Week 2 - GPCRs Flashcards
Give the definition of an ionotropic receptor.
e.g ligand gated ion channels.
- they are a direct exchange of ions through a pore in the ion channel.
Give the definition of a metabotropic receptor.
They are indirectly linked to ion channels through signal transduction mechanisms such as G proteins.
Describe characteristic structural features of GPCRs.
- 7 transmembrane spanning domains
- N terminus at extracellular side (ligand binding)
- C terminus at intracellular side (G-protein binding)
- TM3 is centrally located next to a binding pocket -> which is crucial for ‘transduction’ of ligand binding.
GPCRs can detect a huge diversity of stimuli.
Briefly describe the steps in the mechanim of action of a G-protein.
1) Inactive state when GDP is bound to the alpha subunit.
2) a ligand binds and causes a conformational change in the receptor which activates the G-protein.
3) GDP is then released and the alpha subunit separates from the beta-gamma subunit. The alpha subunit then binds to GTP and becomes active.
4) It binds to target protein in the membrane to elicit a responses within the cell.
How is the rate of G-protein signalling controlled?
Controlled by rate of GDP hydrolysis by Galpha.
Activated G proteins regulate the activity of enzymes which control the levels of second messengers.
What are second messengers?
2nd messengers = small molecules that carry signals inside cells.
In the G-protein mechanism of action, what is important to note about GDP to GTP change?
it is the alpha subunit which DISPLACES GDP to GTP. it does NOT CONVERT it.
What type of G-protein subunit is involved in Uveal melanoma?
- Gq alpha subunit
- leads to blocking of GTP hydrolysis so the subunits are always active –> causes permanent signal transmission.
What type of G-protein subunit is involved in Uveal melanoma?
- Gq alpha subunit
- leads to blocking of GTP hydrolysis so the subunits are always active –> causes permanent signal transmission.
What are the 2 types of effectors for trimeric g-proteins?
Enzymes which make second messengers and ion channels whose gating is regulated either directly (βγ subunits) or indirectly by 2nd messengers and their effectors
What activates and what inhibits the cAMP second messengers system?
what are the steps in the cAMP second messenger system?
Gi alpha subunit inhibit and Gs stimulates it.
- Ligand binds to receptor activating G protein
- α subunit moves (once GDP had been displaced by GTP) and binds to adenylate cyclase in the membrane
- This activated enzyme catalyses formation of a cAMP from ATP
- The cAMP (2nd messenger) activates Protein kinase A
- PKA phosphorylates/activates protein
- Initiates a response within the cell
What are 4 ways in which signalling can be switched off?
Signalling is switched off by…
1) Agonist dissociating from receptor
2) GTPase activity of Gαs
3) cAMP breakdown by phosphodiesterase
4) Dephosphorylation of enzymes
in the cGMP second messenger system, how does it differ to the cAMP system?
Enzyme is guanylate cyclase which can be receptor bound or ‘free’ in the cytoplasm.
Converts guanosine triphosphate (GTP) to 3’, 5’-cyclic guanosine monophosphate (cGMP)
the production of cAMP is regulated by which enzyme?
which enzyme removes cAMP?
adenylate cyclase
Phosphodiesrerase removes it.
GPCRs can activate PLC to generate … and …
IP3 and DAG
IP3 is water soluble and diffses through cytoplasm
DAG is hydrophobic, remains in the membrane
