Signal transduction
60 flashcards covering Signal transduction for the MCAT Biology & Biochemistry section.
Signal transduction is the process by which cells detect and respond to external signals, such as hormones or neurotransmitters, to maintain functions like growth, metabolism, and immune responses. At its core, it involves a series of steps: a signal binds to a receptor on the cell surface, triggering a cascade of internal events that amplify and transmit the message inside the cell. This allows cells to communicate effectively and adapt to their environment, making it essential for understanding how organisms function at a molecular level.
On the MCAT, signal transduction appears in biology and biochemistry passages, often through questions that test your knowledge of pathways, receptors, and second messengers like cAMP or calcium ions. Common traps include confusing similar mechanisms, such as G-protein coupled receptors with enzyme-linked receptors, or overlooking how signals are regulated to prevent errors. Focus on key concepts like signal amplification, cross-talk between pathways, and integration with topics like gene expression or cell signaling disorders. Practice drawing out pathways to solidify your understanding.
Terms (60)
- 01
Signal transduction
Signal transduction is the process by which a cell converts an extracellular signal into a specific cellular response through a series of molecular events involving receptors and intracellular messengers.
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Ligand
A ligand is a signaling molecule that binds to a specific receptor on or in a cell, triggering a response in signal transduction pathways.
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Receptor
A receptor is a protein on the cell surface or inside the cell that binds to a specific ligand, initiating signal transduction by changing its conformation or activity.
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Hydrophilic signal
A hydrophilic signal is a water-soluble molecule that cannot cross the cell membrane and must bind to cell-surface receptors to initiate signal transduction.
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Hydrophobic signal
A hydrophobic signal is a lipid-soluble molecule that can pass through the cell membrane and bind to intracellular receptors to affect gene expression.
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G-protein coupled receptor
A G-protein coupled receptor is a cell-surface receptor that, upon ligand binding, activates associated G-proteins to initiate intracellular signaling cascades.
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Receptor tyrosine kinase
A receptor tyrosine kinase is a cell-surface receptor that dimerizes and autophosphorylates upon ligand binding, activating downstream signaling pathways like the MAPK cascade.
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Ion channel-linked receptor
An ion channel-linked receptor opens or closes an ion channel upon ligand binding, allowing ions to flow across the membrane and alter the cell's electrical state.
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Intracellular receptor
An intracellular receptor is located inside the cell and binds to hydrophobic ligands, often leading to changes in gene transcription.
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Second messenger
A second messenger is a small, non-protein molecule or ion released inside the cell in response to receptor activation, amplifying the signal and triggering further responses.
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cAMP pathway
The cAMP pathway involves adenylyl cyclase converting ATP to cAMP, which activates protein kinase A and leads to various cellular responses like glycogen breakdown.
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IP3 and DAG pathway
The IP3 and DAG pathway is activated by phospholipase C, producing IP3 to release calcium from the ER and DAG to activate protein kinase C, both mediating cellular responses.
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Calcium signaling
Calcium signaling uses calcium ions as a second messenger to regulate processes like muscle contraction and neurotransmitter release by binding to proteins like calmodulin.
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Protein phosphorylation
Protein phosphorylation is the addition of a phosphate group to proteins by kinases, often in signal transduction, to activate or deactivate them and propagate the signal.
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Kinase cascade
A kinase cascade is a series of sequential protein phosphorylations where one kinase activates another, amplifying the signal and leading to specific cellular outcomes.
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Signal amplification
Signal amplification occurs when a single ligand-receptor binding event triggers multiple downstream molecules, allowing a small signal to produce a large cellular response.
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Receptor desensitization
Receptor desensitization is the process where a receptor becomes less responsive to repeated or prolonged ligand exposure, often through phosphorylation or internalization.
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Downregulation of receptors
Downregulation of receptors involves a decrease in receptor numbers on the cell surface, often by endocytosis, in response to sustained signaling to prevent overstimulation.
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Autocrine signaling
Autocrine signaling occurs when a cell releases a ligand that binds to receptors on its own surface, influencing its own behavior in processes like cell growth.
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Paracrine signaling
Paracrine signaling involves a cell releasing a ligand that affects nearby cells, such as in immune responses where cytokines coordinate local activities.
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Endocrine signaling
Endocrine signaling uses hormones released into the bloodstream to affect distant target cells with specific receptors, regulating functions like metabolism.
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Signal transduction in immune cells
Signal transduction in immune cells involves receptors like T-cell receptors that activate pathways to trigger cytokine production and immune responses.
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Apoptosis pathway
The apoptosis pathway is a signal transduction process that leads to programmed cell death, often initiated by receptors like Fas and involving caspases.
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JAK-STAT pathway
The JAK-STAT pathway is activated by cytokine receptors, where JAK kinases phosphorylate STAT proteins, which then enter the nucleus to regulate gene expression.
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MAPK/ERK pathway
The MAPK/ERK pathway is a kinase cascade activated by growth factors, leading to cell proliferation and differentiation through transcription factor activation.
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PI3K/Akt pathway
The PI3K/Akt pathway is triggered by receptor tyrosine kinases, promoting cell survival and metabolism by inhibiting apoptosis and activating nutrient uptake.
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Notch pathway
The Notch pathway involves cell-surface receptors that, upon ligand binding, are cleaved to release an intracellular domain that enters the nucleus and alters gene expression.
