Homeostasis and cell signalling

Question 1

Explain the importance of G protein [3]

Answer 1

1. Relay extracellular signal from receptor to downstream intracellular relay proteins
2. Binds to ligand-bound GPCR
3. Becomes activated when it displaces GDP for GTP
4. Conformational change of G protein
   » alpha subunit dissociates from beta and gamma subunits
5. Translocates along PM&raquo_space; binds to adenylyl cyclase&raquo_space; phosphorylates and activates it

Question 2

Explain the importance of adenylyl cyclase [2]

Answer 2

1. Activated when activated G protein binds to it
2. Catalyses conversion of ATP to cAMP
   » which binds to and activates PKA
   » initiates sequential activation of kinases (PHOSPHORYLATION CASCADE)
3. Small no. of ligands sufficient for SIGNAL AMPLIFICATION
   » large no. of cAMP produced

Question 3

Describe 2 mechanisms of how relay proteins lead to downregulation of blood glucose [2]

Answer 3

1. Result in vesicles carrying GLUT4 transporters moving towards PM to fuse with it
2. Activates glycogen synthase

Question 4

Describe how cells can decrease stimulation of receptor molecules by high [insulin] [2]

Answer 4

1. Reduce no. of available receptors

2. Thus fewer ligands bind to receptors

Question 5

Describe the advantages of a cell signalling system [6]

Answer 5

1. SIGNAL AMPLIFICATION - small no. of ligands needed to trigger a large response
2. MULTIPLE CHECKPOINTS FOR REGULATION
3. MULTIPLE RESPONSES to 1 ligand via multiple signal transduction pathways
4. Activate MANY CELLS simultaneously
5. Ensures SPECIFICITY - specific ligand > specific receptor (conformation) of specific cell
6. Activate GENES in the nucleus without needing to move into the nucleus

Question 6

Explain how mutation (in signal transduction) leads to cancer

Answer 6

1. GIF in K-Ras
   » hyperactive K-Ras remains constitutively bound to GTP (cannot hydrolyse bound GTP)
   » activates effectors constitutively
2. GIF in GPCR
   » mutated GPCR unable to hydrolyse ligand (remains bound)
   » remains activated
   » continuously activates K-Ras

Question 7

Describe a unique structural feature of RTK and relate it to its function [1]

Answer 7

Intracellular domain with Tyr residues that can be phosphorylated and can phosphorylate other Tyr residues during DIMERISATION to activate downstream pathways

Question 8

Explain how a named example of a protein is removed during the differentiation of stem cells [2]

Answer 8

1. Ubiquitination of telomerase

2. Enters proteasome&raquo_space; cleaved