Lecture 12 - G Protein Coupled Receptors (Signal Transduction)

Question 1

What is Signal Transduction?

Answer 1

Its how extracellular signals arrive at cells and cause a response

Question 2

What must cells have if they want to respond to extracellular signalling molecules?

Answer 2

Appropriate receptors

Question 3

Where can receptors be found?

Answer 3

Extracellularly (On plasma membrane)
Intracellularly (Nuclear or in the cytosol)

Question 4

What are some endogenous agonist ligands that act at Adrenoceptors?

Answer 4

Noradrenaline
Adrenaline

Question 5

What are some Exogneous agonist ligands that act at adrenoceptors?

Answer 5

Isoprenaline
Salbutamol (Inhalers)

Propranolol = ANTAGONIST

Question 6

What is an agonist?

Answer 6

Binds to the receptor and activates it

Question 7

What is an antagonist?

Answer 7

Binds to the receptor but does not activate it (BLOCKS ACTION OF AGONISTS)

Question 8

What anti asthma drug affects GPCRs?

Answer 8

Salbutamol

Question 9

What adrenoceptor does Salbutamol act as an agonist to?

Answer 9

B2 adrenoceptor

Question 10

If a drug has a high affinity for a receptor but no efficacy what does this mean in terms of its action?

Answer 10

Antagonist

It binds to the receptor well and blocks it preventing agonists binding to it and acting on it

Question 11

What B adrenoceptor antagonists can be used to treat hypertension?

Answer 11

Propranolol
Atenolol

Question 12

How many polypeptide chains make up a G protein coupled receptor?

Answer 12

1

Question 13

Where is the N terminal end of the polypeptide chain making up a G-protein coupled receptor?

Answer 13

Outside the cell

Question 14

Where is the C terminal end of the polypeptide chain making up a G-protein coupled receptor?

Answer 14

Inside the cell

Question 15

GPCRs have many different ligands, what are these ligands?

Answer 15

Ions
Neurotransmitters
Peptide and non-peptide hormones

Question 16

Where are the 2 binding sites on the G-protein coupled receptors for ligands?

Answer 16

Between 2nd and 3rd transmembrane domain

The N terminal region

Question 17

How many transmembrane domains are there making up a G protein coupled receptor?

Answer 17

7

Question 18

A GPCR normally sits in an off state, once a ligand binds what happens?

Answer 18

A conformational change in shape cause the G Protein associated with it to become activated

Question 19

What molecule which is bound to a G protein determines whether it is activate or inactive?

Answer 19

GTP = On/ACTIVATED
GDP = Off/INACTIVATED

Question 20

How many subunits is a G protein (Guanine-nucleotide binding protein) made up of?

Answer 20

3

Question 21

What are the 3 subunits of a G-protein?

Answer 21

Alpha (a)
Beta (B)
Gamma (γ)

Question 22

Why are G-proteins described as being heterotrimeric?

Answer 22

Made up of 3 different subunits

Question 23

What is the structure of a turned off/inactive G-Protein?

Answer 23

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

Question 24

When a ligand (agonist) binds to the GPCR, what happens to the alpha subunit of the G protein and what does this cause?

Answer 24

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

Question 25

What happens once the A subunit with GTP bound disassociates from the B/γ subunit?

Answer 25

Both the A and B/γ subunit are now activated and can interact with their effector proteins

Question 26

The alpha subunit performs 1 function and the B/γ subunit does ___ function

Answer 26

1

Question 27

What enzyme inactivates the G-protein?

Answer 27

GTPase

Question 28

How does GTPase inactivate the G-protein?

Answer 28

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)

Question 29

How are cellular responses triggered by activation of G proteins?

Answer 29

The alpha and the B/γ subunit go on to interact with specific effector proteins leading to the production of second messengers

Question 30

What are the 3 main types of G protein and which subunit is the important part which interacts with the specific effector proteins?

Answer 30

Gs Gi Gq Alpha subunit

Question 31

What enzyme (effector protein) do Gs and Gi G-proteins interact with?

Answer 31

Adenylyl cyclase

Question 32

What effect does Gs have on the activity of adenylyl cyclase?

Answer 32

Stimulates Gs (s) for (s)timulates

Question 33

What affect does Gs stimulating adenylyl cyclase have on the cell?

Answer 33

More cAMP (second messenger) produced More PKA (Protein Kinase A) activated which activates a range of proteins

Question 34

What effect does Gi have on the activity of adenylyl cyclase?

Answer 34

Inhibits GI (i) for (i)nhibits

Question 35

What affect does Gi inhibiting adenylyl cyclase have on the cell? What second messenger is produced?

Answer 35

Less cAMP (second messenger) produced Less PKA (Protein Kinase A) activated which activates a range of proteins (less activation happens)

Question 36

What enzyme (effector protein) does the G-protein Gq interact with?

Answer 36

Phospholipase C

Question 37

What effect does Gq have on the activity of Phospholipase C?

Answer 37

Stimulates it

Question 38

What affect does Gq stimulating Phospholipase C have on the cell? What second messenger is produced?

Answer 38

More IP3 (Inositol triphosphate ) and DAG produced

Question 39

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?

Answer 39

Gs Adenylyl cyclase +ve chronotropic affect +ve inotropic affect Adrenaline or noradrenaline

Question 40

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?

Answer 40

Gq Phospholipase C Vasoconstriction Adrenaline or noradrenaline

Question 41

What type of receptor signalling pathways are interfered with by Cholera toxin (CTx) and Pertussis toxin (PTx)?

Answer 41

GPCR G protein signalling

Question 42

How does Cholera toxin interfere with the G protein signalling pathway?

Answer 42

Prevents G protein deactivation (continuous effector protein stimulation) Covalently modifies the Alpha subunit so GTPase cant hydrolyse GTP and deactivate the G protein

Question 43

What effect does the Cholera toxin have by preventing the G protein from being deactivated by GTPase?

Answer 43

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

Question 44

How does Pertussis toxin interfere with G-protein signalling? What type of G protein is it affecting?

Answer 44

Prevents the exchange of GDP with GTP on the alpha subunit Gi

Question 45

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?

Answer 45

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)

Question 46

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?

Answer 46

Adenylyl cyclase active for less time so less cAMP made

Question 47

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?

Answer 47

Adenylyl cyclase enzyme active for more time so more cAMP made