Intracellular Signalling - Signalling Pathway Components Flashcards
What can G-proteins bind?
GDP or GTP
What is GTP?
High energy molecule that activates G-proteins
What is function of GTPases?
Hydrolyse GTP into GDP (GDP switches G-protein off)
What is prenylation?
Attachment of lipid-anchored membrane proteins via lipid tails
How are G-proteins switched on?
Active when bound to GTP
- Binding of 1st messenger to receptor induces conformational change in receptor that allows G-protein to bind to receptor
- Stimulates G-protein to exchange its GDP for GTP
- Switches it ‘on’
How are G-proteins switched off?
- G-protein hydrolyses GTP back to GDP
2. Inactive when bound to GDP
What are the different alpha subunits of G-proteins?
- Gi (inhibitory)
- Gs (stimulatory)
- Gg
- G12,13
What are the different alpha subunits of G-proteins coupled to?
Different effector enzymes
What is effect when drug is coupled to activation of Gs?
Stimulates adenylate cyclase –> elevates cAMP
What is effect when drug is coupled to activation of Gi?
Inhibits adenylate cyclase –> reduces cAMP level
What are effects enzymes associated with:
- Gs
- Gi
- Gg
- Adenylate cyclase (stimualtion)
- Adenylate cyclase (inhibition)
- Phospholipase C (stimulation)
What does GTP-bound Gs-alpha subunit bind to?
Binds to and activates adenylyl cyclase which catalyses conversation of ATP to cyclic AMP
What is cyclic AMP broken to to and by?
Broken down to AMP by phosphodiesterases
What is effect of Gg?
Increases DAG and IP3 levels
How does cholera toxin affect Gs?
Affects Gs
- Prevents GTPase activity of Gs so GTP remains bound to Gs so it stays switched ‘on’
- This leads to over-stimulation of adenylate cyclase and accumulation of cyclic AMP
- Elevated cAMP in intestinal epithelial cells increases loss of Cl- ions through chloride channels
- Resultant osmotic gradient leads to water being excreted into intestinal lumen –> diarrhoea and dehydration
What is pertussis toxin?
Whooping cough (affects Gi)
What are effects of pertussis toxin on Gi?
- Inhibits Gi –> prevents GDP/GTP exchange by Gi and Gi protein locked in ‘off’
- Unable to inhibit adenylate cyclase
- Accumulation of cyclic AMP
- Increased insulin secretion and increased sensitivity to histamine
Describe Gs before activation
Alpha subunit of Gs is bound to GDP, in ‘off’ state
Adenyl cyclase is inactive as hasn’t been activated by G-protein
What is adenyl cyclase?
An enzyme with key regulatory roles in essentially all cells
What happens when ligand binds to Gs receptor?
- Causes G-protein to release GDP and swap it for GTP
- Switches G-protein to ‘on’ state
- GTP bound alpha subunit dissociates from the beta and gamma subunits
What can GTP-bound Gs alpha subunit bind to and activate?
Adenyl cyclase –> catalyses conversion of ATP to 2nd messenger cAMP
How is GTP hydrolysed back to GDP? What then happens?
The GTPase activity of the Gs-alpha subunit hydrolyses GTP to GDP (with release of inorganic phosphate, Pi), thus reverting the G-protein back to the “OFF” state
- The GDP-bound alpha subunit re-associates with the beta and gamma subunits
- Cyclic AMP is broken down to AMP by phosphodiesterases
What is effect of Gs stimulation on cAMP?
Stimulates adenyl cyclase and increases cAMP
What is effect of Gi on cAMP?
Inhibits adenyl cyclase and decreases cAMP
Describe the opposing effects of Gs and Gi on cAMP levels
- A drug coupled to activation of Gs stimulates adenylate cyclase and elevates cAMP
- In contrast, a drug that is coupled to activation of Gi inhibits adenylate cyclase and reduces cAMP levels
