GPCR's

Question 1

What are GPCR’s and some of their features?

Answer 1

Receptor that when activated, releases a G-protein to initiate a signalling cascade
Single polypeptide chain
7 transmembrane regions
Extracellular N-terminal

Question 2

What are the binding sites on a GPCR?

Answer 2

N-terminal

Between the 2 and 3 regions

Question 3

What is a G-protein?

Answer 3

Guanine nucleotide binding protein:
Heterotrimeric protein composed of alpha, beta, gamma units
Locked onto the membrane with lipid anchors

Question 4

What structural changes does a G-protein undergo when it is activated?

Answer 4

Activated G-protein exchanges the GDP found on the alpha unit for a GTP
G-protein immediately dissociates into the alpha unit, and a beta-gamma dimer
Both units diffuse laterally along membrane to interact with effector proteins (produce secondary messengers)

Question 5

How is G-protein response controlled?

Answer 5

The alpha unit contains GTPase which hydrolyses GTP back to GDP
When the alpha unit and the beta-gamma dimer unit reform the inactive heterotrimeric G-protein effector proteins are stopped
The activity of GTPase controls how fast the alpha unit can reform with the beta-gamma dimer

Question 6

What are the agonists, receptor type and G-protein of GPCR’s in the autonomic system?

Answer 6

ACh: muscarinic receptors (postganglionic parasympathetics)
M1: Gq
M2: Gi
M3: Gq
M4: GI
Nor-adrenaline: adrenoceptors (postganglionic parasympathetic)
Alpha-1: Gq
Alpha-2: Gi 
Beta-1: Gs
Beta-2: Gs

Question 7

What is the pathway followed by Gq proteins?

Answer 7

Activates Phospholipase C
Phospholipase C cleaves PIP2 (membrane) into IP3 and Diacylglycerol (DAG)
IP3 causes the release of Ca2+ from the sarcoplasmic reticulum
Increased [Ca2+]i and DAG activate PKC (protein kinase C)
PKC phosphorylates proteins

Question 8

What is the pathway followed by Gi/s proteins?

Answer 8

Inhibits/Stimulates Adenylyl Cyclase
Adenylyl cyclase hydrolyses ATP to cAMP
4 cAMP bind to 2 regulatory units on PKA (protein kinase A) activating its 2 catalytic units
The 2 catalytic units phosphorylation proteins

Question 9

What is an antagonist of the Gs protein?

Answer 9

CTx (Cholera Toxin) - prevents termination in the alpha unit (GTPase does not work)

Question 10

What is an antagonist of the Gi protein?

Answer 10

PTx (Pertussis Toxin) - prevents activation in the alpha unit (GDP —> GTP)

Question 11

What example are there of GPCR’s in the body?

Answer 11

Nor-adrenaline binds with Beta-1 receptors (Gs) stimulating PKA
PKA phosphorylates VOCC’s causing the influx of Ca2+
Increasing the contractility of cardiac cells
ACh binds with Muscarinic-3 receptors (Gq) stimulating PKC and causing bronchoconstriction

Question 12

What 3 properties do GPCR’s have that make them good receptors?

Answer 12

Diversity (range of stimuli, receptors, G-Proteins, effectors)
Specificity (one ligand binds to one receptor)
Amplification (significant Intracellular response relative to Extracellular signals)