M&R session 7: signal transduction receptors and effectors Flashcards
What is signal transduction?
The transmission of molecular signals from a cell’s exterior to its interior, responding to external ones t alter its activity in some way.
All cells need receptors to respond to signalling molecules: intracellular for steroids and thyroid hormones, cell-surface for the majority of molecules:
-some receptors can directly alter cellular activity
-many need transduction of initial ligand-binding event by other intracellular signalling components to generate a response
What are the three cell-surface receptor superfamilies?
- LIGAND GATED ION CHANNELS: binding of the ligand ‘gates’ the channel to allow ions to move into or out of the cell. E.g. nAChR
- RECEPTOR TYROSINE KINASES: receptors with intrinsic enzyme activity. Ligand binds, activates enzyme activity that phosphorylates the receptor itself and other substrates. E.g. insulin receptor (though this is an atypical example)
- GPCRs: 7 transmembrane domain receptors. E.g. mAChR. 40% of current prescription drugs exert their effects at GPCRs
Describe some therapeutics which target GPCRs
Agonists: bind to receptor and mimic natural activity of hormone/neurotransmitter to activate receptor, causing intracellular signal transduction. E.g. B2 adrencoceptor agonists such as salbutamol and salmeterol; mu-opioid receptor agonists such as morphine and fentanyl
Antagonists: bind to receptor but have no efficacy so block the effects of agonists. E.g. beta adrenoceptor antagonists such as propranolol and atenolol, D2 dopamine receptor antagonists such as sulpiride and haloperidol
Give some diseases which are caused by defective GPCRs
Retinitis pigmentosa: loss of function to rhodopsin
Nephrogenic diabetes insipidus: loss of function to V2 vasopressin receptor
Familial male precocious puberty: gain of function to LH receptor
Name some stimuli of GPCRs
Light, odours, tastes Ions: H+, Ca2+ Neurotransmitters: ACh, glutamate Hormones: glucagon, adrenaline, TSH Large glycoproteins
There are >800 types of GPCR in the human genome. Describe the common basic structure
Single polypeptide chain of 300-1200 amino acids
7 transmembrane-spanning regions
Intracellular C-terminal and extracellular N-terminal
Where do ligands bind?
To 2-3 of the transmembrane domains, or the N-terminal region/other extracellular domains
Binding site has high affinity and specificity
Describe the effect of GPCR activation
- Must interact with a G-protein
- GPCR-G protein interaction activates the G protein by causing GTP to exchange for GDP on the alpha subunit
- alpha-GTP and betagamma subunits dissociate
- each subunit can interact with effector proteins (second messenger-generating enzymes or ion channels)
How is G protein signalling terminated?
alpha-GTP or betagamma interaction with receptors lasts until the alpha subunit GTPase activity hydrolyses GTP back to GDP. The subunits then reform an inactive heterotrimeric complex
Describe the structure of a G protein
3 distinct subunits by structure, but functionally is a dimer (alpha and beta-gamma subunits).
Alpha subunit has guanine nucleotide binding site to bind GTP, and intrinsic GTPase activity
What causes the G protein to be switched “on” and “off”?
On switch: receptor-facilitated GDP/GTP exchange
Timer/off switch: length of the time taken for GTP hydrolysis on Galpha subunit. Regulated by other cellular proteins
State the receptors adrenaline/noradrenaline act on, the G protein and the effector that are activated
Beta-adrenoceptor-Gs alpha-adenylyl cyclase (stimulates)
Alpha 1-adrenoceptor-Gq alpha-phospholipase C (stimulatory)
Alpha 2-adrenoceptor-Gi alpha-adenylyl cyclase (inhibitory)
At which receptor does light act, and which G protein and effector is activated?
Rhodopsin
Gt (transducin)
Cyclic GMP phosphodiesterase (stimulatory)
At which receptors can acetylcholine act, and which G protein and effector will this activate?
M2/M4 muscarinic: Gi. Adenylyl cyclase (inhibitory)
M1/M3 muscarinic: Gq. Phospholipase C (stimulatory)
How were cholera toxin (CTx) and pertussis toxin (PTx) used to study GPCR-G protein signalling?
Both have similar mechanisms of action: the toxin complex binds to the cell and an enzyme is injected into the cell, causing its harmful effects.
The toxin enzymes are enzymes that make ADP-ribosylate G proteins:
-CTx eliminates GTPase activaste of Gs alpha, so it is irreversibly activated
-PTx interest with GDP/GTP exchange on Gi alpha, so it is irreversibly inactivated
-ADP-ribosylation of Gs alpha by CTx prevents deactivation of Gs protein-mediated signalling, so signalling is not switched off and alpha subunit loses ability to hydrolyse the GTP, increases cAMP, water channels open, symptoms!
-similar mechanism by PTx
