MR 7 Signal Transduction

Question 1

What does G-protein stand for?

Answer 1

guanine nucleotide binding protein

Question 2

Of how many subunits are g-proteins made up?

Answer 2

They are heterotrimeric, being made up of 3 subunits: alpha, beta and gamma. The beta and gamma sub units bind tightly to each other and function as a single unit

Question 3

Which sub unit binds to guanine?

Answer 3

The alpha sub unit has a nucleotide-binding site, which binds GTP and slowly hydrolyses it to GDP (GTPase activity)

Question 4

Under basal conditions where is the G protein and in what form?

Answer 4

The inner face of the plasma membrane, in its heterotrimeric form with GDP bound to the alpha sub unit

Question 5

What happens when an agonist binds to the receptor?

Answer 5

The activated receptor has a high affinity for the GDP bound G protein and a protein-protein interaction occurs causing GDP to be released and GTP bound in its place (receptor acts as guanine nucleotide exchange factor GEF). Once GTP has bound affinity of receptor for both alpha-GTP and beta-gamma subunits is decreased and they release and split apart being able now to interact with effectors

Question 6

What terminates G-protein signalling?

Answer 6

The GTPase on the alpha sub unit hydrolyses the GTP to GDP and then the alpha-GDP sub unit and the beta-gamma sub unit reform an inactive heterotrimeric complex

Question 7

If we think of G proteins as on/off switches and timers what represents each?

Answer 7

On switch is receptor facilitated GDP->GTP exchange

Timer/off switch is length of time for GTP hydrolysis

Question 8

What does Gs(stimulatory) do and what signal and receptor does it respond to?

Answer 8

Stimulates adenylyl cyclase to make cAMP e.g in glycogenolysis and lipolysis
signal- adrenaline/noradrenaline
beta-adrenoceptor

Question 9

What does Gi(inhibitory) do and what signal and receptor does it respond to?

Answer 9

Inhibits adenylyl cyclase so less cAMP is made (also stimulates K+channel) e.g slowing of cardiac pacemaker
signal- acetylcholine
M2 muscarinic receptor

Question 10

What does Gq do and what signal and receptor does it respond to?

Answer 10

Stimulates phospholipase C in smooth muscle contraction
Signal- acetylcholine
M3 muscarinic

Question 11

What does Gt (transducin) do and what signal and receptor does it respond to?

Answer 11

Stimulates Cyclic GMP phosphodiesterase giving visual excitation.
Signal- light
Receptor rhodopsin

Question 12

How many alpha beta gamma sub unit combinations are there?

Answer 12

over 1000

Question 13

What advantage does the high g protein and receptor diversity give?

Answer 13

Extracellular signal working via specific receptor activates only a signle or small sub-population of g-proteins and effectors in cell to give a specific response

Question 14

How collectively do the cholera and pertussis toxins affect G-protein function?

Answer 14

They ADP-ribosylate specific g proteins

Question 15

What G protein does CTx affect and what does it do?

Answer 15

Affects Gs-alpha, eliminating GTPase activity meaning Gs-alpha irreversibly activated

Question 16

What G protein does PTx affect and what does it do?

Answer 16

Affects Gi-alpha intefering with the GDP/GTP exchange so Gi-alpha becomes irreversibly inactivated

Question 17

What is retinitis pigmentosa caused by?

Answer 17

loss of function (or gain of function) mutation to rhodopsin

Question 18

What is nephrogenic diabetes insipidus caused by?

Answer 18

loss of function mutation to V2 vasopressin receptor

Question 19

What is familial male precocious puberty caused by?

Answer 19

gain of function mutation to luteinizing hormone receptor

Question 20

Which g proteins is adenylyl cyclase affected by/

Answer 20

Gs and Gi

Question 21

What does adenylyl cyclase do?

Answer 21

hydrolyses cellular ATP to generate cAMP which interacts with cyclic AMP-dependent protein kinase (PKA) which phosphorylates other proteins within cell to affect activity

Question 22

What effects can increasing cAMP have?

Answer 22

increased glycogenolysis and gluconeogenesis in the liver, increased lipolysis in adipose tissue, relaxation of smooth muscle, positive inotropic and chronotropic effects in the heart

Question 23

What does phospholipase c do and what does its product do?

Answer 23

Hydrolyses membrane phospholipid PIP2 to IP3 when activated by Gq
IP3 interacts with receptors on ER to allow Ca2+ release into the cytoplasm from the ER

Question 24

What causes the receptor desensitisation phenomenon?

Answer 24

Whilst activated receptors are susceptible to a varitey of protein kinases which phosphorylate the receptor and prevent it activating further g-proteins

Question 25

What factors deactivate g protein coupled receptor pathways?

Answer 25

Once receptor has productively interacted with a g-protein agonist binding is weakened and likely to dissociate
Receptor desensitisation phenomenon
active lifetime of alpha-GTP limited by cellular factors stimulating the GTPase activity of subunit
Basal state is favoured and cells contain high activity enzymes that break down second messengers
Enzymatic cascades downstream to second messenger activation act to oppose effect

Question 26

How does Gi effect chronotropy in the heart?

Answer 26

Activation of M2 muscarinic receptors by Ach release from parasympathetic nerves stimulates Gi which increases the open probability of K+. The increased K+ permeability causes hyperpolarisation slowing the intrinsic firing rate giving a negative chronotropic effect

Question 27

How does Gs affect inotropy in the heart?

Answer 27

Activation of beta1 adrenoceptors increases the amount of VOCCs via Gs both directly and indirectly via cAMP and PKA which phosphorylates and activates VOCCs

Question 28

How a g-proteins used in arteriolar vasoconstriction?

Answer 28

noradrenaline acts on alpha1 adrenoceptors to stimulate phospholipase C and IP3 production via Gq.
The IP3 releases ER Ca2+ and initiates a contractile response

Question 29

How do g proteins influence neurotransmitter release?

Answer 29

modulates neurotransmitter release as the gamma-beta sub unit inhibits some VOCCs reducing Ca2+ influx and reducing neurotransmitter release