CB28 Cell Signalling 1

Question 1

Describe basic principles of intracellular signal transduction

Answer 1

Cell signalling is the transmission of information into the cell

• Molecule binds to receptor protein
• signal is transduced inside the cell
• message is carried via second messenger/activated enzyme
• activate target

Question 2

What are the three types of receptors

Answer 2

Channel coupled receptor
Enzyme coupled receptors
G protein coupled receptors

Question 3

Describe the general mechanism of G protein coupled receptors

Answer 3

G proteins are amplifiers, timers & molecular switches

• Signal molecule binds to receptor protein
• causes conformational change & opens G protein binding site
• GDP is replaced by GTP
• beta & gamma subunits are displaced
• alpha subunit is released from receptor & binds to effector
• g-protein is a GTPase so will break down GTP into GDP (and return to original state)

Question 4

Explain the role of g-proteins as amplifiers & timers

Answer 4

• Alpha subunit & GTP break away from g-protein receptor, allowing another G protein to attach (amplifies signal)
• G protein is a weak GTPase so interacts with effector before eventually hydrolysing GTP into GDP & returning to its original state

Question 5

What is cAMP

Answer 5

Cyclic adenosine monophosphate is a second messenger

Question 6

What is the role of cAMP

Answer 6

• The activated G protein activates adenylate cyclase (effector) which causes the conversion of ATP → cAMP
• cAMP binds to the regulator of kinase A & removes catalytic subunit so its active site is free
• cAMP dependent kinases phosphorylase enzymes in the liver which convert glycogen → glucose

Question 7

Describe the consequences of constitutive activity of adenylate cyclase

Answer 7

• cholera taxin, ADP-ribose binds to the g-protein preventing it from being broken down
• constant g-protein & adenylate cyclase activity
• large increase in cAMP in intestinal epithelium causing loss of water → diarrhoea & dehydration