Cell Signalling

State the four main stages of cell signalling

1. Ligand-receptor interaction.

2. Signal transduction (phosphorylation cascade and signal amplification).

3. Cellular response (change in gene expression).

4. Signal termination.

Explain the need for cell surface receptors

1. Ligands for cell surface receptors are water-soluble and too large to pass through the hydrophobic core of the phospholipid bilayer.

Describe the role of protein kinase

1. Protein kinase is an enzyme which transfers phosphate group from ATP to a protein (phosphorylation), causing a conformational change in the protein and producing an ADP molecule.

Define phosphorylation cascade

1. Phosphorylation cascade is a sequence of signalling pathway events where one kinase phosphorylates another kinase, causing a chain reaction leading to the phosphorylation of thousands of proteins.

Describe the role of protein phosphatase

1. Protein phosphatase is an enzyme that removes phosphate groups from phosphorylated protein kinases to return them back to their inactive state after the cellular response is carried out.

Explain the need for the termination of phosphorylation cascade

1. The termination of phosphorylation cascade is necessary so that it enables the cell to respond again the next time an extracellular signal binds to the receptor.

Describe the significance of signal amplification

1. Each activated relay molecule of the cell signalling pathway can activate many more molecules in the next step of the pathway.

2. This results in a much greater number of activated products than in the preceding step.

3. Hence, a low concentration of signal molecules bound to the cell surface receptors initially is sufficient to result in a much greater cellular response through multiple signal cascades.

4. This signal amplification effect is exemplified by the phosphorylation cascade involving several protein kinases.

What are second messengers?

1. Second messengers are non-protein molecules or ions which are water soluble and readily spread throughout the cell via diffusion.

What is the function of second messengers?

1. Second messengers relay signals from the activated receptor to relay molecules of signalling pathways.

2. They activate the relay molecules.

3. They are involved in signal amplification.

4. Examples of second messengers are cyclic AMP and calcium ions.

What is the function of cyclic AMP?

1. Activated adenyl cyclase catalyses the conversion of ATP to cyclic AMP.

2. Cyclic AMP acts as a second messenger and activates protein kinase A.

3. Activated protein kinase A phosphorylates and activates other protein kinases, leading to a phosphorylation cascade and eventually a cellular response.

Describe how a signal is terminated

1. Dissociation of ligand from receptor.

2. Degradation of the ligand, second messenger or other relay molecules.

• GTPase in G protein (Ras protein) hydrolyses bound GTP to GDP.

• Phosphodiesterase converts cAMP to AMP.

• Protein phosphatase removes phosphate groups form phosphorylated protein kinases and other proteins.

3. Desensitisation of receptors.

• Number of active receptors decreases via endocytosis of receptors.

Describe the significance of cell signalling

1. Signal amplification.

• A low concentration of signal molecules bound to the cell surface receptors initially is sufficient to result in a much greater cellular response through multiple signal cascades.

2. Able to trigger a cellular response without signal molecule entering the cell.

• Binding of the signal molecule to a cell surface receptor is able to activate genes in the nucleus within the cell.

• The signal molecule does not need to pass through the phospholipid bilayer of the cell surface membrane to effect a cellular response.

3. One signal molecule is able to result in different cellular responses from the same cell.

• A single signal molecules of is capable of activating more than one signal transduction pathway due to branching of the pathway.

Describe the structure of G protein coupled receptor

1. GPCR is made of one polypeptide chain.

2. It is folded into seven alpha helices which are embedded in the transmembrane region of the cell surface membrane.

3. It consists of one extracellular ligand binding site and one intracellular G protein binding site.

4. Hydrophobic amino acid residues of alpha helices interact with hydrophobic tails of phospholipids via hydrophobic interactions.

5. Hydrophilic amino acid residues of ligand binding site and G protein bind site interact with hydrophilic phosphate heads of phospholipids via hydrophilic interactions.

Describe the structure of G protein

1. It is made up of three protein subunits (three polypeptide chains).

2. It is activated when bound to GTP.

3. It is inactivated when bound to GDP.

4. It contains GTPase which hydrolyses bound GTP to GDP.

Describe the mechanism of action of G protein coupled receptor

Before binding of signal molecule

1. GPCR and G protein are in the inactive form.

2. Inactive G protein is bound to GDP.

Activation of GPCR and G protein

1. Shape of ligand is complementary to shape of ligand binding site of GPCR.

2. Binding of ligand to ligand binding site of GPCR results in a conformational change in GPCR and activates the GPCR.

3. Shape of G protein is complementary to shape of GPCR binding site.

4. Binding of activated GPCR to inactive G protein causes a GTP molecule to displace the GDP molecule and activates the G protein.

Activation of adenylyl cyclase

1. Activated G protein dissociates from the receptor and moves along the cell surface membrane.

2. Shape of activated G protein is complementary to shape of active site of adenyl cyclase.

3. Binding of activated G protein to adenyl cyclase results in a conformational change in adenyl false and activates adenyl cyclase.

4. Activated adenyl cyclase triggers the signal transduction pathway leading to a cellular response.

Return to inactive form and signal termination

1. GTPase hydrolyses the bound GTP to GDP, causing the G protein to return to its inactive form.

2. Inactive G protein dissociates from adenyl cyclase which becomes inactive.

3. Signal is terminated and G protein is now ready to be reactivated by another activated receptor.

Describe the structure of receptor tyrosine kinase

1. It is made of two polypeptide chains.

2. Each polypeptide chain has one alpha helix embedded in the transmembrane region of the cell surface membrane, one extracellular ligand binding site and one intracellular tail with multiple tyrosinase residues.

