2.2 Intracellular Signalling Pathways

Question 1

Describe the effects of the G protein - G,alpha,s - on Adenylyl Cyclase

Answer 1

1. G,alpha,s-GTP interacts with AC
2. Triggers conversion of ATP==>cAMP (cyclic AMP)
3. cAMP goes off into cytoplasm and binds to a different protein

Question 2

What protein does cAMP go off into the cytoplasm and often attach to?

Answer 2

1. Cyclic AMP dependent protein kinase (PKA)
2. cAMP binds to regulatory subunit of PKA
3. Catalytic subunits of PKA release
4. These go off and phosphorylate target proteins in cell

Question 3

Describe the effects of the G protein - G,alpha,i - on Adenylyl Cyclase

Answer 3

1. G,alpha,i-ATP prevents activation of AC

2. cAMP not made and so other proteins are not be activated

Question 4

Describe the effects of the G protein - G,alpha,q - on phospholipase C (PLC)

Answer 4

1. G,alpha,q interacts with PLC
2. PIP2 ==> IP3 + DAG
3. IP3 interacts with IP3 receptor on ER allowing release of Ca2+
4. DAG goes on to interact with Protein Kinase C (PKC)

Question 5

What is Signal Amplification?

Answer 5

1. Key feature in many cell signalling pathways
2. Small changes in extracellular signals elicit significant changes in cellular behaviour
3. E.g. One receptor, one G protein, few AC, lots more cAMP, loads of PKA etc

Question 6

Describe what happens with G,alpha,s if increasing contractility of inotropy in heart.

Answer 6

1. G,alpha,s-GTP affects AC
2. AC - ATP ==> cAMP
3. cAMP goes on to attach to regulatory subunits of cAMP dependent protein kinase (PKA)
4. Catalytic subunits released
5. Go on and trigger VOCC
6. Let in more Ca2+
7. Bigger contraction

Question 7

Give an example where the BETA GAMMA subunit is used to trigger a cascade of events instead.

Answer 7

1. BETA GAMMA interacts with VOCC
2. Inhibits specific types of voltage operated Ca2+ channels opening
3. Reduces Ca2+ influx
4. Reduces NT release
5. Less pain

E.g. Morphine

Question 8

Why is it important to regulate amount of intracellular calcium?

Answer 8

Used as a second messenger so can go and trigger things!

Potentially harmful

Question 9

How is calcium increased intracellularly (2)? What are these mechanisms opposed by (3)?

Answer 9

1. Movement of Ca2+ across plasma membrane
2. Release of Ca2+ from ER/SR
3. Relative impermeability of cell membrane to Ca2+
4. Pumps and transport proteins move Ca2+ out of cytoplasm
5. Ca2+ buffer protein by binding

Question 10

High and low concentration of calcium where?

Answer 10

High extracellularly

Low intracellularly

Question 11

Mechanisms that maintain low [Ca2+]

Answer 11

1. ATP Dependent
   - PMCA - use 1 ATP to transfer 1 Ca2+ out of cell
   - SERCA - use 1 ATP to transfer Ca2+ into ER/SR
2. Transporter mechanism
   - NCX - Na+, Ca2+ exchanger. 3Na+ in for 1 Ca2+ out

Question 12

What mechanisms are used to increase intracellular [Ca2+] via cell membrane

Answer 12

1. LGIC - ligand gated ion channel, activated by NT, influx of Ca2+
2. VOCC - voltage operated Ca2+ channel, depolarisation allowing Ca2+ to flow down conc gradient

Question 13

What mechanisms are used to increase intracellular [Ca2+] via ER/SR

Answer 13

Ca2+ induced Ca2+ release (CICR) - ryanodine receptors

IP3R - Inositol 1,4,5-triphosphate receptors

• G,alpha,q-GTP interacts with PLC
• PIP2 ==> IP3 + DAG
• IP3 interacts with IP3R on ER allowing release of Ca2+
• DAG goes onto interact with PKC