Lecture 22: G-protein Coupled Receptors

Question 1

Neuromuscular Transmission = 3

Answer 1

1. In SKELETAL muscle ACh
   stimulates N-cholinergic
   receptors.
2. NICOTINIC receptors are a ligand gated ion channel.
3. Opening of the channel causes
   a motor endplate potential
   (EPSP)

Question 2

Ligand Gated Ion Channels: 4

Answer 2

1. Many neurotransmitters act on
   ligand gated ion channels.
   ACh on N-cholinergic receptors
   - arrival of an electrical stimulus triggers release of acetylcholine receptor on the muscle cell…
   (NICOTINIC)
2. GLUTAMATE on the NMDA receptor
   - …activating the entry of sodium, which causes a local membrane depolarisation
3. Binding opens the ion channel, either hyperpolarises or
   depolarises (depending on the ion passing through the channel
   - acetylcholinesterase degrades the transmitter, terminating the signal.

Question 3

Other Transmitters and Hormones?
= 4

Answer 3

1. Are *M-cholinergic receptors (MUSCARINIC) ion channels?
2. Parasympathetic transmission
3. What about a and b adrenergic receptors
4. Sympathetic transmission and adrenaline

Question 4

Not Ligand Gated Channels: 5

Answer 4

1. Stimulation of cardiac beta1
   -receptors increases heart
   rate
2. Circulating adrenaline cause GLYCOGEN BREAK DOWN IN SKELETAL MUSCLE
3. Ion channels PRODUCE BINARY RESPONSES
   hyperpolarization or depolarization.
4. ADRENALINE DOES NOT CAUSE MUSCLE CONTRACTION OR RELAXATION
   so it cannot be acting through changes in
   membrane potential.
5. MUSCARINIC receptors, a and b adrenergic receptors are NOT LIGAND GATED ION CHANNELS

Question 5

G-protein Coupled Receptors (GPCR)

Answer 5

G-protein Coupled Receptors (GPCR)

Question 6

Heterotrimeric G-proteins
- 4

Answer 6

Receptor R: consists of 7 membrane-spanning segments.

1. Ligand binds, receptor activates
2. Receptor interacts with the G protein to promote a conformational change and the exchange of GDP for GTP.
3. G protein dissociates from the receptor
4. alpha-GTP and By subunits dissociate.

Question 7

G-protein Coupled Receptors PATHWAY 12

Answer 7

1. G-protein coupled receptors = (GPCR)
2. Receptor is a transmembrane protein (with 7 membrane domains).
3. Bind hormone or neurotransmitter etc on
   extracellular side and when activated binds to
   HETEROTRIMERIC G-PROTEINS.
4. Heterotrimeric =3 DIFFERENT PROTEINS MAKE UP FINAL STRUCTURE
5. G-proteins bind guanine diphosphate (GDP) and
   guanine triphosphate (GTP).
6. INACTIVE bound to GDP, ACTIVATED bound to GTP
7. Binding of GPCR to G-protein cause release of GDP.

8.Release of GDP allows binding of GTP and activates the G-protein

9. Disassociation of alpha subunit from beta and y.
10. ACTIVE ALPHA SUBUNIT (and sometimes by subunit) binds to membrane bound enzymes and activates them
11. A SECOND MESSENGER is produced that takes the signal into the cytosol.
12. Three main groups of G-proteins

Question 8

GPCR Types
AGONIST:
ACh

Answer 8

Receptor: M1, M3, M5

G-protein: Gq

Typical action: Ip3

Question 9

GPCR Types
Agonist: ACh

part 2

Answer 9

Receptor: M2, M4

G-protein: Gi

Typical action: Inhibit cAMP

Question 10

GPCR Types
Agonist: NA , Adr

Answer 10

Receptor: alpha-adrenergic

G-protein: Gq

Typical action: Ip3

Question 11

GPCR Types
Agonist: NA , Adr
part 2

Answer 11

Receptor: beta-adrenergic

G-protein: Gs

Typical action: Activate cAMP

Question 12

**Gs Vs Gi pathways

1. Stimulatory agonist (e.g., epinephrine)
2. Inhibitory agonist (e.g., adenosine)

Answer 12

1. Stimulatory agonist (e.g., epinephrine)
2. Receptor (By)
3. Stimulation (alpha, s)
4. Adenylyl cyclase
5. Inhibitory agonist (e.g., adenosine)
6. Receptor (B,y)
7. ai Inhibition
8. Adenylyl cyclase

Question 13

cAMP

Answer 13

cyclic adenosine monophosphate
(cAMP)