3. It is inactive when the two polypeptide subunits are separate.

4. It is active when the two polypeptide subunits are linked as a dimer.

Describe the mechanism of action of receptor tyrosine kinase

Before binding of signal molecule

1. RTK is in its inactive form where the two subunits are separate.

2. Shape of ligand is complementary to shape of ligand binding site of RTK.

Binding of ligand

1. Binding of ligand to ligand binding site of RTK causes the two subunits to associate closely with each other to form a dimer (dimerisation).

Activation of RTK

1. Dimerisation activates the tyrosine kinase region of each subunit.

2. Activated tyrosine kinase region of each subunit phosphorylates the tyrosine residues on the intracellular tail of the other subunit (cross phosphorylation)

3. Both subunits are phosphorylated and the receptor is fully activated.

Triggering of signal transduction pathways and signal termination

1. Relay proteins in the cell can now bind to phosphorylated tyrosine residues on the receptor.

2. Bound proteins undergo a conformation change and become activated.

3. Each activated relay protein then triggers a signal transduction pathway which eventually leads to a cellular response.

4. One RTK dimer activates many intracellular relay proteins hence triggering different signal transduction pathways and cellular responses simultaneously.

5. After the specific cellular responses is carried out, the signal is terminated by protein phosphatases or endocytosis of the receptor and ligand.

What triggers the secretion of insulin?

1. When blood glucose level increases beyond 90mg per 100ml of blood, the increase in blood glucose level is detected by islets of Langerhans in the pancreas.

2. This triggers the secretion of insulin by beta cells of islet of Langerhans via exocytosis into the bloodstream for transport to target cells.

3. This triggers the uptake of glucose from the blood into target cells, hence decreasing the blood glucose level back to the normal level of blood glucose.

What happens during ligand receptor interaction of insulin signalling?

1. At the target cells, insulin binds to insulin receptor tyrosine kinase which results in a conformational change to activate the insulin receptor tyrosine kinase.

2. The tyrosine kinase of each subunit phosphorylates the tyrosine residues on the intracellular tail of the other subunit.

3. Both subunits are now phosphorylated.

4. The receptor is fully activated.

What happens during signal transduction of insulin signalling?

1. Different relay proteins bind to specific phosphorylated tyrosine residues.

2. The tyrosine kinase phosphorylates and activates these relay proteins.

3. Phosphorylation cascade is triggered.

4. More than one signal transduction pathway can be activated due to the activation of different relay proteins.

What happens during cellular response of insulin signalling?

1. Activation of the last protein of each signalling pathway results in a cellular response.

2. One cellular response is the increase in the amount of glucose transporters on the cell surface membrane.

3. This increases the permeability of the cell surface membrane to glucose, hence increasing the uptake of glucose into target cells.

4. Other cellular responses include increased rate of glycogenesis, increased rate of protein synthesis, increased rate of lipogenesis and decreased rate of gluconeogenesis.

How does signal termination of insulin signalling occur?

1. Dissociation of insulin from insulin receptor tyrosine kinase.

2. Degradation of insulin.

3. Desensitisation of insulin receptor tyrosine kinase.

Describe briefly how blood glucose level is regulated by glucagon

1. When blood glucose concentration decreases below 90mg per 100ml of blood, the decrease in blood glucose concentration is detected by islets of Langerhans in the pancreas.

2. This triggers the secretion of glucagon by alpha cells of the islets of Langerhans via exocytosis into the bloodstream to be transported to the target cells (e.g. liver cells).

3. This then triggers the release of glucose from target cells into the blood, hence increasing the blood glucose concentration back to the normal level of blood glucose.

Describe what happens during ligand-receptor interaction of glucagon signalling

1. At the target cells, glucagon binds to ligand binding site of glucagon GPCR, resulting in a conformational change to activate the GPCR.

2. G protein binding site of activated GPCR binds to an inactive G protein, causing a GTP molecule to displace the GDP molecule and activate the G protein.

3. The activated G protein dissociates from the receptor, moves along the cell surface membrane and binds to adenylyl cyclase, resulting in a conformational change to activate adenylyl cyclase.

Describe what happens during signal transduction of glucagon signalling

1. The activated adenylyl cyclase catalyses the conversion of ATP to cyclic AMP.

2. cAMP acts as a second messenger and activates protein kinase.

3. The activated protein kinase then activates other protein kinase in a phosphorylation cascade.

4. This eventually results in the activation of the last protein of a signal transduction pathway.

Describe what happens during cellular response of glucagon signalling

1. The activated last protein catalyses the breakdown of glycogen to glucose.

2. This increases the release of glucose into the bloodstream and restores the normal level of blood glucose.

3. Another cellular response is the inactivation of glycogen synthase that reduces the rate of glycogenesis.

Distinguish between glucagon and insulin signalling

1. Glucagon signalling is triggered when blood glucose concentration decreases below 90mg per 100ml of blood while insulin signalling is triggered when blood glucose concentration increases above 90mg per 100ml of blood.

2. Target cells of glucagon signalling are liver cells while target cells of insulin signalling are liver, muscle and adipose cells.

3. Glucagon signalling involves G protein coupled receptor which is made of one polypeptide chain while insulin signalling involves Receptor tyrosine kinase which is made of two polypeptides that forms a linked dimer.