G-protein coupled receptors part 2

Question 1

What do activated G proteins do

Answer 1

They regulate the activities of enzymes that control the levels of second messengers, including:

-Hydrophobic lipids confined to the membrane in which they are generated
-Small soluable molecules that diffuse through the cytoplasm
-Calcium ions

Question 2

1. CAMP second messenger system activated by Adenylate cyclase

Answer 2

-Made of 10 isoforms
-Membrane anchored enzyme by two homologous domains that come together to form 12 transmembrane domains
-cAMP signalling imaged in the cytoplasms and nucleus of a neuron
-Can be activated by G alpha s or G alpha i

Question 3

What are the steps in the cAMP second messenger system

Answer 3

1. Ligand binds to receptor activating G protein
2. alpha subunit moves and binds to adenylate cyclase in the membrane once GDP has been displaced by GTP
3. This activated enzyme catalyses formation of cAMP from ATP
4. The cAMP activates protein kinase A
5. PKA phosphorylates/ activates protein
6. Initates a response within the cell

Question 4

Give an example of cAMP second messenger system

Answer 4

B2 adrenoreceptor regulation of metabolism in the liver and skeletal muscle

-Signalling is switched off by agonist dissociating from receptor, GTPase activity of Gas, cAMP breakdown by phosphodiesterase and dephosphorylation of enzymes

Question 5

What’s the cGMP second messenger system

Answer 5

Similar to cAMP expect it uses the enzyme guanylate cyclase which can be receptor bound or free in the cytoplasm.

It converts guanosine triphosphate to 3’-5’ guanosine monophosphate