Introduction To Receptors

Question 1

What are the three superfamilies for cell surface receptors and what is the main one in the body ?

Answer 1

1) G protein coupled receptors ( eg in muscarinic ACH receptors)
2) ligand gated ion channels ( nicotinic ACH receptors)
3) receptors with intrinsic enzyme activity ( eg insulin receptors)

Question 2

Give one example of an agonist medication that works on G-protein couple receptors ?

Answer 2

Salbutamol (blue inhaler) which works on beta 2 adrenoceptors which causes relaxation of airways in order to help deal with asthma.

Question 3

Give an example of an G protein coupled receptors antagonist ?

Answer 3

• Atropine : this binds to M2 receptors in the heart : preventing ACH from binding.This helps those with bradycardia.

Question 4

Describe the structure of G-protein coupled receptors ? ( GPCR)

Answer 4

• one single polypeptide chain
• Extracelllular N terminal
• intracellular C terminal
• 7- transmembrane spanning regions

Question 5

What two regions in G-protein coupled receptors can ligands bind to ?

Answer 5

1) for some , the ligand binding site is within the transmembrane domains
2) for some , the N terminal form the ligand binding site.

Question 6

Describe in steps , how G protein coupled receptors bring about a cellular response ?

Answer 6

1) ligand bind to receptors on the N terminal or in the transmembrane domains.
2) G protein coupled receptors response to ligands by activating changing their 3D shape.
3) this causes activation of G proteins
4) this causes G proteins to exchange GDP for GTP on the alpha sub unit.
5) the Alpha - beta/gamma complex immediately dissociates . Where each subunit interacts with their effectors.
6) Interaction with effectors lasts until the alpha sub unit GTPase hydrolyses GTP back to GDP. Alpha and gamma/beta subunit then reform to form their heterotrimeric complex.

Question 7

What is the structure of a G protein ?

Answer 7

Alpha subunit joined with a beta/gamma sub unit which are bound together.

Question 8

Give an example of how adrenaline /noradrenaline can act on different G protein coupled receptors in the heart to bring about different responses

Answer 8

1) binds to B-adrenoceptor which activates the Ga(s-stimulate) protein which activates ardenyl Cyclase.
2) binds to a2-adrenoceptor which activates Ga(I) protein which inhibits the activation of adenylyl cylcase
3) binds to a1 adrenoceptor which activates ga(q) which activates phospholipase C.

Question 9

How does cholera toxin interfere with G protein functioning ?

Answer 9

• cholera toxin is present in the gut - mainly.
• it prevents GTP from reforming back into GDP. And so alpha subunit does not join with the beta/gamma sub unit. And so continual activation of effectors occurs.
• this causes an increased amount of water to be released from intestinal cells. This results in diorreah and dehydration.

Question 10

How does pertussis toxin interfere with G protein functioning ?

Answer 10

• it prevents the GDP from being converted into GTP. .