L10: GPCRs

Question 1

GPCRs general structure

Answer 1

• 7 transmembrane alpha-helical regions
• G proteins interact w/ a-helices 5 and 6
• Some have an extra short a-helix, H8, which runs parallel to PM on cytosolic face w/ palmitic acid attached

Question 2

Principal GPCR grps

Answer 2

Family A:
- Biological amines, light odorants, peptides or chemokines, purines, ligands
- Basic GPCR structure
Family B:
- Peptides
- Large EC domain anchored to H1 at N-terminus; binds to peptide ligands, positions in receptor binding site
Family C:
- Biological amines, glutamate, Calcium
- Large VFD (Venus fly trap domains) on cytosolic side, ligand trapped by domains -> perception of ligand

Question 3

Beta adrenergic receptor substrate

Answer 3

• Epinephrine (fight or flight; increase glycogen breakdown, increase release of fatty acids) interacts w/ 15 aas on 4 TM a-helices

Question 4

Heterotrimeric G-proteins

Answer 4

• a, b, y subunits
• Associated w/ beta-adrenergic receptor and activation of adenylate cyclase
• Guanyl nucleotide BS (binds either GDP or GTP)
• Molecular switches (switched ‘on’ w/ GTP, ‘off’ w/ GDP and forms full trimer - exchange of GTP or GDP)
• Act as molecular timers depending on intrinsic GTPase activity on a-subunit

Question 5

B-adrenergic rec. pathway

Answer 5

• Receptor active; activates heterotrimeric G protein
  -> exchanged GDP for GTP
• a-subunit released, targets next effector (adenylate cyclase)
• Activation by a-subunit causes v. rapid rise in conversion of ATP to cAMP
• cAMP activates protein kinase A
• Amplification at all stages! (10^-10 stimulus in EC environment up to 10^-6 conc.)

Question 6

How is adenylate cyclase activated?

Answer 6

• Large pp; 12 TMS a-helices, 2 overall domains, each containing catalytic domain which give enzyme its activity

Question 7

Regulating the pathway

Answer 7

• G protein cycle switching off cycle through intrinsic GTPase activity on a-subunit (‘molecular switch’)
• Dissociation of ligand from receptor
• Phosphorylation by GCPR kinase (BARK) and binding of Arrestin
• Endocytosis
• Inhibitory effects of other GPCRs (e.g. inhibiting cAMP activation) -> Gi protein can bind to and inhibit adenylate cyclase

Question 8

Alpha1-adrenergic pathway

Answer 8

• Ligand binds to receptor, activates Gq protein, exchange of GDP for GTP on a-subunit
• Activated, a-subunit released and activates phospholipase C

Question 9

Phospholipase C

Answer 9

• Involved in calcium signalling
• Hydrolyses PIP2 into lipid (DAG) and cytosolic (IP3) component, both of which are important second messengers in calcium signalling
• Activate PKC