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3.D.3 Signal Transduction
Signal transduction pathways link
signal reception with cellular
response.
Many chemical messengers cannot
cross the plasma membrane.
Signal transduction is the process by
which an extracellular signal is
converted to an intracellular reponse.
Signaling begins with the recognition of a
chemical messenger, a ligand, by a
receptor protein.
A ligand can be:
•
•
•
•
•
Ions
Proteins (hormones)
Neurotransmitters
Steroids
Many other types
Different receptors recognize different
ligands in a specific one-to-one
relationship.
When a ligand binds to a receptor, it
causes the receptor protein’s shape to
change (conformation change).
The conformation change of the
receptor initiates transduction of the
signal.
Types of
Receptors:
G Protein
Receptors
Ligand-gated
Ion Channels
Receptor
Tyrosine Kinases
G-protein linked receptors
The ligand binds to the receptor.
The receptor is attached to a G-protein,
which is bound to a GDP molecule.
The binding of the ligand to the receptor
causes a conformation change in the
receptor. The GDP attached to the Gprotein is replaced with a GTP.
The G-protein disassociates from the
receptor.
The G-protein binds to and activates of
adenylate cyclase, which in turn produces
the second messenger cyclic AMP (cAMP).
The cAMP phosphorylates a protein
kinase, which begins a phosphorylation
cascade that amplifies the signal.
Ligand-gated Ion Channels:
Model: the Sodium Channel
The ligand that binds to the ion-gated
sodium channel is the neurotransmitter
acetylcholine.
The sodium channel is closed when
acetylcholine is not bound to the receptor.
The sodium channel opens when
acetylcholine is bound the receptor site.
Sodium is then able to diffuse into the
cell down its concentration gradient.
Receptor Tyrosine Kinases (RTK):
Model: Growth Factor Hormone
Inactivate RTKs are disassociated.
When growth factor binds to
neighboring RTKs, they combine to
become a cross-linked dimer.
Cross-linking activates the tyrosine kinase
activity. Each RTK in the dimer phosphorylates
multiple tyrosines on the other RTK. This
process is called cross-phosphorylation.
RTKs activate relay proteins that are able to
active multiple signal transduction
pathways at once.
Signaling cascades often amplify the
incoming signals, with the result of
appropriate responses by the cell.
Second messengers are often essential
to the function of the cascade.
Second messengers can include cyclic
GMP, cyclic AMP, calcium ions (Ca2+),
and inositol triphosphate (IP3)
A signal relayed by a signal transduction pathway may
trigger an increase in calcium in the cytosol.
Pathways leading to the release of calcium involve
inositol triphosphate (IP3) and diacylglycerol (DAG) as
additional second messengers.
Many signal transduction pathways cause:
• Protein modifications such as methylation
that change the signaling process and regulate
cell processes.
• Phosphorylation cascades that amplify the
signal.
An Example Signal Transduction Pathway
Learning Objectives:
LO 3.36 The student is able to describe a model
that expresses the key elements of signal
transduction pathways by which a signal is
converted to a cellular response. [See SP 1.5]
References:
• https://highered.mcgrawhill.com/sites/0072507470/student_view0/chapter17/anim
ation__membranebound_receptors_that_activate_g_proteins.html
• https://highered.mcgrawhill.com/sites/0072507470/student_view0/chapter17/anim
ation__second_messenger__camp.html
• http://highered.mcgrawhill.com/sites/0072495855/student_view0/chapter2/anima
tion__receptors_linked_to_a_channel_protein.html
• http://www.dnatube.com/video/539/Receptor-TyrosineKinase-Activation-and-Signalling