Signal Transduction Flashcards
Why do we need signal transduction?
As most things that affect cell activity or function do not enter cells Instead they act on membrane-bound receptors that control signalling proteins via the production of secondary messengers
What is the function of secondary messengers ?
They mediate cell activity (including gene expression)
What are the 2 ways that a signal can be transduced ?
1). Via proteins inside the cell which can change its behaviour (fast response - Seconds to Minutes) 2). Via making new proteins by transcription in order to change a cells behaviour (Slow - Minutes to Hours)
How do signalling proteins generally act ?
Many signalling proteins act as molecular switches
How many kinases and phosphates does the human genome encode?
Approx 520 kinases and Approx 150 phosphates
What are the two common pathways used to activate/deactivate signalling proteins ?
1). Signalling via phosphorylation; - The signal goes into the cell via the receptor and activates protein kinase which coverts ATP to ADP and adds a phosphate onto the target protein to activate it - The phosphate is cleaved off the activated protein using protein phosphatase and this releases phosphate so it can bind back onto ADP to form ATP and the protein is deactivated 1). Signalling via GTP binding; - The signal goes into the cell via the receptor and activates GTP binding which coverts GDP to GTP which binds onto the target protein to activate it - The GTP is attached to the activated protein is hydrolysed using GTP hydrolysis which releases a phosphate, consequently leaving GDP binded to the protein and the protein is deactivated
What are the two main types of kinases ?
Tyrosine kinase and Serine/Threonine kinase
What are the two main types of GTP-binding proteins ?
Trimeric G proteins and Monomeric GTPases
What does ligand binding do ?
- Ligand binding activates a G-protein which in turn activates or inhibits another protein, which is often an enzyme which generates a specific second messenger
What does GPCR’s stand for?
G-protein coupled receptors
What series of steps occurs when activating G-protein-coupled receptors?
- An extracellular ligand binds to then G-protein-coupled receptor protein found in the plasma membrane
- This activates the inactive G-signal transducing protein (this is known as the G-protein activation) which inhibits/activates a down stream inactivated effector enzyme
- An inactivated effector enzyme (like adenylyl cyclase or phospholipase c) binds onto the now activated form of G-protein (this is known as complex formation)
- The Active effector (G-protein + Enzyme) generates “second messengers” (like cAMP; inositol 1,4,5-triphosphate; 1,2 - diacylglycerol)
What do all G-protein-coupled receptors have?
7 membrane spanning regions; - with their amino termini (N-terminus) facing extracellularly - with their carboxyl termini (C-terminus) facing intracellularly from the plasma membrane
How can many of the cell-surface receptors be described?
Many of the cell-surface receptors are coupled to trimeric signal-transducing proteins (composed of three different subunits - Alpha, Beta and Gamma) that bind to either GTP or GDP
Describe the mechanisms of action of G-protein signal transduction?
- The ligand binds to the receptor and changes its conformation, causing the receptor to bind to the G-alpha (Ga) protein, during this GDP is displaced by GTP which binds to the G-alpha (Ga) protein
- This triggers G-Beta-Gamma (GBy) to release from the Gs trimeric protein (Containing G-alpha protein and G-Beta-Gamma protein joined together)
- This release of G-Beta-Gamma (GBy) activates downstream pathways
- These pathways only last for a short period of time as GTP which is bonded to G-alpha hydrolyses back to GDP in seconds, causing G-Beta-Gamma (GBy) to bind back onto G-alpha and the inactivation of the enzyme adenylate cyclase
What is GTP required for?
GTP is required for the ligand-induced stimulation of Adenylate Cyclase (enzyme)
What couples to G-alpha ?
Glucagon
What are the 3 things that is required for a signal transduction system?
1). A receptor (e.g - Glucagon) 2). A transducer (G-protein) 3). An amplifier (adenylate cyclase) which generates a large amount of second messenger
What is the role of Adenylate cyclase ?
Amplifies signals and produces large amounts of second messengers
What is the structure of G-protein-coupled receptors (GPCR) ?
Their receptors consists of 7 transmembrane helices (hydrophobic amino acids) which sit in the plasma membrane - which are Beta helices (called Beta 2 adrenergic receptor) They interact with heterotrimeric G-protein complex on the extracellular side (has Alpha, Beta and Gamma subunits)
How many types of G-alpha subunits is there?
20
What are the 3 main types of G-alpha classes and their functions?
Gaq (or Gq) - Stimulates phospholipase C Gs - Stimulates adenylate cyclase, increases cAMP Gi - Inhibits adenylate cyclase, decreases cAMP
What is unique about all GPCR’s (G-protein-coupled receptors) ?
All GPCR’s exhibit preferential association with a particular G-protein (or subset of G-protein) GCPRs signal via GTP-mediated activation of G proteins
How do GPCR’s signal and draw a diagram ?
- A signalling molecule activates a G-protein linked receptor which activates a G-protein subunit
- This activates the enzyme phospholipase C which liberates two signalling molecules from Phosphatidylinositol 4,5-biphosphate (PIP2), a phospholipid found in the lipid bilayer
- This creates inositol 1,4,5 triphosphate (IP3) and diacyglycerol (DAG)
- The inositol 1,4,5 triphosphate (IP3) activates the Calcium channel on the endoplasmic reticulum causing a calcium influx inside the cell to activate PKC (Protein kinase C) and the binding of the diacyglycerol (DAG) also further activates the Protein kinase C (both substances are required to activate it)
What does a rise in cytosolic calcium levels cause PKC (Protein kinase C) to do?
A rise in cytosolic calcium levels causes PKC to bind to the cytosolic leaflet of the plasma membrane, where it can be activated by the membrane-associated DAG and/or Calcium PKC then phosphorylates a wide variety of substrate proteins on serine and threonine residues
How does an increase cAMP activate the cAMP-dependent protein kinase ?
cAMP binds to the regulatory subunits found in the cAMP-dependent protein kinase and causes the catalytic subunits to detach at the pseudosubstrate site, allowing the substrate site to be free to bind to cAMP
What is Cholera Toxin ?
Cholera toxin is an oligomeric complex (has repeating units of monomers) which after cleavage becomes active and enters the intestinal epithelial cells to stimulate G-alpha S The over stimulation of cAMP production results in a release of water and ions including Sodium, Potassium, Chlorine and Bicarbonate ions into the lumen of the small intestine This leads to rapid fluid loss and dehydration
Explain what happens in the enzyme-linked receptor; Receptor Tyrosine Kinases (RTK’s)
1) . The Receptor Tyrosine Kinases (RTK’s) are inactive and apart from one another
2) . A signalling molecule in the form of a dimer (An oligomer made of 2 monomers joined together) binds to the 2 RTK’s, consequentially brining them close to one another
3) . Due to how close the 2 RTK’s are their tyrosine kinase domains are touching one another and are stimulating kinase activity
4) . These kinases are activated and add phosphate groups to the tyrosine amino acids
5) . Intracellular signalling proteins bind to the phosphorylated tyrosine. This causes a signal relayed by activating signalling proteins into the cells interior
