Signal Transduction I

Question 1

describe the steroid receptor

Answer 1

• an example of signal transduction by hormones that may cross the cell membrane to gain access to a specific intracellular receptor
  
  • ​steroid hormones such as testosterone, estrogen, cortisol
• These hormone molecules are hydrophobic and usually work by up-regulating gene expression at the level of DNA transcription

Question 2

what is the response time for steroids?

Answer 2

• Works through gene expression
  
  • comparatively slow; may take hours or days

Question 3

describe a gated ion channel

Answer 3

• 2 types: ligand or voltage-gated
• Ligand gated example:
  
  • binding of neurotransmitter causes channel to open
  • results in rush of ions through channel
  • alters membrane potential promoting or inhibiting nerve impulse transmission

Question 4

describe a receptor enzyme (catalytic receptor)

Answer 4

• The Receptor Tyrosine Kinase is an example
  
  • insulin binds to outside of cell, which changes the structure of the tyrosine kinase inside the cell and activates it
  • autophosphorylation of the inside part of receptor

Question 5

what are the outcomes of insulin signaling

Answer 5

• GLUT4 for glucose uptake
• transcription
• glycolysis
• protein synthesis

Question 6

describe G-protein coupled receptor (GPCR)

Answer 6

• Receptors indicate receipt of a signal through the production of a “second messenger” inside the cell
• Second messengers (which are _{<strong>small molecules</strong>}) trigger a cascade of intracellular events in response to the binding of a hormone to its receptor
• examples:
  
  • adenylate cyclase system (cAMP)
  • calcium/phosphatidylinositol system (IP3, DAG, Ca2+)

Question 7

describe the structure of GPCRs

Answer 7

• Heptahelical receptors
• They interact with heterotrimeric G-proteins
  
  • bound to GDP = OFF
  • conformational change when ligand binds outside, GDP releases and GTP binds = ON

Question 8

α1-adrenergic receptor

Answer 8

• works through Gαq protein
  
  • in response to a ligand binding to GPCR, Gαq dissociates and binds to GTP
  • it then activates phospholipase c (PLC)
  • creates DAG
  • creates IP3
    
    • goes to endoplasmic reticulum and releases Ca2+
  • Ca2+ along with DAG activate protein kinase C
    
    • “protein kinase c activated by calcium”

Question 9

describe the 2 types of ß-adrenergic receptors

Answer 9

1. Gαi = g-alpha inhibitory
2. Gαs = g-alpha stimulatory

• Depending on which is activated, it will activate/deactivate adenylate cyclase to either make more/less cAMP
• once cAMP is made, it will activate enzyme protein kinase A
  
  • protein kinase A = cyclic AMP dependent protein kinase

Question 10

list the steps in GPCR signalling

Answer 10

1. Ligand binds to G-protein coupled receptor
2. The activated receptor interacts with G-protein causing it to release GDP and then bind GTP
3. GTP bound form stimulates the trimer to dissociate into the alpha subunit and the beta-gamma dimer
4. the GTP bound alpha subunit moves and activates adenylate cyclase
5. the intrinsic GTP ►GDP + P GTPase actvity of the G-protein ensures signalling is short lived. The enzyme phosphodiesterase hydrolyzes cAMP to AMP
6. cAMP binds to the regulatory subunit of PKA thereby allowing catalytic kinase subunits to activate

Question 11

describe the function of cholera toxin

Answer 11

• an enzyme that ADP-ribosylates Gαs resulting in continuously active Gαs (GTPase activity is destroyed, so Gα​s is locked in GTP bound state)
  
  • effect is increased cAMP in intestine
  • uncontrollable dystentery

Question 12

describe the function of pertussis toxin

Answer 12

• an enzyme that ADP-ribosylates Gαi resulting in a continuously inactive Gαi
  
  • Effect is increased cAMP in respiratory-tract cells
  • respiratory distress and whooping cough

Question 13

describe the phosphoinositide system

Answer 13

1. Gqα activates PLC
2. PLC cleaves PIP₂ to IP₃ and DAG
3. IP₃ causes Ca2+ release from ER
4. IP3 and Ca2+ activate the PKC signalling kinase for cellular responsesity
5. PKC requires DAG, Ca2+ and phospholipids (membrane) for mamixal activity

Question 14

describe the role of cGMP

Answer 14

• cGMP is the second messenger for smooth muscle relaxation and vasodilation
• a rise in cGMP activates protein kinase G (PKG) which phosphorylates several protein targets resulting in smooth muscle relaxation and vasodilation
  
  • cGMP formed by guanylate cyclase

Question 15

name the 2 types of guanylate cyclase

Answer 15

• An integral membrane enzyme that gets activated directly by binding to a signal molecule (such as atrial natriuretic factor)
  
  • does not require a G-protein
• A soluble guanylate cyclase in smooth muscle that is activated directly by NO produced by the adjacent endothelial cells
  
  • does not require a G-protein

Question 16

describe the synthesis of NO

Answer 16

• acetylcholine activates receptors in the endothelium
• this activates an isozyme of phospholipase C and results in the release of IP3, followed by a rise in intracellular calcium
• calcium binds to a small protein called calmodulin which in turn associates with NO synthase and activates it which produces NO in endothelial cells
• NO (which is a gas and can diffuse through adjacent cells) activates guanylate cyclase which results in a rise in cGMP
• this activates PKG, and subsequent muscle relaxation