Unit 8.1 : Signal Transduction Pathways - I Flashcards
4 Steps: Gs-protein-coupled receptors that signal through the second messenger cAMP
(1) Ligand binding activates the receptor
(2) The heterotrimeric Gs protein associates with the receptor and is activated by the replacement of GDP for GTP.
(3) The a subunit dissociates from the by subunits and associates with adenylyl cyclase.
(4) Active adenylyl cyclase leads to the production of the second messenger cAMP (adenylyl cyclase converts ATP)
Gs-protein-coupled receptors that signal through the second messenger cAMP:Turning the signal off requires (2):
(i) HYDROLYSIS of GTP on the a subunit to GDP and Pi
(ii) conversion of cAMP to AMP by a PHOSPHODIESTERASE ENZYME
cAMP is formed from
ATP
**adenyl cyclase turns ATP into cAMP
cAMP is formed from ATP by:
a CYCLIZATION REACTION that removes two phosphate groups from ATP and joins the “free” end of the remaining phosphate group to the sugar part of the ATP molecule.
The conversion of cAMP to AMP is performed by a
phosphodiesterase enzyme
The conversion of cAMP to AMP is performed by a phosphodiesterase enzyme that
reduces one of the ester bonds.
The diverse effects of cAMP in animal cells are mediated by the action of
cAMP-dependent protein kinase (protein kinase A = PKA).
The inactive form of PKA is a:
tetramer
The inactive form of PKA is a tetramer, consisting of
two regulatory (R) and two catalytic (C) subunits.
pka activation leads to:
the phosphorylation of downstream target proteins modulating their activity
The inactive form of PKA is a tetramer, consisting of two regulatory (R) and two catalytic (C) subunits.
Each R subunit has:
two binding sites for cAMP
–> one pka can bind 4 camp
Each R subunit has two binding sites for cAMP. Upon binding of cAMP to the regulatory subunits, the catalytic subunits
in their active form are released and can phosphorylate downstream target proteins.
Adrenaline-mediated rise in cAMP is important in
mediating the body’s response to stress, such as fright or heavy exercise, when all tissues have an increased need for glucose and fatty acids.
Adrenaline-mediated rise in cAMP is important in mediating the body’s response to stress, such as fright or heavy exercise, when all tissues have an increased need for glucose and fatty acids.
The responses are dictated by:
the types of receptors found in these cells and the types of signaling pathways that are activated.
Adrenaline binds to
adrenergic receptors.
b-adrenergic receptors are:
Gs receptors
a-adrenergic receptors are
Gi receptors.
adrenaline -> heart: major response
increase in heart rate and force of contraction (increased blood supply (nutrients) to tissues
adrenaline -> skeletal muscle:major response:
glycogen breakdown
adrenaline -> fat:major response:
fat breakdown
Glycogen is a:
POLYMER of GLUCOSE and is the major storage form of this sugar.
Glycogen Synthesis:
Glucose is coupled to UDP and then incorporated into the glycogen polymer by GLYCOGEN SYNTHASE.
Glycogen Breakdown:
GLYCOGEN PHOSPHORYLASE
removes glucose as glucose-1-phosphate when glucose is needed.
What are the two enzymes involved in glycogen synthesis and breakdown
Synthesis: Glycogen Synthase
Breakdown: Glycogen Phosphorylase
Glycogen metabolism: tissue specific regulation: what happens to glucose 1 phosphate in muscles vs in liver:
in muscles: glucose 1 phosphate is converted to glucose 6 phosphate
in liver: phosphate is removed from glucose-1-phosphate and glucose absorbed in blood
Adenylyl cyclase
an enzyme that converts ATP to cyclic AMP
Glycogen metabolism: One target of PKA is glycogen synthase. The phosphorylated form is;
inactive and this prevents glycogen production. It can be restored to its active form by the actions of a phosphatase called protein phosphatase 1.
phosphorylated form of glycogen synthase is:
inactive (and this prevents glycogen production)
phosphorylated glycogen synthase is inactive: it can be restored to its active form by:
the actions of a phosphatase called protein phosphatase 1
Another target of PKA is phosphorylase kinase. Once this kinase is phosphorylated, it is
activated. This activation is also reversible by protein phosphatase 1.
PHOSPHORYLATED phosphorylase kinase is :
active
phorylated phosphorylase kinase is active, and can be reversed by:
phosphatase 1
The pka has two types of pathways:
(1) fast (second - minutes)
(2) slow (minutes to hours)
The PKA pathway has fast and slow responses: fast responses involve:
Signal transduction in the cytosol involving activation of PKA,
phosphorylation of glycogen synthase, phosphorylase kinase and glycogen phosphorylase.
