cAMP-binding domains of RI and RII?
What is the homology percentage between the cAMP-binding domains of RI and RII?
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What is the role of forskolin (Fsk) in the activation of adenylyl cyclase (AC)?
What is the role of forskolin (Fsk) in the activation of adenylyl cyclase (AC)?
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Which structure is NOT a characteristic of adenylyl cyclase (AC)?
Which structure is NOT a characteristic of adenylyl cyclase (AC)?
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How does forskolin affect cyclic AMP levels in research?
How does forskolin affect cyclic AMP levels in research?
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What effect does forskolin have on heart rate?
What effect does forskolin have on heart rate?
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What happens to the configuration of AC when it is activated?
What happens to the configuration of AC when it is activated?
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Which of the following statements correctly describes the effect of Gαs on AC?
Which of the following statements correctly describes the effect of Gαs on AC?
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What is the primary output of the reaction catalyzed by adenylyl cyclase?
What is the primary output of the reaction catalyzed by adenylyl cyclase?
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Which domains of adenylyl cyclase are crucial for its catalytic activity?
Which domains of adenylyl cyclase are crucial for its catalytic activity?
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How do Fsk and Gαs interact in their functional roles?
How do Fsk and Gαs interact in their functional roles?
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Which statement correctly describes Gαs and Gαi?
Which statement correctly describes Gαs and Gαi?
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Which actions are primarily mediated by cAMP?
Which actions are primarily mediated by cAMP?
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What role does Gβγ play in adenylyl cyclase activity?
What role does Gβγ play in adenylyl cyclase activity?
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Which types of actions are categorized as non-PKA-mediated?
Which types of actions are categorized as non-PKA-mediated?
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What can uncontrolled cAMP-dependent pathways ultimately lead to?
What can uncontrolled cAMP-dependent pathways ultimately lead to?
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What differentiates the action of Gαi from Gαs in the context of adenylyl cyclase?
What differentiates the action of Gαi from Gαs in the context of adenylyl cyclase?
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What is the function of CAM in relation to adenylyl cyclase?
What is the function of CAM in relation to adenylyl cyclase?
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Study Notes
Bioc 325: Lecture 4
- Learning Objectives: Define effector systems and second messengers downstream of Gα. Appreciate the role of these systems in extracellular signal processing.
GPCR Downstream Signaling
-
Effector Systems:
- Cyclases (AC, GC)
- Phospholipases
- Phosphodiesterases (PDEs)
- BARKS
- RhoGEFs
- Ion channels
-
Second Messengers:
- Ions (Ca2+, K+)
- cAMP, cGMP
- Arachidonic acid
- Phosphoinositides
- Diacylglycerol
- Gases (NO, CO)
-
Second messenger-dependent Protein Ser/Thr Kinases:
- AGC family (PKA, PKG, & PKC)
- PKB/Akt
- CAMK: Ca2+/calmodulin-activated protein kinase
Relay Proteins and Second Messengers
- Relay molecules (effectors, kinases) and second messengers pass signals intracellularly.
- Relay molecules are proteins requiring activation and do not diffuse quickly.
- Second messengers are small, water-soluble, easily diffusible non-protein molecules that activate relay proteins allosterically.
- cAMP and Ca2+ are common second messengers.
Two Main Effector Systems Downstream of GPCRs
| Effector Enzymes Coupled to GTP-binding Proteins | Associated 2nd messenger | 2nd messenger-dependent Ser/Thr Kinase |
|---|---|---|
| Adenylyl cyclase (AC) | cAMP | PKA |
| Phospholipase C (PLC) | IP3, DAG | PKC |
Main Pathways Downstream of GPCRs
- AC-cAMP-PKA pathway
- PLC-(IP3 & DAG)-PKC pathway
Enzymatic Activity of AC
- AC: converts ATP to 3',5'-cyclic AMP (cAMP).
- cAMP is hydrolyzed to 5'-AMP by Phosphodiesterase (PDE).
- Theophylline and cAMP are involved in the process.
Regulation of AC Activity
- Stimulatory hormones (Epinephrine, Glucagon, ACTH) activate adenylate cyclase (E).
- Inhibitory hormones (PGE₁ Adenosine) inhibit adenylate cyclase.
Mammalian AC
- 9 membrane-bound isoforms activated by Gαs.
- Type 1 is purified from bovine brain.
- Gαs activates all AC isoforms in the GTP-bound form.
- All isoforms (except AC9) are activated by forskolin (Fsk).
Mammalian AC (Isoform Information)
- Data on isoforms and their distribution in various tissues and activation.
AC Classes
- Sequence similarities are 50% at the amino acid level.
- Structure and TM domains resemble those of transporters.
- 93% conserved in the Clα and C2α domains
Classification of Mammalian AC
- Three main classes of ACs with specific activation properties.
- Details of each class are provided.
Structural Domains of AC
- 12-helical transmembrane domains (6 at M1 and 6 at M2).
- Two catalytic domains (C1 and C2) each with two subdomains (a & b).
- C- and N-terminals are cytoplasmic.
- Inactive conformation: Clα and C2α domains are separate.
What is Fsk?
- Forskolin (Fsk) is a diterpene from the Indian Coleus plant used to increase cAMP levels.
- Fsk is a strong activator for AC.
- Fsk has a positive inotropic effect, increasing heart rate and lowering blood pressure.
- Fsk has gastrointestinal (GI) and central nervous system (CNS) effects which are non-specific.
- Fsk is a hydrophobic activator that glues the two domains of the active core (Head to tail fashion), assisting GTP-Gαs binding as Fsk.
