GPCRs

Question 1

Describe basic structure of GPCRs

Answer 1

7 transmembrane domains
-arranged into a barrel like pore where the ligand binds

N terminus outside
C terminus inside

Question 2

What portion of GPCR binds ligand?

Answer 2

The extracellular and/or transmembrane domain.

the T1 and T7 loop back on each other, forming a barrel-like structure

Question 3

What portion of the GPCR confers G-protein binding specificity?

Answer 3

The cytoplasmic i-loops.

carboxyl terminal ends

Question 4

Define GPCR agonist vs. antagonist

Answer 4

Agonist: binding triggers receptor activation
Antagonist: binding blocks receptor activation by antagonists (competitive)

Question 5

How do GPCRs activate heterotrimeric G-proteins?

Answer 5

1. Ligand binds to GPCR
2. Galpha releases GDP, binds GTP. == ACTIVATION
3. Alpha and beta-gamma subunits dissociate, become available to effector proteins

Question 6

how do GPCR activate heterotrimeric G proteins?

Answer 6

G Proteins:
alpha subunit - binds GTP/GDP
beta/gamma subunits

-ligand binding to the GPCR gives the G proteins ability to exchange GDP for GTP

Question 7

What is the rate limiting step of the GPCR cascade?

Answer 7

nucleotide exchange (GDP–> GTP)

activates cellular pathway

Question 8

Steps of GTP hydrolysis cycle of G protein signaling

Answer 8

1. ligand binds
2. alpha subunit exchanges GDP for GTP
3. alpha-GTP unit dissociates from beta-gamma
4. both units act on effector proteins
5. Intrinsic alpha GTPase activity hydrolyzes GTP–> GDP

Question 9

What proteins can help in Catalyzing the exchange of:
GDP –> GTP?
GTP –> GDP?

Answer 9

GDP –> GTP? GEF

GTP –> GDP? GAPs

Question 10

How does cholera toxin affect the G-protein molecular switch?

Answer 10

The CTX ribosylates the G-subunit, and locks it in the GTP-bound ACTIVE state. Cannot be deactivated so you get constitutively expressed cAMP

Question 11

How does bordella pertussis affect the G-protein molecular switch?

Answer 11

The PTX binds to the C-terminal side and locks the G-protein in the heterotrimeric INACTIVATED state. The G-protein cannot be activated, leads to whooping cough.

Question 12

Within the ANS, symp and parasymp have what kind of transmission via what signaling molecules?

Answer 12

Symp: Adrenergic via NE and epinephrine

Parasymp: Muscarinic cholinergic via ACh

Question 13

function of second messengers in receptor signaling

Answer 13

Second messengers are molecules that relay signals from the cell surface to targets inside the cell. They
often are involved in amplification of the signal.

Question 14

Example of how second messengers are activated in beta adrenergic receptors (heart)?

Answer 14

1. NE binds the receptor, causing GDP/GTP exchange
   - –> activation of alpha unit
2. alpha-GTP unit binds to adenylyl cyclase (AC) to activate it.
3. AC makes cAMP, a second messenger
4. which in turn activate protein kinase A (PKA).
5. PKA phosphorylates Ca channels, leading to influx of Ca.

These steps lead to contraction of cardiac muscle.

Question 15

What effect would metaprolol (beta1 selective antagonist) have on heart rate?

Answer 15

Decrease heart rate

• blocks activation of G protein with GTP and prevents increase in cAMP

Question 16

Example of how second messengers are activated in alpha 1 adrenergic receptors (peripheral vasculature)

Answer 16

1. NE binds, causes the exchange of GDP/GTP
   
   • –> activation of alpha unit
2. alpha-GTP interacts with the PLC molecule
3. PLC cleaves membrane lipid PIP2.
4. Results in 2 second messengers DAG and IP3
5. Both DAG and IP3 stimulates Ca2+ (another second messenger)

These steps lead to contraction of smooth muscles.

Question 17

What effect would prazosin (an alpha1 selective antagonist) have on blood pressure?

Answer 17

decrease blood pressure (block vasoconstriction response)

• prevents creation of DAG and IP3

Question 18

Explain how receptor activation leads to signal termination through receptor desensitization

Answer 18

The signaling pathway that activates G proteins, also activates receptor desensitization. One of the
binding partners of the beta-gamma subunit after it dissociates from the alpha-GTP unit is a kinase
called GRK.

phosphorylation recruits beta arrestin, which further prevents Gproteins from associating with receptor

Question 19

GRK

Answer 19

kinase that phosphorylates the cytoplasmic loops of the G protein coupled receptor. This results in its
inability to interact with G proteins. = desensitization

Question 20

What effect would prazosin (an alpha1 selective antagonist) have on blood pressure?

Answer 20

decrease blood pressure (block vasoconstriction response)

Question 21

Explain how receptor activation leads to signal termination through receptor desensitization

Answer 21

The signaling pathway that activates G proteins, also activates receptor desensitization. One of the
binding partners of the beta-gamma subunit after it dissociates from the alpha-GTP unit is a kinase
called GRK.

Question 22

GRK

Answer 22

kinase that phosphorylates the cytoplasmic loops of the G protein coupled receptor. This results in its
inability to interact with G proteins.

Question 23

Atropine

Answer 23

parasympathetic antagonist/ ach antagonist / muscarinic antagonist:  that prevents activation of G protein with GTP.

Blocks parasympathetic (which depresses HR) --> then muscle contractions increase = faster heart rate

Question 24

PDE inhibitors

Answer 24

ie: Caffeine

target phosphodiesterases (PDE) of the message pathway to activate signaling. PDE inhibitors are able
to extend signaling.

normally PDEs degrade cAMP by
cleaving it to AMP, which is not capable of activating PKA, and thus terminate signaling

Question 25

1. Would inhalation of ipratropium (muscarinic antagonist) relieve acute asthma symptoms?

Would inhalation of albuterol (beta2-selective agonist) add to or counter effects of ipratropium?

Answer 25

1. Yes
   block parasympathetic constriction (favor airway opening)
2. add to
   block muscarinic pathway and activate adrenergic pathway

( antagonist of one pathway, and agonist of other pathway would add to)