WEEK 5: SIGNALLING IN CELLS

Question 1

State the 6 major types of receptors

Answer 1

1.Receptors which form transmembrane ion channels, opened by signal binding.

2.Receptors which activate production of a second messenger within the cell, working via a trimeric G protein.

3.Receptors whose inner domain is an enzyme activated when the signal molecule binds.

4.Receptors which bind and activate a cytoplasmic enzyme when the signal molecule binds.

5.Cytoplasmic or nuclear receptors activated by hydrophobic signal molecules which enter the cell.

6.Cytoplasmic receptors activated by small gas molecules acting as signals.

Answer 2

The acetyl choline receptor forms what is known as a ligand gated ion channel. Others of the same type are:

			Ligand 		               Ion transmitted

EXCITATORY RECEPTORS:

Acetylcholine (nicotinic receptor) Na / K

Glutamate (NMDA receptor) Na / K
(non NMDA receptor)

Serotonin (5-OH tryptamine) Na / K
T3 receptor)

INHIBITORY RECEPTORS:

*G-amino butyric acid Cl
(A class receptor)
*Glycine Cl

Question 3

Describe how receptors which activate a trimeric G-protein work.

Answer 3

When receptors of this type bind their activating messenger (cognate ligand, in the language of the subject), they change shape, opening a binding site for a G-protein, which is activated by the receptor.

The activated G protein now activates an effector enzyme, which is another intrinsic transmembrane protein. This enzyme synthesises an intracellular messenger molecule (the second messenger) which carries the message into the cell.

The second messenger is produced from a substrate which is readily available in the cell.

Question 4

State the 5 G-protein types, their effectors and second messengers.

Answer 4

G-protein Effector enzyme Second Messenger
1.Gs Adenylate cyclase cyclic AMP

2.Gi Phospholipase C Inositol
triphosphate +
diacyl glycerol
3.Gt GMP phosphodiesterase cyclic GMP
(transducin)
(Light detection in the eye)

4.Golf adenylate cyclase cyclic AMP
(smell and taste)

5.Gk potassium channel protein K+

Question 5

Describe how the The Gs Protein work.

Answer 5

1.The Gs protein activates adenylate cyclase, which produces cyclic 3’5’AMP from ATP

2.cAMP is simply an ‘on’ switch. The receptor for cAMP is an enzyme called protein kinase A.

3.It will phosphorylates a target protein, bringing about the cellular effect of the signal

Question 6

State the 4 effects that can be brought by cAMP.

Answer 6

1. The activity of an enzyme can be changed. The enzyme will be activated or inactivated by phosphorylation.
2. The synthesis of a protein can be increased. This will usually be by activation of gene transcription.
3. The permeability of a membrane can be changed. This will usually involve the opening of a transmembrane channel. Often this will be an ion channel, and will stimulate, or more often inhibit, the firing of a neuron.
4. The release of preformed material may be stimulated. Some hormone release is stimulated via a cAMP signal mechanism. The release of thyroid hormone from granules in response to TSH stimulation is an example.

Question 7

State the enzyme that switches off the cAMP response.

Answer 7

Cyclic AMP is broken down to 5’AMP by the enzyme cAMP dependent phosphodiesterase.

Question 8

Describe the mechanism of Gi protein system.

Answer 8

1.Activates a membrane enzyme called phospholipase C.

2.Targets a membrane phospholipid, phosphatidyl inositol 4,5 bis phosphate.

3.It splits it producing inositol 1,4,5 tri phosphate (IP3) and 1,2 diacyl glycerol.

4.Both these molecules can function as second messengers.

5.
*IP3 it binds to a calcium channel protein, opening the channel.
- Ca ++ binds to a detector protein called calmodulin.

*1,2 DAG activates an enzyme called protein kinase C.