G-protein coupled receptors

Question 1

What mechanism do all GPCR’s act through?

Answer 1

7 pass transmembrane G-protein coupled mechanism

Question 2

Which cytosolic loops are involved in signal transduction?

Answer 2

Helix 3, 5 and cytosolic loops interact with coupled G protein.

Question 3

What is an example of a ligand for a GPCR?

Answer 3

Adrenaline/ retinal

Question 4

What two types of receptors does adrenaline bind?

Answer 4

beta2-adrenergic receptor and alpha2-adrenergic receptor

Question 5

How does ligand biding to GPCR shift cytosolic loops?

Answer 5

Helix 5 and 6 move relative to eachother changing conformation of C3 (cytosolic loop 3)
C3 binds and activates G protein

Question 6

where is beta2-adrenergic receptor found?

Answer 6

Found on smooth muscle cells. Binds on the heart muscle cells to increase heart contraction rate, increases blood flow to tissues
also binds hepatic and adipose tissues. triggers release of fatty acids and sugars.

Question 7

where is alpha2-adrenergic receptor found?

Answer 7

Found in muscle cells lining blood vessels. Cuts off circulation to skin kidneys and intestines. Supplies energy to major locomotor muscles in response to bodily stress

Question 8

What is the result of adrenaline binding a b2adrenergic receptor?

Answer 8

Adrenaline binding activates Gsalpha g-protein which stimulates adenylate cyclase activity, increases cytosolic conc of cAMP

Question 9

What is the result of adrenaline binding a a2adrenergic receptor?

Answer 9

binding stimulates release of Gialpha g-protein which inhibits adenylate cyclase decreasing cytosolic cAMP conc

Question 10

Which receptor has a higher affinity for adrenaline?

Answer 10

beta-2-adrenergic

Question 11

How does the GTPase molecular switch work in GPCRs?

Answer 11

ligand binding shifts cytosolic loop 3 in receptor. This shift is passed on to the coupled G protein which causes GDP to dissociate from Ga, and for GTP to bind Ga.
Ga-GTP dissociates from the complex and binds to membrane targets such as membrane bound enzymes or ion channels (in adrenaline binds adenylate cyclase)

Question 12

What are the downstream effects of GPCR signaling in hepatic cells?

Answer 12

Gsalpha-GTP activates adenylate cyclase to increase cytosolic cAMP levels.
Increased cAMP allows protein kinase A to dissociate from its inactive complex.
Activated protein kinase A phosphorylates metabolic enzymes (eg glycogen breakdown enzymes) and phosphorylates transcription factor CREB.
Also inactivates myosin light chain kinase in smooth muscle cells.

Question 13

How are GPCR’s inactivated?

Answer 13

PKA phosphorylates the receptor which desensitises it (feedback)
BARK phosphorylates ligand bound beta receptors leading to B-arrestin binding and endocytosis.

Question 14

What is an agonist (ligand analogue?)

Answer 14

An agonist mimics a ligand by binding the receptor and causing the normal response? (mimetic)

Question 15

What is an example of an agonist of adrenaline in medical field?

Answer 15

B2 selective agonists eg turbutaline used as treatment for asthma - bind to b2 receptors on smooth muscle cells lining bronchial passages and stimulate muscle relaxation.

Question 16

What is an antagonist (ligand analogue)?

Answer 16

Antagonist is competitive inhibitor. Binds receptor in place of ligand but does not cause activation

Question 17

What is an example of an antagonist of adrenaline used in medical field?

Answer 17

beta blockers (practolol) bind to beta1 receptors on heart muscle cell. Normal ligand binding would increase heart rate but antagonist binding decreases heart rate.

Question 18

What are the key features of a b-adrenergic ligand analogue?

Answer 18

Side chain containing NH groups determines affinity for the receptor.
A catechol ring is required for adenylate cyclase activation (ie agonist)

Question 19

what is an example of a light sensitive GPCR?

Answer 19

Rhodopsin (in the beta-adrenergic family)

Question 20

What is the ligand for Rhodopsin?

Answer 20

Retinal (technically part of the molecule)

Question 21

What type of molecule is retinal?

Answer 21

Retinal is a chromophore. The structure is altered by light.

Question 22

What happens when light is detected on Rhodopsin?

Answer 22

Light causes cis to trans isomerisation of retinal. This shifts cytosolic loop on opsin part of the protein.
This activates the coupled G protein

Question 23

What G protein is coupled to rhodopsin?

Answer 23

Gt = G transducin

Question 24

What is the effector enzyme of G transducin in rhodopsin pathway?

Answer 24

PDE (phosphodiesterase)

Question 25

What is the action of the effector enzyme in the Rhodopsin pathway?

Answer 25

PDE converts cGMP back to GMP. This reduces cytosolic levels of cGMP blocking cGMP gated ion channels.
This prevents release of the neurotransmitter glutamine.

Question 26

What is, in short, the optical signal sent by the Rhodopsin pathway?

Answer 26

The lack of release of the neurotransmitter glutamine is the optical signal.

Question 27

How is the rhodopsin pathway turned off?

Answer 27

Phosphorylated by the rhodopsin kinase to downregulate ability to activate Gt.

Question 28

What happens to Rhodopsin in very bright light?

Answer 28

Arrestins bind rhodopsin and there is no activation of Gt - Gt is endocytosed to other parts of the cell.