Lecture 22 Signal Transduction 1

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Lecture 22Signal Transduction 1
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Important Concepts in Signal Transduction

• Primary messengers
• Membrane receptors
• Second messengers
• Amplification
• Signal termination
• 7TM receptors
• G proteins
• Adenylate cyclase - Protein Kinase A
• Phospholipase C- Protein Kinase C, Ca2 Channels

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Cells respond to the environmentSignal
Transduction
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The term signal transduction refers to the
biochemical mechanism responsible for
transmitting extracellular signals inside the
cell, which ultimately lead to the activation of
target proteins that control metabolic pathways
or regulate gene expression.
Courtesy Roger Miesfeld
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Steps of Signal Transduction

• 1. Signal molecule (primary messenger, first
  messenger, ligand) travels to the cell.
• 2. Primary messenger binds to the extracellular
  domain of a receptor protein and initiates a
  structural change in the receptor which is
  propagated across the membrane.
• Membrane receptors sense the stimulus and
  transfer info across the membrane.
• Exception some molecules, for example steroid
  hormones, move across membranes, bind to proteins
  and act, generally at the nucleus.
• Most molecules are too polar or too large to
  cross the membrane, so the stimulus does not
  enter without membrane receptors.
• Generally, receptors are intrinsic (integral)
  membrane proteins with extra- and intracellular
  domains.

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Steps of Signal Transduction

• 3. Receptor protein stimulates signaling proteins
  
• 4. Second messengers amplify the signal
• Free to diffuse
• Cross talk between pathways exists
• 5. Second messengers bind to additional signaling
  proteins
• 6. Signal is propagated, often by a protein
  kinase cascade

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Steps of Signal Transduction

• 7. Target proteins are affected (activated,
  inhibited)
• Transcription factors
• Metabolic enzymes
• Cytoskeletal proteins
• Transport proteins
• Etc.
• 8. Signal is terminated
• Phosphatases

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The biochemical basis for signal transduction
involves three primary mechanisms

• 1) protein conformational changes
• 2) covalent protein modifications
• 3) altered rates of gene expression

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First/primary messengers are small diffusible
biomolecules. These can be produced through
endocrine mechanisms and act at a distance. Or
they can function locally as paracrine or
autocrine signals.
Courtesy Roger Miesfeld
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Small molecules act as diffusible signals
First/Primary Messengers
Human growth hormone and insulin are peptide
hormones Cortisol is a steroid that is derived
from cholesterol Epinephrine, also known as
adrenaline, is a derived from the amino acid
tyrosine Acetylcholine is a neurotransmitter that
binds the acetylcholine receptor NO is produced
by deamination of L-arginine
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Small molecules act as diffusible signals
Second Messengers
Second messengers amplify the receptor-generated
signal Fine tuning Rapid production of maximum
response
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One of the best characterized second messengers
is cyclic AMP (cAMP).
Produced by the enzyme adenylate cyclase from
ATP. Receptor activation of adenylate (adenylyl)
cyclase generates large amounts of cAMP, which in
turn, binds to and activates downstream signaling
proteins such as cAMP dependent protein kinase A
(PKA). Importantly, the intracellular
concentration of cAMP is carefully controlled by
the relative levels of receptor-activated
adenylate cyclase and soluble forms of cAMP
phosphodiesterase (PDE) which converts cAMP to
AMP.
Courtesy Roger Miesfeld
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The intracellular concentration of cAMP is
carefully controlled Relative levels of
receptor-activated adenylate cyclase and soluble
forms of cAMP phosphodiesterase (PDE) which
converts cAMP to AMP.
Courtesy Roger Miesfeld
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Another second messenger important in signal
transduction is cyclic GMP (cGMP)
Produced from GTP by the enzyme guanylyl cyclase.
The cGMP analog Sildenafil, also know as Viagra,
is used to treat sexual dysfunction by inhibiting
the activity of cGMP phosphodiesterase (PDE). The
molecular structure of sildenafil is similar to
cGMP and binds tightly to cGMP PDE.
Courtesy Roger Miesfeld
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The Kissing Bug, Rhodnius prolixus, delivers NO
to their victims by injecting heme-containing
proteins called nitrophorins that carry NO into
the wound along with their saliva.
Courtesy Roger Miesfeld
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Some Other Second Messengers diacylglycerol
(DAG), inositol 1,4,5-trisphosphate (IP3) and
calcium ion (Ca2).
Intracellular levels of DAG, IP3 and Ca2 are
controlled by the activity of a membrane
associated enzyme called phospholipase C (PLC).
Receptor-mediated activation of phospholipase C
leads to cleavage of the membrane phospholipid
phosphatidylinositol 4,5-bisphosphate (PIP2) to
form DAG and IP3.
Courtesy Roger Miesfeld
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The term signal transduction refers to the
biochemical mechanism responsible for
transmitting extracellular signals inside the
cell, which ultimately lead to the activation of
target proteins that control metabolic pathways
or regulate gene expression.
Courtesy Roger Miesfeld
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5 Major classes of receptor proteins the
gatekeepers of the cell
Courtesy Roger Miesfeld
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Seven-Transmembrane-Helix Receptors (7TM
receptors)
All seven-transmembrane-helix receptors are
coupled to G Proteins G Protein Coupled
Receptors or GPCRs
Fig. 14-4
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ß-Adrenergic receptor signal transduction pathway
Binding of ligand on the extracellular domain of
the ß- adrenergic receptor causes a structural
change on the cytoplasmic side This structural
change causes a change in an associated
signal-coupling protein called a G-protein
(guanyl nucleotide binding protein) The change
in the G-protein involves GDP vs. GTP binding
affinity. The G-protein activates adenylate
cyclase, increasing cAMP levels
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Inactive form of G protein heterotrimer binds GDP
Notice the three different subunits here
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G Protein Activation
Receptor conformational change causes structural
change in the G protein GDP leaves, GTP
binds GTP binding cause a structural change in
the G proteinmolecular switch ß? dimer then
dissociates and activated a subunit goes off to
affect other proteins
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The human genome contains 15 a, 5 ß and 10 ?
subunits, leading to many different possible
combinations (1000) and functions
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The activated Ga subunit binds to other proteins
to activate them
Gas binds to adenylate cyclase, activating it,
leading to increased levels of cAMP, which turns
on Protein Kinase A.
Fig. 14-7
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Examples from the adrenergic systems
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Glycogen is a storage form of glucose. Glycogen
is broken down to glucose when the body needs
energy. Glucose levels in the blood are tightly
controlled. Glycogen synthesis (anabolism) and
degradation (catabolism) are highly regulated by
hormone signaling. The liver is one of two
major storage depots for glycogen, which in
response to epinephrine or glucagon signaling,
provides an important source of glucose for
export throughout the body when dietary glucose
is limiting. Epinephrine is the fight or flight
hormone. Glucagon is released by the pancreas
and has been called the hunger hormone because it
signals low blood glucose levels.
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Examples from the adrenergic systems
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Switch II, undergoes a conformational change in
the presence of GTP and is critical for
stimulation of adenylate cyclase activity
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How does cAMP binding activate the
phosphorylating function of protein kinase A?
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Stimulation of PKA signaling events in liver
cells by epinephrine
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Stimulation of PKA in liver cells by epinephrine
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Stimulation of PKA signaling events in liver
cells by epinephrine
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Activation of phospholipase C