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Wnt pathway
The Wnt pathway stabilizes beta-catenin when Wnt ligands bind to Frizzled receptors, allowing beta-catenin to enter the nucleus and regulate cell fate genes.
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Hedgehog pathway
The Hedgehog pathway is activated when Hedgehog ligands bind to Patched receptors, relieving inhibition of Smoothened and leading to gene transcription for development.
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TGF-beta pathway
The TGF-beta pathway involves serine/threonine kinase receptors that phosphorylate SMAD proteins, which then regulate genes involved in cell growth and differentiation.
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G-protein activation mechanism
G-protein activation occurs when a ligand binds to a GPCR, causing the G-protein to exchange GDP for GTP and dissociate into subunits that propagate the signal.
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Beta-arrestin function
Beta-arrestin binds to phosphorylated GPCRs to desensitize them and can also act as a scaffold for other signaling proteins, linking to pathways like MAPK.
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Adenylyl cyclase role
Adenylyl cyclase is an enzyme that converts ATP to cAMP in response to G-protein activation, serving as a key step in many signal transduction pathways.
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Phosphodiesterase
Phosphodiesterase breaks down cAMP or cGMP to terminate signaling, ensuring signals are transient and cells can respond to new stimuli.
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Calmodulin binding
Calmodulin binding to calcium ions changes its shape, allowing it to activate enzymes like kinases in calcium-dependent signal transduction pathways.
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CREB transcription factor
CREB is a transcription factor phosphorylated by protein kinase A in the cAMP pathway, binding to DNA to promote genes involved in memory and metabolism.
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Cross-talk between pathways
Cross-talk between pathways occurs when components of one signal transduction pathway influence another, leading to integrated cellular responses.
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Feedback loops in signaling
Feedback loops in signaling are regulatory mechanisms where pathway products inhibit or enhance earlier steps, maintaining homeostasis or amplifying responses.
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Oncogenic mutations in RTKs
Oncogenic mutations in receptor tyrosine kinases can cause constitutive activation, leading to uncontrolled cell growth and cancer development.
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Signal termination mechanisms
Signal termination mechanisms include receptor internalization, phosphatase activity, and second messenger degradation to reset the cell after signaling.
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Receptor internalization
Receptor internalization is the process where activated receptors are endocytosed into the cell, leading to degradation or recycling and ending the signal.
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Ubiquitination of receptors
Ubiquitination of receptors marks them for degradation by the proteasome, a key step in terminating signal transduction and preventing prolonged responses.
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Scaffolding proteins in signaling
Scaffolding proteins organize components of signal transduction pathways, ensuring efficient and specific signal propagation by holding kinases in proximity.
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Adaptor proteins like Grb2
Adaptor proteins like Grb2 link phosphorylated receptors to downstream effectors, such as Ras, in pathways like those activated by growth factors.
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Ras protein
Ras is a small GTPase that, when activated, triggers kinase cascades like MAPK, playing a central role in cell growth and division signals.
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Raf kinase
Raf kinase is the first kinase in the MAPK cascade, activated by Ras to phosphorylate MEK and propagate signals for cell proliferation.
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MEK and ERK
MEK and ERK are kinases in the MAPK pathway where MEK phosphorylates ERK, which then enters the nucleus to regulate genes for growth and differentiation.
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Insulin receptor signaling
Insulin receptor signaling involves RTK activation that leads to glucose uptake and metabolism through pathways like PI3K/Akt, regulating blood sugar levels.
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Glucagon signaling
Glucagon signaling via GPCRs increases cAMP levels, activating protein kinase A to promote glycogen breakdown and gluconeogenesis in the liver.
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Epinephrine effects via GPCRs
Epinephrine binds to GPCRs like beta-adrenergic receptors, activating adenylyl cyclase to increase heart rate and mobilize energy stores.
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Steroid hormone action
Steroid hormones bind to intracellular receptors, forming complexes that act as transcription factors to regulate gene expression over hours to days.
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Thyroid hormone signaling
Thyroid hormones bind to nuclear receptors, influencing metabolism by altering gene transcription for proteins involved in energy production.
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Vitamin D signaling
Vitamin D signaling involves binding to intracellular receptors that regulate genes for calcium absorption and bone health.
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Retinoic acid signaling
Retinoic acid signaling uses nuclear receptors to control development and differentiation by modulating the expression of specific genes.
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Common trap: Confusing GPCRs and RTKs
A common error is confusing GPCRs, which activate G-proteins, with RTKs, which autophosphorylate; GPCRs often use second messengers, while RTKs use tyrosine phosphorylation.
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Strategy for drawing signaling pathways
To draw signaling pathways, start with ligand binding, note key intermediates like second messengers, and end with cellular responses, ensuring to include amplification steps.
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Example of signal amplification
In the cAMP pathway, one ligand can activate multiple G-proteins, leading to production of many cAMP molecules and widespread activation of protein kinase A.
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Role of GTP in G-proteins
GTP binding to G-proteins activates them by causing subunit dissociation, while its hydrolysis to GDP inactivates them, regulating the duration of the signal.
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Hydrolysis of GTP
Hydrolysis of GTP by the G-protein's intrinsic GTPase activity turns off the signal, allowing the G-protein to reassociate and prepare for the next activation.
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Effect of toxins on signaling
Toxins like cholera toxin permanently activate G-proteins by inhibiting GTP hydrolysis, leading to uncontrolled cAMP production and secretory diarrhea.