The PKA pathway has fast and slow responses: slow response (minute to hours) include:
cAMP-inducible gene transcription
cAMP- inducible gene transcription: The PKA pathway can regulate gene expression by:
activating transcription factors that bind to cAMP response elements (CREB) in the DNA.
cAMP- inducible gene transcription:All cAMP-responsive genes have a
DNA sequence upstream of the gene called a cAMP response element (CRE).
cAMP- inducible gene transcription:What happens after CREB gets activated?
CREB then dimerizes and binds to the CRE sequence. This then recruits a protein complex composed of CBP and p300 and other proteins that modify the DNA such that it becomes transriptionally active.
pka pathway slow response (3 steps):
- free catalytic subunit of protein kinase A translocates to the nucleaus through nuclear pores
2.it phosphorylates and activates a transcription factor called CREB - CREB dimerizes and binds to CRE sequence, this the recruits a protein complex and other proteins that modify DNA such that it becomes transcriptionally active
CREB
cAMP response element binding proteins
All cAMP-responsive genes have a
DNA sequence upstream of the gene called a cAMP response element (CRE).
cAMP-Inducible Gene Transcription
PKA Translocation: The free catalytic subunit of PKA translocates to the nucleus.
CREB Activation: PKA phosphorylates and activates CREB (cAMP response element-binding protein).
CRE Sequences: cAMP-responsive genes have a CRE sequence upstream of the gene.
Transcriptional Activation:
CREB dimerizes and binds to the CRE sequence.
This recruits a protein complex composed of CBP and p300, which modify the DNA to make it transcriptionally active.
Turning off the adrenaline/glucagon signal (4):
- Affinity between receptor and ligand decreases in the presence of
activated Gsa, limiting the number of Gsa proteins that are activated.à - Hydrolysis of GTP on Gsa is increased by adenylyl cyclase (like a GAP
protein) - Phosphodiesterase converts cAMP to AMP, lessening the signal.
- The b-adrenergic receptor itself becomes a substrate for both PKA and another kinase called BARK (b-adrenergic receptor kinase) that desensitize it.
Some second messengers are derived from phosphatidylinositol. The inositol group can be phosphorylated by several kinases at either
the 4 or 5 position of the ring
A phospholipase C enzyme can then hydrolyze the inositol ring releasing two second messengers
diacylglycerol (DAG) and inositol-1,4,5- triphosphate (IP3).
A phospholipase C enzyme can then hydrolyze the inositol ring releasing two second messengers: diacylglycerol (DAG) and inositol-1,4,5- triphosphate (IP3).The second messengers DAG and IP3 then trigger separate downstream events ;
(1) DAG remains membrane associated
(2) IP3 is hydrophilic and can diffuse in the cytosol
G-protein coupled receptors also signal through phospholipids and Ca2+
Two phospholipase C (PLC) isoforms:
- PLC-b – Activated through Gq-associated receptors via the Gqa subunit.
Vasopressin, acetylcholine and thrombin signal through such receptors. - PLC-g – activated through receptor tyrosine kinases (RTKs)
Gq vs Gs or Gi?
Gq is a type of G-protein alpha subunit that activates a different signaling pathway than Gs or Gi.
Instead of regulating adenylate cyclase and cAMP, Gq activates phospholipase C (PLC), which triggers the IP₃/DAG pathway.
__ is a protein that binds to calcium ions
calmodulin
In the calcium bound state: calmodulin binds to:
other CaM kinases and other proteins to elicit diverse responses
once calmodulin binds CaM kinases and other proteins it elicits diverse responses such as:
(1) Phoshorylates the myosin light chain
(2) activation of PDE 1 reducing levels in cytosol
(3) Phosphorylation and activation of CREB protein: transcriptipn of various genes
Describe the 3 steps involved: Regulation of Muscle Contraction by Ca 2+ and CaM:
(1) A rise in intracellular Ca2+ causes Ca2+ to bind to CaM
(2) CaMCa2+ activates myosin light chain kinase (MLCK) (a CaM kinase)
(3) MLCK phosphorylates the light chain of myosin which then binds to actin to initiate muscle contraction
Regulation of CREB Transcriptional activity by Ca2+ and CaM (4):
(1) A rise in intracellular Ca2+ causes Ca2+ to bind to CaM
(2) CaM Ca2+ activates CaMKII ( a CaM kinase)
(3) CamKII enters the nucleus through the pores and phosphorylates CREB
(4) CREB dimerizes and binds to DNA at CRE sequences, and recruits p3000 and CBP to activate gene transcription
Regulation of cAMP levels by Ca2+ and CaM (3):
(1) A rise in intracellular Ca2+ causes Ca2+ to bind to CaM
(2) CaM Ca2+ activates calcium-calmodulin dependant phosphodiesterase 1 (PDE1), highly expressed in brain, heart, lung.
(3) PDE1 converts cAMP (produced by adenyl cyclase) to AMP, downregulating cAMP=dependant signaling pathways
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