CB28: Cell Signalling 1 Flashcards
How are signals propagated?
Signal binds to the receptor. Receptor generates a message that is sent to the right location at the right time.
What are the 2 types of signals and what are their requirements?
- Hydrophobic signals don’t require receptors because they can cross the membrane.
- Hydrophilic signals bind to a receptor and relay their message by causing a conformational change.
Define second messengers.
Small chemicals that are used in a signalling cascade to transmit information.
How does amplification increase signalling ?
One signal binds to oen receptor, but one receptor can recruit multiple kinases that can activate many second messengers.
How is signalling stopped?
- Signal is down-regulated by endocytosis of receptor + destruction by lysosomes.
- Auto-inhibitory/self limiting factors are released.
- Antagonistic mechanisms break down signals or interrupt signalling pathways.
What are the differences between ion-gated channels, G-protein coupled receptors and enzyme-linked receptors ?
- Ion-gated channels: use ions to transmit messages. Signal = ion flux. Ion channels open when ligand binds.
- G-protein coupled receptors: use G-proteins to transmit messages. Binding of ligand causes a conformational change in the ligand so the G-proteins can bind.
- Enzyme-linked receptors: use enzymes to transmit messages. Have kinase domains that are the switches for the receptors.
What is another name for enzyme-coupled receptors?
Catalytic receptors
What is the general mechanism of action for G-protein coupled receptors?
1) The ligand binds to the G-protein coupled receptor.
2) This causes a conformational change in the receptor that reveals the binding site for the G-protein.
3) The G-protein binds to the receptor and the GDP associated to the G-protein is displaced & replaced by GTP.
4) This causes the G-protein to detach from the receptor and causes the alpha subunit to move away from the beta and gamma subunits.
5) The alpha subunit interacts with second messengers/effectors to produce a response.
How do G-proteins act as mediators?
They are only activated when GDP is displaced from the alpha subunit, which only occurs after the G-protein has bound to the receptor.
How do G-proteins act as amplifiers?
One G-protein coupled receptor can interact with many G-proteins.
How do G-proteins act as timers?
G-proteins act as GTPase (reverts them to the inactive state) but they work slowly.
What does cAMP stand for ?
Cyclic Adenosine Monophosphate.
How is cAMP formed?
Activated G-protein binds to adenylate cyclase –> converts ATP to cAMP.
How is cAMP degraded?
Receptor gets endocytosed so signal for cAMP decreases.
Phosphodiesterase breaks down cAMP.
How does the cholera toxin affect cAMP levels?
Toxin chemically modifies alpha subunit of G-protein so it can’t function as a GTPase anymore. G-protein is always turned on and it continuously binds to adenylate cyclase which leads to continuous formation of cAMP. This means water is lost by efflux through ion channels.
