GPCR functions

Question 1

K+ channels

Answer 1

• acetylcholine binds MAR (receptor)
• GTP binds Galpha, whole complex dissociates
• Gby binds K channel (effector protein)
• opens channel an K release to outside
• slows rate of muscle contraction

Question 2

Regulation of Adenylyl Cyclase (AC)

Answer 2

• catalyses production of cAMP

- stimulatory and inhibitory G protein complex compete

Question 3

Cholera - AC

Answer 3

• toxin maintains Galpha stimulatory protein in an active state (GTP bound)
• Galpha constitutively bound to AC
• massive increase in cAMP levels. Leads to phos CFTR
• Cl pumped from lumen to intestine, water from intestine to lumen
• massive dehydration

Question 4

Whooping cough - AC

Answer 4

• toxin maintains Galpha inhibitory protein in an inactive state
• prevents it from inhibiting AC = massive amounts of cAMP= CFTR activation
• ions out, mucous/water retained in lungs

Question 5

cAMP activates Protein kinase A (PKA)

Answer 5

• PKA has 2 regulatory and active domains
• active = cAMP bound to Regulatory domain. CO-OPERATIVE BINDING
• when active, the sets of domains dissociate

Question 6

Intracellular second messengers

Answer 6

• cAMP: activates PKA
• cGMP: activates PKG, opens cation channels in rod cells
• DAG: activates PKC
• IP3: opens Ca+2 channels in ER

Question 7

Regulation of glycogen metabolism by cAMP

Answer 7

High cAMP= PKA active
- phosphorylation of GPK, GP= glycogen breakdown
- phos of GS and IP (inhibitor of phosphatase)
Low cAMP= phosphatase active
- GPK and GP= inactive
- GS active

Question 8

Activation of gene transcription by GPCR

Answer 8

• genes regulated by PKA contain cAMP response element (CRE)

- activation of the CREB transcription factor (CRE-binding protein) by phosphorylation in nucleus

Question 9

CREB signalling pathway

Answer 9

1. Activation of G protein
2. Activation of AC–> increased cAMP
3. cAMP activates PKA
4. PKA catalytic domain translocates to the nucleus–> phosphorylates CREB
5. P-CREB binds CRE as a dimer. Also binds CPB/P300 co-activator –> recruits RNApoly2

Question 10

Phospholipase C (PLC)

Answer 10

• cleaves PIP2 into IP3 and DAG

Question 11

IP3/DAG signalling pathway

Answer 11

1. Galpha binds PLC: PIP2–> DAG + IP3
   - DAG remains bound on PM
2. IP3 binds Ca+2 channels, opens.
   - Ca+2 from ER lumen –> cytosol
3. Ca+2 binds PKC–> binds DAG on PM
   - leads to phos of substrates downstream

Question 12

Calmodulin

Answer 12

• Ca+2 binding protein, regulate the activity of other proteins

Question 13

NO,cGMP, PKG pathway

Answer 13

1. Acetylcholine binds GPCR
2. PLC–>IP3–> Ca+2/Calmodulin
3. NO synthase–> NO into smooth muscle cells
4. NO receptor catalyses GTP–>cGMP + Pi
5. cGMP–>PKG–>relaxation of muscle cells

Question 14

Viagara

Answer 14

• inhibits the PDE that catalyses cGMP–>GMP–>PKG needed for muscle relaxation
• erection maintained, never relaxes

Question 15

NO Receptors

Answer 15

• activate GC to increase cGMP and PKG activity = vasodilation of smooth muscle cells and increased blood flow