Active Conformation of AC
- The Clα and C2α domains contact each other to form binding sites for ATP, Fsk, Gas and Gi.
- Dimer formation between Clα and C2α characterizes the active AC conformation.
Head To Tail Binding of Clα and C2α
- Head-to-tail binding of the C1α and C2α domains is described.
The C1 and C2 domains
- The C1 and C2 domains are sufficient for ATP to cAMP conversion but with low activity.
- The association rate and activity of domains are enhanced 100-fold by Fsk or Gαs.
- Fsk and Gαs are synergistic; one enhances the other's affinity.
Crystal Structure of Activated C1α-C2α dimer of AC
- Detailed information about the crystal structure of the activated dimer.
Different Binding Sites on AC
- Gas facilitates active site closure around ATP.
- Gᵧ binds to C2α and facilitates conformational changes for cooperative enzyme stimulation.
- Fsk facilitates active site closure around ATP.
- Gai stabilizes a more open conformation.
3D Structure of the Binding Sites of AC
- Detailed information on various parts.
Important Domains of AC
- CAM associates with C1b for regulation of Ca2+-sensitive AC (1 and 8).
- Gai acts only on AC 1, 5, and 6; different binding sites than Gαs.
- Gβγ activates AC2 when Gαs is bound.
cAMP-mediated Cellular Responses
- Many cell responses are mediated by cAMP, including increased heart rate, cortisol secretion and glycogen/fat breakdown.
- Uncontrolled cAMP-dependent pathways can lead to cancer development or progression.
cAMP Actions
- PKA-mediated actions and non-PKA-mediated actions.
- cAMP-dependent exchange protein (Epac) exchange.
PKA
- Nomenclature: cAMP-dependent protein kinase (also called protein kinase A).
- Function: phosphorylates proteins at Ser/Thr residues.
Structure of PKA
- 2 catalytic subunits (α, β, γ) with tissue-specific isoforms.
- 2 regulatory subunits (RI and RII), with isoforms and similarities in cAMP-binding domains.
- Subunits' origins differ: RI in skeletal muscles and RII in cardiac muscles.
Structure-Function Relationship of PKA
- Inactive PKA in a stable tetramer (C2R2) with blocked substrate binding sites in C subunits
- Activation by cAMP binding causes tetramer dissociation, freeing active C subunits (monomers) and opening substrate binding sites.
Actions of cAMP-activated PKA
- Cytosolic actions (short-term regulation), e.g., glycogenolysis regulation.
- Nuclear actions (long-term regulation), e.g., transcription factor (CREB) phosphorylation and gene expression.
Role of PKA in the Control of Glycogen Breakdown
- Following exercise or starvation, glycogen breakdown occurs through ẞ2-adrenergic receptor activation and cAMP signaling.
- PKA phosphorylates phosphorylase kinase and glycogen synthase.
- This enhances glycogenolysis and inhibits glycogen synthesis.
Activated PKA
- Favors glycogen breakdown.
- Blocks glycogen synthesis.
- Results in enhanced glycogenolysis.
Role of PKA in the Control of Glycogen Breakdown: Signal Amplification
- Signal amplification occurs via phosphorylation mechanisms involving kinases.
- Each kinase can activate multiple kinase enzymes in a cascade (geometric increase).
Signal Amplification by Phosphorylation: Importance of Kinases
- Explains the mechanism of signal amplification by enzymes.
PKA-mediated cAMP Actions
- Illustrates PKA-mediated cAMP actions on various cellular organelles.
PKA-Induced Activation of CREB Transcription Factor
- PKA catalytic subunit phosphorylates CREB within the nucleus.
- CREB activation leads to gene expression, specifically binding to CRE in promoter regions.
PKA-Induced Activation of CREB Transcription Factor: Extracellular Space
- Visual representation of the extracellular space.
Interaction with non-PDZ scaffold proteins
- AKAPs (A-kinase anchoring proteins) are scaffolds for PKA-receptor interactions, needed for receptor phosphorylation by PKA.
- AKAPs are involved in desensitization, decreasing subsequent receptor signal.
AKAP79
- AKAP79 associates with β2-ARs, recruiting PKA, PKC, and PP2B.
- This integration of signaling pathways modulates cellular function.
PKA-Induced Activation of ERK1/2
- PKA phosphorylates β2-ARs diverting attention from Gs to Gi signaling.
- Phosphorylation of Gβγ initiates downstream reactions and ERK activation.
- This leads to subsequent nuclear signals.
Epac
- Nomenclature: Exchange protein directly activated by cAMP (a GEF).
- Function: a guanine nucleotide exchange factor (GEF).
- Structure: has homology to Ras-GEF and Rap1-GEF, with cAMP-binding domains.
- Two isoforms: Epac1 and Epac2.
Epac-mediated cAMP actions
- (Detailed cAMP actions mediated by Epac).
Epac is Involved in Signaling of Multiple Receptors
- (Comprehensive illustration of Epac involvement in various receptor signaling pathways).
Epac
- Epac is a CAMP-binding domain that functions as a molecular switch to control diverse biological functions regulated by cAMP levels.
- The presence of two cAMP effectors (EPAC and PKA) allows for more specific and integrated control of cAMP signaling pathways in a spatial and temporal manner to modulate cellular function.
Bioc 325: Lecture 5
- Topic: Effector Systems and Second Messengers Downstream of GPCRs (Part 2)
- Focus: PLC-(IP3 & DAG)-PKC pathway
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Description
This quiz explores the role of protein kinase A (PKA) in glycogen metabolism, including the effects of cAMP and key signaling pathways. Test your understanding of how glycogen breakdown is regulated and the significance of second messengers in cellular signaling.
