Intracellular Signalling Pathways - Receptor/Effector Mechanisms

Question 1

What is signal transduction?

Answer 1

A ligand binds to a receptor. The receptor can either be intracellular or on the plasma membrane. The receptor then becomes activated and activates a transducer which then sets of a cascade of reactions.

Question 2

There are three superfamilies of cell-surface receptors. Which?

Answer 2

GPCRs g-protein coupled receptors.
Ligand-gated (receptor-operated) ion channels
Receptor with intrinsic enzymatic activity

Question 3

Give an example of a GPCR.

Answer 3

Muscarinic acetylcholine receptor

Question 4

Give an example of a ligand-gated ion channel.

Answer 4

Nicotinic acetylcholine receptor
GABA
IP3 which is however not on the cell surface.

Question 5

Give an example of a receptor with intrinsic enzymatic activity.

Answer 5

Receptor tyrosine kinases

Question 6

Give examples of agonists of GPCRs. Name the receptor and the agonist.

Answer 6

Beta2 adrenoceptors - salbutamol and salmeterol

u-opioid receptor agonists - morphine and fentanyl

Question 7

Give examples of antagonists of GPCRs. Name the receptor and the antagonist.

Answer 7

Antagonists bind to the receptor but do not activate it. Block the effects of agonists.
Beta1 adrenoceptors - propanolol, atenolol
D2 dopamine receptor - haloperidol, sulpiride

Question 8

Outline the structure of GPCRs.

Answer 8

7 transdomain proteins. A C-terminal intracellular and an N-terminal extracellular.

Question 9

Where can the binding site for a ligand be on a GPCR protein?

Answer 9

For some receptors the ligand binding site is formed by the transmembrane domains
Less commonly the N-terminal can be the ligand binding site.

Question 10

What is a g-protein?

Answer 10

A guanine nucleotide-binding protein.

Question 11

What does the guanine nucleotide part mean?

Answer 11

That it binds to GTP and GDP. Depending on whether it is bound to GTP or GDP tells us if its active or not.

Question 12

What is the structure of a g-protein?

Answer 12

It has an alpha subunit which is fairly large, than another subunit which is split into two subunits which are beta and gamma. The beta and gamma subunits act as a single functional unit.

Question 13

Does GDP or GTP activate the g-protein?

Answer 13

GTP does. GDP is usually bound to it.
GDP = off
GTP = on

Question 14

Briefly outline how GPCR to g-protein activation works.

Answer 14

A ligand binds to the receptor. This causes a g-protein to bind to the receptor due to conformational change of the receptor. The g-protein with a GDP attached is now exchanged to a GTP instead. The GTP turns on the g-protein and this causes the alpha subunit to dissociate from the beta/gamma subunits. The different subunits are now turned on and can interact with an effector protein to create a response either stimulating or inhibiting.

Question 15

How does termination of G-protein signalling work?

Answer 15

The GTP which is still attached to the alpha subunit.
The alpha subunit then interacts with GTPase. This causes an inorganic phosphate group to dissociate from the GTP and the alpha subunit. Now the alpha subunit can bind to the beta/gamma subunit again.

Question 16

What makes g-proteins selective?

Answer 16

There are 1000 possible alpha-beta/gamma combinations because the human genome encodes 20 alpha, 5 beta and +12 gamma subunits.
This combination makes the selectivity for effector proteins higher.

Question 17

An activated GPCR interacts with specific types of G protein. Which subunit is the primary determinant?

Answer 17

The alpha subunit.

Question 18

Why is the selectivity important in g-proteins?

Answer 18

This makes one ligand and one receptor to activate a specific g-protein which will then interact with a specific effector which will then bring about a specific cellular response.
This is important because it means that not a lot of ligands and a lot of receptor will bring about similar responses.

Question 19

What receptors do adrenaline work on? What kind of alpha-subunit is related to the GPCR? What effector does it either stimulate or inhibit?

Answer 19

Adrenaline is an agonist for b-adrenoceptors. The alpha subunit is ‘S’ (Galphas) and it stimulates the effector adenylyl cyclase.
Adrenaline is also an agonist for alpha2-adrenoceptors. The alpha subunit here is ‘I’ (Galphai) and it inhibits the effector adenylyl cyclase.
Adrenaline is also an agonist for alpha1-adrenoceptors where the alpha subunit is ‘q’ (Galphaq). The effector is phospholipase C and it it stimulated.

Question 20

What different types of GPCR do adrenaline bind to?

Answer 20

Alpha1-adrenoceptors
Alpha2-adrenoceptors
Beta1-adrenoceptors
Beta2-adrenoceptors

Question 21

What is alpha1-adrenoceptors effector? Does it inhibit or stimulate?

Answer 21

Phospholipase C

Stimulates

Question 22

What is alpha2-adrenoceptors effector? Does it inhibit or stimulate?

Answer 22

Adenylyl cyclase

Inhibits

Question 23

What is beta1-adrenoceptors effector? Does it inhibit or stimulate?

Answer 23

Adenylyl cyclase

Stimulates

Question 24

What is beta2-adrenoceptors effector? Does it inhibit or stimulate?

Answer 24

Adenylyl cyclase

Stimulates

Question 25

What is QISS QIQ?

Answer 25

An acronym for the adrenoceptors and muscarinic receptors.
QISS:
Q = Gaq (alpha1) stimulates phospholipase C
I = Gai (alpha2) inhibits
S = Gas (beta1) stimulates
S = Gas (beta2) stimulates
QIQ:
Q = M1 stimulates phospholipase C
I = M2 inhibits adenylyl cyclase
Q = M3 stimulates phospholipase C

Question 26

What can interfere with G protein function?

Answer 26

Cholera toxin and pertussis toxin.

Question 27

Where does pertussis toxin interfere?

Answer 27

The exchange of GDP to GTP to activate the g-protein. This means that there is an inhibition.

Question 28

Where does cholera toxin interfere?

Answer 28

On the GTPase which is supposed to terminate the stimulation of adenylyl cyclase/cAMP and PKA. This means that the stimulation is on at all times which can be very dangerous.