Intracellular Signalling

Question 1

What are the examples of intracellular signals? (2)

Answer 1

Ions, gases proteins, second messengers

Question 2

How can Extracellular signals result in action in the cell?

Answer 2

The ligand binds to transmembrane receptors (which are normally hydrophobic), which undergoes conformational change to initiate intracellular singllaing.

For Action: Signal can be transducted or amplified via different intracellular signalling molecules to effector proteins for cell response.

Question 3

How are signal molecules interpreted? (3)

Answer 3

1. Post-translational Modification e.g Phosphorylation
2. G protein binding to GDP or GTP.
3. Second messengers/activators such as Ca2+ and cAMP.

Question 4

Describe Protein Phosphorylation. What enzymes are involved? (3)

Answer 4

1. Phosphorylation involves addition of phosphate group to a protein.
2. KINASE enzyme phosphorylates proteins and PHOSPHATASE dephosphorylates.

Question 5

What are the amino acids that can function as protein kinases? Why? (3)

Answer 5

1. Serine
2. Threonine
3. Tyrosine.

They have free hydroxyl groups.

Question 6

What are the examples of Serine/Threonine Kinases? (4)

Answer 6

1. Ca2+/calmodulin-dependent protein kinases (CaM kinases)
2. Protein Kinase A (PKA)
3. Protein Kinase C (PKC)
4. Mitogen-activated protein kinases (MAPK)

Question 7

What are examples of Tyrosine Kinases? (2)

Answer 7

1. Non receptor tyrosine Kinases e.g Src Family Kinases

2. Receptor tyrosine kinases (RTKs). e.g Epidermal growth factor receptor.

Question 8

GTP binding proteins? what are they and how do they work? (3)

Answer 8

1. Small individual proteins involved in MOLECULAR switch mechanism.
2. In GDP (Guanosine-5’-diphosphate (GDP)) bound state switch is off. Guanine exchange factors (GEFs) exchange GDP for GTP.
3. In GTP (Guanosine-5’-triphosphate (GTP)
   ) bound state swtich is on. GTPase activating proteins (GAPs) help hydrolyse GTP back to GDP.

Question 9

Ras is a small GTPase, what happens when a mutation causes loss of Ras activity?

What condition is this associated with? (2)

Answer 9

The intracellular signalling stay activated because the molecular switch has not been turned off.

Associated with cancers. e.g Colorectal cancer.

Question 10

Describe a Voltage-gated ion channel (3)

Answer 10

It is made up of:
1. an Alpha subunits with 4 homologous domains which open in response to voltage.

2. Each domain has 6 transmembrane region. The 4th domain has amino acids with positive Rgoups that sense voltage and move in response.
3. 4 beta subunits which traffic the channel and regulate its kinetic properties.

Question 11

Describe a Ligand gated ion channel (3)

Answer 11

1. These have a receptor and channel which transverse membrane.
2. The channel opens in response to a ligand binding to receptor. The receptors are classified based on corresponding agonist.

Question 12

Describe Nicotinic acetylcholine receptor (3)

Answer 12

1. Receptor is composed of 5 subunits (2 alpha, 1 beta, 1 gamma and 1 omega).
2. The ‘M2’ region each subunit forms the channel.
3. 2 acetylcholine bind to alpha subunits causing M2 helices to move and open channel.

Question 13

Calcium Signalling:
1. What type of events are regulated by Calcium

2. How are Ca2+ ion movement regulated
3. What type of channels do Ca2+ pass through?

Answer 13

1. Skeletal muscle contraction, secretion, transcription factor activities.
2. CaM kinase and Calcineurin phosphatase mediate Ca2+
3. Ligand or voltage channels.

Question 14

Describe G-protein coupled receptors. How do they work? (5)

Answer 14

1. They have 7 transmembrane domains but do not form pores.
2. The G-protein has 3 subunits (1 alpha, 1 beta and 1 gamma), and floats freely intracellularly.
3. When a signal binds to G-protein, the internal conformation changes, releasing GDP for alpha subunit.
4. GTP replaces GDP. This activates the G protein, dissociating the Beta and gamma subunits which can each initiate further signalling.
5. GTP is then hydrolysed back to GDP and subunits recombine to associate with another GPCR.

Question 15

How do GPCRs result in Protein Kinase A activation?

What Receptor is this important in? (3)

Answer 15

1. GPCR -> Activated Gs alpha Protein -> Adenyl cyclase -> cAMP -> Protein Kinase A.
2. Occurs in Beta 2 Adrenegic receptor to relax blood vessels in Skeletal muscle.

Question 16

How do GPCRs result in Phospholipase C activation?

How does Activating Phospholipase C result in activation of Protein Kinase C?

Where does this happen?
(4)

Answer 16

Occurs in M1 acetylcholine receptor for secretion.

1. GPCR -> Activated G alpha Protein -> Phospholipase C.
2. Phospholipase C -> Diacylglycerol;

Phospholipase C -> IP3-> Ca2+ from ER

3. Diacylglycerol, Ca2+ from ER -> Protein Kinase C.

Question 17

How does GPCRs activate K+ ion channel?

What cell is this important? (4)

Answer 17

GPCR -> activated G alpha protein -> Beta and Gamma units attach to K+ channel, causing it to open -> K+ moves out of cell.

Important in Muscarinic M2 acetylcholine receptor in heart.

Question 18

How does Cholera interfere with GPCR and how does it cause damage to cells? (3)

Answer 18

1. Cholera toxin inhibits GTPase activity in G alpha protein, leading uncontrolled stimulation of Adenyl cyclase.
2. This causes water and Cl- to move out of cells lining intestine.
3. Results in diarrhoea, dehydration and death.

Question 19

How does Whooping Cough interfere with GPCR and how does it cause damage to cells? (3)

Answer 19

1. Whooping cough toxin inhibits GTPase activity in G alpha protein, leading uncontrolled stimulation of Adenyl cyclase.
2. This prevents association of G protein with GCPR again.
3. Prolonged signal results in constant coughing.

Question 20

How do receptor tyrosine kinases work? (3)

Answer 20

The ligand binds to the receptors:

1. Receptors dimerise,
2. Activating tyrosine kinase (through phosphorylation) in cytoplasmic region in response to signal.