Session 3

Question 1

For each of the G protein cascades

1. M1 receptor
2. Alpha-2 receptor
3. Beta-1 receptor

Name their respective
A. G protein α-subunit
B. Effector
C. Substrate
D. Intracellular messenger
E. Increased/Decreased

Answer 1

1) A - Gαq
B - Phospholipase C
C - PIP2
D - IP3
E - Increased IP3

2) A - Gαi
B - Adenylyl cyclase
C - AMP
D - cAMP
E - Decreased cAMP

3) A - Gαs
B - Adenylyl cyclase
C - AMP
D - cAMP
E - Increased cAMP

Question 2

Regarding the Gαs subunit, once stimulated, what target protein will the intracellular messenger created act upon?

A: Phospholipase C
B: βγ heterodimer
C: IP3 receptors
D: PKA
E: Adenylyl cyclase

Answer 2

D: PKA

Gαs targets adenylyl cyclase to create cAMP. cAMP has many target effector proteins, but the only one in this list is PKA.

Question 3

What will be the clinical consequences on the heart if you stimulate β-1 receptors?

Answer 3

β-1 receptors when stimulated cause increased cAMP (via stimulation of adenylyl cyclase). This in turn acts upon PKA, which increases activity of VOCC. This will mean more calcium enters the cell, causing increased force of contraction.

Question 4

With this in mind, what will happen to the heart if the patient is given a drug that inhibits activity of β-1 receptors (‘beta blockers’)?

A: Reduced filling capacity
B: Reduced force of contraction
C: Increased heart rate
D: Increased blood pressure
E: Increased force of contraction

Answer 4

B: Reduced force of contraction

Inhibiting β-1 receptors will mean less cAMP production, therefore not as much PKA/VOCC to be stimulated, causing reduced calcium to enter the cell. This leads to reduced force of contraction.

Question 5

What is the significance of a disease process that prevents conversion of GDP to GTP?

Answer 5

No GTP will be created, therefore the subunits cannot dissociate in order to target their effector proteins. This will mean reduced signalling cascade.

Question 6

What is the significance of a disease process that prevents activity of GTPase?

Answer 6

GTPase converts GTP to GDP, acting like the ‘off’ switch. With no ‘off’ switch (GDP), the subunits are effectively always activated, therefore there is no regulation of the signalling cascade.

Question 7

Define the words ‘agonist’ and ‘antagonist’.

Answer 7

Agonist: A ligand that binds to the receptor and activates it (leading to intracellular signal transduction events)

Antagonist: A ligand that bind to the receptor, but does not activate it (it blocks the effects of agonists at the receptor)

Question 8

Briefly explain how the toxin produced by Bordetella pertussis prevents changes in intracellular cAMP levels.

Answer 8

Pertussis toxin catalyses the ADP-ribosylation of the αi subunits of the heterotrimeric G protein. This prevents the G proteins from interacting with G protein-coupled receptors on the cell membrane, thus interfering with intracellular communication. The Gi subunits remain locked in their GDP-bound, inactive state, thus unable to inhibit adenylate cyclase activity, leading to increased cellular concentrations of cAMP.

Question 9

The mnemonic ‘QISS QIQ’ can be used to aid learning the type of GPCR under investigation.
Complete the list by stating the receptor name and their natural ‘endogenous’ ligand.
Q: Adrenoceptor (A)
I: Adrenoceptor (B)
S: Adrenoceptor (C)
S: Adrenoceptor (D)
Q: (E) 1
I: (F) 2
Q: (G) 3
QISS ligand = (H)
QIQ ligand = (I)

Answer 9

A. α1
B. α2
C. β1
D. β2
E/F/G. Muscarinic
H. Adrenaline and noradrenaline
I. Acetylcholine

Question 10

Which of the following statements is true about a G-protein coupled receptor?

A: It contains five transmembrane hydrophobic sections
B: There are more extracellular loops than intracellular loops
C: The ligand binding region is within the plasma membrane
D: The binding region for the G-protein involves two extracellular loops
E: The N-terminal chain is extracellular and the C-terminal chain is intracellular

Answer 10

E. The N-terminal chain is extracellular and the C-terminal chain is intracellular

Most GPCRs are 7 transmembrane receptor proteins, the number of extracellular and intracellular loops are the same, the ligand binding domain is extracellular and often involves the N-terminal chain, whilst the binding region of the G-protein involves one of the intracellular loops and
the C-terminal chain. Therefore, answer E is correct.