Transduction Mechanisms

Question 1

Describe the transduction mechanism of LGICs, i.e. nAChR

Answer 1

1. nAChR combines the function of an ACh receptor and a cation channel
2. Binding of ACH results in a conformational change in the receptor protein
3. This causes the ion channel to open and Na+ and K+ to flow into or out of the cell.
4. No other proteins are involved in generating the response
5. Inward current causes depolarization of the muscle membrane and initiation of a muscle action potential within 1ms of ACh binding to receptor.

Question 2

Where else can the LGIC transduction response be observed?

Answer 2

In Artificial lipid membranes to which purified AChR protein has been added

Question 3

Give two examples of an excitatory LGIC.

Answer 3

nAChR;

AMPA

Question 4

Give two examples of an inhibitory LGIC.

Answer 4

GABA(A);

GlyR

Question 5

What is the most common type of plasma membrane receptor

Answer 5

GPCR

Question 6

How much of the human genome encodes for GPCRs?

Answer 6

Roughly 1%

Question 7

GPCRs are the target for what percentage of current therapeutic drugs?

Answer 7

> 50%

Question 8

What are the three types of protein involved in the G-protein activation cycle?

Answer 8

1. The receptor (GPCR)
2. The G-protein
3. The effector (e.g. AC)

Question 9

Describe the mechanism of the G-protein activation cycle.

Answer 9

1. At rest, the alpha subunit of the G(s)-protein binds GDP
2. When the receptor is activated, its affinity for G(s) increases
3. The receptor interacts with the G(s)-protein. The alpha subunit catalyzes the exchange of GDP for GTP
4. The GTP-bound G-protein acts as the first messenger and interacts with an effector molecule (e.g. AC)
5. Adenylate Cyclase becomes activated, producing cAMP
6. The GDP stays bound to the G-protein, so the G-protein reverts to the resting state (i.e. stage 1). This switches off the G-protein’s function

Question 10

What are the two main consequences of the three-stage response of the G-protein activation cycle?

Answer 10

1. Compared to the response of LGICs (roughly 0.3 sec), a G-protein-linked response is slow and long-lasting; (roughly 1.3 sec)
2. This process has in-built amplification - a single receptor can activate several G-protein molecules sequentially while stimulated by an agonist; one G-protein molecule can interact with several effector molecules before being switched off; and one effector molecule can produe many secondary messenger molecules (if the effector is an enzyme)

Question 11

State the functions of G(s) proteins.

Answer 11

Activates adenylate cyclase (AC);

Activates Ca2+ channels

Question 12

State the functions of G(i) proteins.

Answer 12

Inhibits AC;

Activates K+ channels

Question 13

State the functions of G(q/11) proteins.

Answer 13

Stimulates PLC-beta;

Activates K+ channels

Question 14

State the functions of G(12/13) proteins.

Answer 14

Regulates Rho family GTPase signalling and cytoskeleton remodeling

Question 15

State the functions of G(t) proteins.

Answer 15

Activates cGMP phosphodiesterase in vertebrate rod photoreceptors

Question 16

State the functions of G(o) proteins.

Answer 16

Stimulates PLC-beta;
Activates K+ channels;
Inactivates Ca2+ channels

Question 17

Give an example of an agonist and receptor related to G(s)

Answer 17

Noradrenaline <===> beta-adrenoceptor

Question 18

Give an example of an agonist and receptor related to G(i) and G(o)

Answer 18

Acetylcholine <===> M2-muscarinic;

Noradrenaline <===> alpha(2)-adrenoceptor;

Enkephalin/morphine <===> mu/delta-opiate

Question 19

Give an example of an agonist and receptor related to G(q)

Answer 19

Noradrenaline <===> alpha(1)-adrenoceptor;

Acetylcholine <===> M1-muscarinic

Question 20

What kind of effect is created by the GPCR-related transduction mechanisms involved in the heart and in smooth muscle?

Answer 20

Heart <===> Antagonistic effects

Smooth muscle <===> Synergistic effects

Question 21

How is the force of cardiac contraction increased in the heart?

Answer 21

1. NA acts on beta-adrenoceptors to increase the force and rate of cardiac contraction.
2. Both effects result from G(s)-medicated stimulation of AC and the formation of cAMP.
3. cAMP increases the force of contraction by activating protein kinase A, which phosphorylates CA2+ channels in the myocardial cell membrane
4. This increases the extent to which they open during the cardiac action potential, and so increases Ca2+ entry.

Question 22

How is the RATE of contraction increased in the heart?

Answer 22

1. Does not depend on phosphorylation; instead, cAMP directly facilitates the opening of the pacemaker in the SA node, accelerating the spontaneous discharge rate
2. ACh acts on the M2 receptor to oppose the action of NA
3. M2 couples to G(i) to produce two effects: inhibits AC to reverse stimulatory action of NA, and opens a set of K+ channels in the membrane of SA node cells
4. Efflex of K+ ions then hyperpolarizes the cells and slows their rate of sponetaneous discharge - therefore with ACh, it is the G-protein which has two effects

Question 23

Describe the GPCR-related transduction mechanisms involved in smooth muscle.

Answer 23

In some smooth muscle cells, both ACh and NA have the same effect:

1. Both ACh and NA induce contraction because the two receptors involved (M1 muscarinic and alpha-adrenergic) activate G(q).
2. This activates PLC and two products of phospholipid breakdown (IP3 and DAG) act synergistically to cause muscle contraction.

This is an example of convergent effects mediated by different receptors.

Question 24

Describe the basic structure of receptor tyrosine kinases (RTKs).

Answer 24

There are over 90 RTKs distributed into 20 families.

Most are single membrane spanning proteins

Most are single subunit receptors but some exist as complexes, such as the insulin receptor.

Question 25

Describe the mechanism of receptor tyrosine kinases (RTKs).

Answer 25

1. Platelet-derived growth factor (PDGF) binds to a receptor, leading to receptor dimerisation.
2. This causes a conformational change which renders the active sites within the kinase domain available to bind ATP.
3. This leads to the autophosphorylation of tyrosine residues.
4. The phosphorylated tyrosine residues act as docking sites for effector molecules containing src homology domains (SH2, SH3 domains)
5. SH2 domains specifically recognize the phosphorylated state of tyrosine residues, thereby allowing SH2 domain-containing proteins to localize to tyrosine-phosphorylated sites

Question 26

Describe the basic structure of (nuclear) intracellular DNA-binding receptors.

Answer 26

These receptors are generally found in the cytoplasm or in the nucleus.

Their ligands are membrane permeable (e.g. steroid hormones)

The receptors have ligand binding, DNA binding and transcription activating domains