• Human liver cells contain a1 adrenergic receptors
• bind epinephrine and signal glycogen degradation
  through a second messenger pathway linked to a
  phosphorylation cascade.
• a1 adrenergic receptors are coupled to a
  heterotrimeric G protein containing Gaq
• activates the enzyme phospholipase C (PLC)
  through a mechanism very similar to Gas
  stimulation of adenylate cyclase activity
• Phospholipase C is a membrane associated protein
• catalyzes the hydrolysis of phosphatidylinositol
  4,5-bisphosphate (PIP2) to form the second
  messengers DAG and IP3.

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PLC produces second messengers DAG and IP3
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Function of second messengers IP3 DAG

• IP3 binds to calcium channels on the endoplasmic
  reticulum
• causes a rapid increase in intracellular Ca2
  levels.
• Released Ca2 binds to protein kinase C (PKC)
• stimulates its association with DAG at the plasma
  membrane resulting in activation of the PKC
  kinase function
• Ca2 binds to calmodulin, activating
• phosphorylase kinase
• calmodulin dependent kinase
• PKC and calmodulin dependent kinase
• phosphorylate and inactivate glycogen synthase
• Calmodulin-activated phosphorylase kinase
  stimulates glycogen degradation by activating
  glycogen phosphorylase

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Stimulation of PKC signaling events in liver
cells by epinephrine
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G proteins reset themselves
Possesses GTPase activity
The ß? dimer then re-associates with the a
subunit, preventing it from further propagating
the signal Ready to start all over again.
Fig. 14-9
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Dont forget the 7TM receptor,it must be reset
as well, or it activates more G protein
Fig. 14-10
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Summary of G protein coupled receptor signaling
1. Receptor-mediated activation of GDP-GTP
exchange in Ga subunits.2. Ga stimulation of an
effector enzyme that generates 2nd
messengers.3. Activation of a phosphorylation
cascade by 2nd messenger signaling.4.
Inactivation of Ga by effector stimulation of the
intrinsic GTPase activity.5. Signal duration is
controlled by loss of 2nd messengers and receptor
desensitization.
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What happens when receptors arent reset?

• Dopamine receptor and cocaine / amphetamines
• Opiate receptor and heroin
• Serotonin 5-HT1A receptor and MDMA (Ecstasy)
• These are all G protein coupled receptors
  (GPCRs)!
• First you get high, then you become addicted.
• Why? Because the dopamine D2 receptor and the
  opiate and the 5-HT1A receptors bind these drugs
  much more tightly than they do their natural
  ligands. They dont get reset properly, the whole
  signaling cascade downstream is messed up.
• After exposure, more receptor is needed to get
  the normal physiological response. The body
  responds by altering gene expression and receptor
  levels in the brain.
• One exposure to MDMA or Meth permanently alters
  brain function! There is no going back. This is a
  huge social problem right now.

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