Signalling Mechanisms

Question 1

Why are G proteins heterotrimeric?

Answer 1

They are formed of 3 subunits (alpha, beta, gamma).

Question 2

How are GPCRs activated?

Answer 2

When the ligand binds, it causes a conformational change, which causes the alpha subunit to bind to GTP instead of GDP. The alpha subunit dissociates from the beta and gamma subunits and regulates target proteins in the membrane (e.g adenylate cyclase to produce cAMP). The GTP is hydrolysed to GDP and everything goes back to the start.

Question 3

How many transmembrane regions does the alpha helix of the GPCR have?

Answer 3

7.

Question 4

Which enzyme inactivates cAMP?

Answer 4

Phosphodiesterase.

Question 5

How do hydrophobic ligands activate receptors?

Answer 5

Diffuse into cytosol and bind to receptor (dissociates from inhibitory protein complex). Ligand-receptor complex binds to target gene promoter to act as transcription factor.

Question 6

How does ionotropic signalling work?

Answer 6

A ligand binds to an allostearic site, or a certain membrane potential, causes a conformational change in integral proteins which opens a pore.

Question 7

Name 4 common second messengers.

Answer 7

Calcium ions.
Cyclic nucleotides e.g cyclic adenosine monophosphate (cAMP)
Inositol 1,4,5 - trisphosphate
Lipids e.g diacylglycerol

Question 8

What is the process of receptor tyrosine kinase activation?

Answer 8

A ligand molecule binds to each of the pair of receptor tyrosine kinases, causing them to move together and form a cross-linked dimer.
Cross-phosphorylation occurs as each RTK phosphorylates the tyrosine on the other.
Proteins with a specific domain are able to bind to the phosphorylated tyrosines.