• cAMP is a major second messenger

Question 14

How is cAMP produced?
What does Gs, Gi, Gq do?
= 5

Answer 14

1. Gs ACTIVATES ADENYLATE CYCLASE
2. Gi INHIBITS ADENYLATE CYCLASE
3. Adenylate cyclase (AKA adenyl cyclase, adenylyl cyclase
   or AC) MAKES CYCLIC ADENOSINE MONOPHOSPHATE
   (cAMP) from ATP.
4. cAMP is a major second messenger
5. Gq activates phospholypase and makes inositol
   triphosphate (Ip3) as a second messenger

Question 15

Gs and Gq pathways =4

Answer 15

1. Gs and Gi signal
   through cAMP
   SIGNLLING MOLECULE
2. Gq signals through
   Ip3
   CELL-SURFACE RECEPTOR

ENZYME
INTRACELLULAR MEDIATOR

3. Ip3 releases Ca2+
   from the ER (SR)
4. Gq effects through
   changes in Ca2+
   TARGET PROTEIN

Question 16

Cardiac Pacemaker Cells =3

Answer 16

1. Pacemaker potential
   (phase 4)
2. Fall in potassium
   current IK and increase in calcium and funny current ICa, If cause pacemaker
3. If channel
   hyperpolarizarion,
   cyclic nucleotide
   activated channel
   (HCN)

Question 17

Understanding Sympathetic Chronotropy =9

Answer 17

1. Sympathetic nerve stimulation, releases NORADRENALINE
   (NA) increases heart rate (positive chronotropy)
2. Circulating ADRENALINE (Adr) increases heart rate
3. b1-receptors bind NA and Adr
4. b1-receptors are Gs
   linked increases cAMP
5. cAMP binds to HCN ion channels increasing the rate
   they open.
6. Hyperpolarisation triggers opening of HCN
7. HCN is a Na+ channel opened by hyperpolarization
8. More If channel open, faster depolarization
9. Cell reaches threshold faster, faster pacemaker

Question 18

Protein Kinase A (A-kinase) = 3

Answer 18

Direct actions of cAMP on ion channels or final target are rare.

Most effects of cAMP produced by PROTEIN KINASE A (PKA)

PKA phosphorylates proteins changing their activity

Question 19

Glycogen Break Down = 3

Answer 19

1. Skeletal muscles store GLUCOSE as GLYCOGEN a carbohydrate.
2. During exercise muscles break down glycogen releasing
   glucose used as fuel
3. Adrenaline causes muscle to turn glycogen into glucose

Question 20

Control of Glycogen Breakdown

Answer 20

1. Inactive A-kinase
2. Protein Kinase A
3. cAMP
4. Active A-kinase
5. inactive phosphorylase kinase — (ATP) (ADP) –> aCTIVE PHOSPHORYLASE KINASE WITH PHOSPHATE.
6. inactive glycogen phosphorylase —(ATP) –(ADP)–> Active glycogen phosphorylase with phosphate
7. GLYCOGEN —> GLUCOSE-1-PHOSPHATE \
8. GLYCOLYSIS

Question 21

Glycogen Break Down: 5

Answer 21

1. Adrenaline activation of b2 -adrenergic receptors, activates Gs
   and andenylate cyclase.
2. Produces cAMP that activates PKA
3. PKA phosphorylates and activates phosphorylase kinase.
4. Which in turn phosphorylates and activates glycogen
   phosphorylase.
5. Glycogen phosphorylase turns glycogen into glucose.2.

Question 22

PKA AND PPI glycogen breakdown =4

Answer 22

1. PKA also inactivates glycogen synthase that turns glucose into glycogen.
2. PKA inactivates the phosphoprotein phosphatase 1
   (PP1).
3. PP1 is the enzyme that turns off glycogen
   phosphorylase and phosphorylase kinase
4. PP1 Is the enzyme that turns on glycogen synthase

Question 23

GPCR signalling = 8

Answer 23

1. Controls enzymes through phosphorylation.
2. Many steps allows amplification of the signal.
3. One receptor activates many adenlyate cyclase enzymes which each produce lots of cAMP.
4. One PKA activates many phosphorylase kinase molecules, each of
   which activates many glycogen phosphorylase enzymes.
5. Protein kinases have multiple targets
6. Turning on one pathway typically turns off antagonistic pathways
7. A phosphates and a kinase often antagonise each other
8. Phosphatases dephosphorylate, kinases phosphorylate

Question 24

SUMMARY = 7

Answer 24

1. Many neurotransmitters and hormones do not use
   ligand gated ion channels
2. M-cholinergic receptors, a & b adrenergic receptors are
   GPCR
3. Three kinds of G-protein linked to GPCR; Gi, Gs and Gq
4. Gs activates adenylate cyclase produces cAMP, Gi inhibits adenylate cyclase
5. b-receptors increase heart rate through the binding of cAMP to HCN ion channels
6. Most effects of cAMP through PKA
7. Protein kinases phosphorylate target proteins turning
   them on or off