ICPP 2

Question 1

What are the 3 superfamilies of cell surface receptor??

Answer 1

1. GPCR
2. Ligand gated ion channels
3. Receptors with intrinsic enzymatic activity, e.g tyrosine kinases

Question 2

Give 2 examples of GPCR AGONISTS

Answer 2

1. Anti-asthma drugs which bind to the beta-2 adrenoceptor to call smooth muscle relaxation. Salbuatmol and Salmeterol
2. Opioids which bind to the mu-opioid receptor and inhibit calcium channels from opening, reducing NT release.

Question 3

Give an example of GPCR antagonist.

Answer 3

Cardiovascula beta-adrenoceptor inhibitors. Propanolol.

Question 4

What is the basic structure of a GPCR?

Answer 4

A single polypeptide chain with 7-transmembrane domains.

Extracellular N-terminal and intracellular C-terminal

Question 5

What are the 2 regions of GPCR’s that ligands can bind to?

Answer 5

1. Ligand binding site between domains 2-3

2. N-terminal region

Question 6

Following ligand binding, what exchange occurs in the G-proteins?

Answer 6

Exchange of GDP for GTP on the alpha subunit, causing a further conformational change which stimulates the alpha subunit to dissociate from the beta-gamma subunits.

Question 7

What is the structure of the g-protein?

Answer 7

It consists of 3 subunits (heterotrimeric): alpha, beta and gamma.

Question 8

How is a signal from a GPCR terminated?

Answer 8

The alpha subunit of the g-protein has GTPase activity and hydrolyses GTP back to GDP. This then reassociate with the beta and gamma subunits to form an inactive heterotrimeric complex.

Question 9

What determines the specificity of a GPCR signal?

Answer 9

GPCR will preferentially interact with a specific type of g-protein, the alpha subunit is the primary determinant.
The g-alpha subunit also binds to specific effector proteins, creating a specific cellular response.

Question 10

How do cholera toxin and pertussis toxins MOA differ?

Answer 10

Cholera toxin inhibits GTPase activity, preventing signal inactivation.
Pertussis toxin inhibits GDP-GTP exhange and therefore inhibits subunit dissociation.

Question 11

Give 3 examples of Gs coupled receptors.

Answer 11

B-adrenoceptors.
D1 dopamine receptors
H2 histamine receptors.

Question 12

Give 3 examples of Gi coupled receptors.

Answer 12

alpha- 2 adrenoceptors
D2 Dopamine receptors
mu-opioid receptors

Question 13

What is the enzyme stimulated/inhibited by Gs and Gi respectively?

Answer 13

Adenylyl Cyclase. Converts ATP to cAMP (second messenger) when activated.

Question 14

How does cAMP exert most of its actions?

Answer 14

through cAMP-dependent kinase- PKA.

Question 15

What is the structure of PKA and how does this relate to cAMP-activation?

Answer 15

It consists of 4 subunits, 2 catalytic subunits bound to 2 regulator subunits. When cAMP binds to the regulator subunits, conformational change release the catalytic subunits which can then phosphorylate target proteins in the cell.

Question 16

What is the concentration of calcium intracellularly, extracellular and in the ER/SR?

Answer 16

Extracellular= 1mM
Intracellular = 100 nM
ER/SR= 2 x 10^-4 M

Question 17

Describe which GPCR increases inotropy of the heart.

Answer 17

Adrenaline/Noradrenaline binds to beta-1-receptors in heart muscle. These are Gs coupled GPCR’s which in turn activate adenylyl cyclase and increase cAMP levels. This activates PKA which phosphorylates VOCC, causing influx of calcium and increased contractility.

Question 18

Explain how noradrenaline can cause smooth muscle constriction, and which receptor type is involved.

Answer 18

NA binds to alpha 1 adrenoceptors on vascular smooth muscle cells (stimulates PLC..IP3 and DAG) to cause constriction.
DAG activated PKC which gives sustainability.

Question 19

Which receptor does ACh interact with to cause bronchoconstriction?

Answer 19

M3 muscarinic receptor, Gq coupled so activates PLC leading to Ip3 and DAG.

Question 20

Explain how GPCR’s can modulate neurotransmitter release.

Answer 20

In both CNS and PNS, neurotransmitter release is often modulated by pre-synaptic GPCRs.
The BETA-GAMMA subunits inhibit VOCC, reducing calcium influx and NT release.

Question 21

What muscle types use CICR?

Answer 21

Smooth muscle and Cardiac muscle. Not skeletal muscle.

Question 22

Name examples of calcium buffers and explain their role.

Answer 22

Calbindin (cytosol), Calsequestrin (SR), calreticulin (SER).

They bind to calcium and slowly release it to control calcium levels.

Question 23

What is calmodulin and how does it play a role in calcium regulation?

Answer 23

It is a calcium sensor. Calmodulin binds up to 4 molecules of calcium, causing a conformational change which allowed it to interact with proteins that can’t interact directly with Ca2+.
It modulates PMCA to increase Ca2+ efflux by increasing its calcium sensitivity.