Lecture 12 - G Protein Coupled Receptors (Signal Transduction) Flashcards
What is Signal Transduction?
Its how extracellular signals arrive at cells and cause a response
What must cells have if they want to respond to extracellular signalling molecules?
Appropriate receptors
Where can receptors be found?
Extracellularly (On plasma membrane)
Intracellularly (Nuclear or in the cytosol)
What are some endogenous agonist ligands that act at Adrenoceptors?
Noradrenaline
Adrenaline
What are some Exogneous agonist ligands that act at adrenoceptors?
Isoprenaline
Salbutamol (Inhalers)
Propranolol = ANTAGONIST
What is an agonist?
Binds to the receptor and activates it
What is an antagonist?
Binds to the receptor but does not activate it (BLOCKS ACTION OF AGONISTS)
What anti asthma drug affects GPCRs?
Salbutamol
What adrenoceptor does Salbutamol act as an agonist to?
B2 adrenoceptor
If a drug has a high affinity for a receptor but no efficacy what does this mean in terms of its action?
Antagonist
It binds to the receptor well and blocks it preventing agonists binding to it and acting on it
What B adrenoceptor antagonists can be used to treat hypertension?
Propranolol
Atenolol
How many polypeptide chains make up a G protein coupled receptor?
1
Where is the N terminal end of the polypeptide chain making up a G-protein coupled receptor?
Outside the cell
Where is the C terminal end of the polypeptide chain making up a G-protein coupled receptor?
Inside the cell
GPCRs have many different ligands, what are these ligands?
Ions
Neurotransmitters
Peptide and non-peptide hormones
Where are the 2 binding sites on the G-protein coupled receptors for ligands?
Between 2nd and 3rd transmembrane domain
The N terminal region
How many transmembrane domains are there making up a G protein coupled receptor?
7
A GPCR normally sits in an off state, once a ligand binds what happens?
A conformational change in shape cause the G Protein associated with it to become activated
What molecule which is bound to a G protein determines whether it is activate or inactive?
GTP = On/ACTIVATED
GDP = Off/INACTIVATED
How many subunits is a G protein (Guanine-nucleotide binding protein) made up of?
3
What are the 3 subunits of a G-protein?
Alpha (a)
Beta (B)
Gamma (γ)
Why are G-proteins described as being heterotrimeric?
Made up of 3 different subunits
What is the structure of a turned off/inactive G-Protein?
Alpha subunit has GDP bound to it
B and γ subunit tightly bound
Alpha and B/γ subunit bound to each other
The subunits all have a high affinity for each other
When a ligand (agonist) binds to the GPCR, what happens to the alpha subunit of the G protein and what does this cause?
GDP on A subunit is exchanged for GTP
Affinity of the alpha subunit for the B/γ subunit decreases
The A subunit disassociates From the B/γ subunits
What happens once the A subunit with GTP bound disassociates from the B/γ subunit?
Both the A and B/γ subunit are now activated and can interact with their effector proteins
The alpha subunit performs 1 function and the B/γ subunit does ___ function
1
What enzyme inactivates the G-protein?
GTPase
How does GTPase inactivate the G-protein?
It hydrolyses/removes a phosphate from GTP on the Alpha subunit converting it back to GDP
This makes the A subunit have a high affinity for the B/γ subunit so the rebind to each other again (inactive heterotrimeric complex)
How are cellular responses triggered by activation of G proteins?
The alpha and the B/γ subunit go on to interact with specific effector proteins leading to the production of second messengers
What are the 3 main types of G protein and which subunit is the important part which interacts with the specific effector proteins?
Gs
Gi
Gq
Alpha subunit
What enzyme (effector protein) do Gs and Gi G-proteins interact with?
Adenylyl cyclase
What effect does Gs have on the activity of adenylyl cyclase?
Stimulates
Gs (s) for (s)timulates
What affect does Gs stimulating adenylyl cyclase have on the cell?
More cAMP (second messenger) produced
More PKA (Protein Kinase A) activated which activates a range of proteins
What effect does Gi have on the activity of adenylyl cyclase?
Inhibits
GI (i) for (i)nhibits
What affect does Gi inhibiting adenylyl cyclase have on the cell?
What second messenger is produced?
Less cAMP (second messenger) produced
Less PKA (Protein Kinase A) activated which activates a range of proteins (less activation happens)
What enzyme (effector protein) does the G-protein Gq interact with?
Phospholipase C
What effect does Gq have on the activity of Phospholipase C?
Stimulates it
What affect does Gq stimulating Phospholipase C have on the cell?
What second messenger is produced?
More IP3 (Inositol triphosphate ) and DAG produced
What type of G-protein is associated with B1- adrenoceptors in the heart?
What effector protein is stimulated?
What affect does this have on the heart?
What ligand binds to this GPCR/B1 adrenoceptor?
Gs
Adenylyl cyclase
+ve chronotropic affect
+ve inotropic affect
Adrenaline or noradrenaline
What type of G-protein is associated with a1- adrenoceptors in the vascular smooth muscle?
What effector protein is stimulated?
What affect does this have on the vascular smooth muscle?
What ligand binds to this GPCR/a1 adrenoceptor?
Gq
Phospholipase C
Vasoconstriction
Adrenaline or noradrenaline
What type of receptor signalling pathways are interfered with by Cholera toxin (CTx) and Pertussis toxin (PTx)?
GPCR G protein signalling
How does Cholera toxin interfere with the G protein signalling pathway?
Prevents G protein deactivation (continuous effector protein stimulation)
Covalently modifies the Alpha subunit so GTPase cant hydrolyse GTP and deactivate the G protein
What effect does the Cholera toxin have by preventing the G protein from being deactivated by GTPase?
Effector proteins continuously stimulated (lots of cAMP made) since its a Gs protein
PKA constantly activating the Cystic Fibrosis Transmembrane Conductance Regulator (CFTR) which pumps Cl- out of the cell. As a result water follows the chloride
Excess water moved into the gut lumen causing diarrhoea
How does Pertussis toxin interfere with G-protein signalling?
What type of G protein is it affecting?
Prevents the exchange of GDP with GTP on the alpha subunit
Gi
What effect does the Pertussis toxin preventing GDP to GTP exchange on the alpha subunit have?
What type of G protein is it inhibiting from working and therefore what is its affect?
G protein never becomes activated and never works on effector proteins
Means process become uncontrolled since the G protein it affects is Gi (less cAMP made)
If the GTPase enzyme activity is fast, what effect does this have on the effector protein and levels of second messenger if the G-protein is Gs?
Adenylyl cyclase active for less time so less cAMP made
If the GTPase enzyme activity is slow, what effect does this have on the effector protein and levels of second messenger if the G-protein is Gs?
Adenylyl cyclase enzyme active for more time so more cAMP made
