Signal Transduction

Question 0

List some functions that G proteins are responsible for

Answer 0

Muscle contraction
Stimulus-secretion coupling
Catabolic/anabolic processes
Light
Smell
Taste

Question 1

What do G protein-coupled receptors (GPCRs) do? And how? (in general)

Answer 1

Alter the activity of effectors (enzymes or ion channels)

Do this by activating one or more types of guanine nucleotide binding proteins.

Question 2

Structure of a G-protein

Answer 2

Heterotrimeric (3 distinct subunits stuck together)
3 distinct subunits α, β, γ
α has a guanine nucleotide binding site

Question 3

Structure of a GPCR?

Answer 3

Single polypeptide chains
7 transmembrane spanning domains
Extracellular N terminal
Intracellular C terminal

Question 4

What regions of the GPCR are responsible for binding?

Answer 4

2-3 of the transmembrane domains
Or
The N-terminal region

Question 5

How does interaction of the G protein with the GPCR activate it?

Answer 5

Causes GTP to exchange for GDP on the G protein α subunit.

GDP -> GTP

Question 6

What happens after the G protein has been activated?

Answer 6

α-GTP and βγ dissociate.

Each interact with effector proteins

Question 7

What stops the two sub units interacting with the effector proteins? What happens next?

Answer 7

Until the α-GTPase activity hydrolysed GTP back to GDP.

α-GDP and βγ then reform an inactive heterotrimeric complex.

Question 8

What does Gs stimulate? What kind of pathway is it?

Answer 8

Adenylyl cyclase to produce cAMP

Stimulatory

Question 9

What do Gi do? Type of pathway?

Answer 9

Inhibits adenylyl cyclase so there is less cAMP

Inhibitory

Question 10

What do Gq proteins do?

Answer 10

Interact with the membrane bound enzyme PIP2 to generate the 2nd messengers InsP3 and DAG

Question 11

What does Gt (transducin) do?

Answer 11

Rhodopsin activates it which in turn activates the enzyme that hydrolyses cGMP ➡️ 5’-GMP

Question 12

What does the pertussis toxin do to GPCRs?

Answer 12

Contains the enzyme ADP-ribosyl transferase which modifies and inactivates Gi-type proteins. This uncouples receptor-effector linkage.

Question 13

What does the cholera toxin do?

Answer 13

Contains an enzyme which specifically modifies Gs-type proteins, leading to irreversible activation.

Question 14

What is retinitis pigmentosa caused by?

Answer 14

A loss of function mutation to rhodopsin.

Question 15

What is nephron epic diabetes insipidus caused by?

Answer 15

Loss of function mutation to V2 vasopressin receptor

Question 16

What is familial male precocious puberty caused by?

Answer 16

A gain of function mutation (where the receptor is effective without the ligand) to the lute using hormone receptor.

Question 17

What activates and what inhibits adenylyl cyclase?

Answer 17

Activated via Gs and inhibited by Gi

Question 18

What does adenylyl cyclase do? What does this lead to?

Answer 18

Hydrolyses ATP to generate cAMP
cAMP interacts with a specific protein kinase (PKA)
PKA then phosphorylates a variety of other proteins within the cell to affect activity.

Question 19

What effects does adenylyl cyclase have on the cell?

Answer 19

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 20

What is phospholipase C

Answer 20

An enzyme that hydrolyses membrane phospholipid PIP2 ➡️ IP3

Question 21

What activates phospholipase C?

Answer 21

Gq

Question 22

What does IP3 do?

Answer 22

Interacts with specific intracellular receptors on the ER to allow Ca2+ to leave the ER lumen and enter the cytoplasm.

Question 23

What is cyclic GMP phosphodiesterase and what does it do?

Answer 23

A specialised mechanism found in the photoreceptors of the retina.
Rhodopsin is excitated by a photon of light which causes Gt to regulate the breakdown of the second messenger cGMP
Causes channel closure and membrane hyperpolarisation.

Question 24

What happens when cGMP levels are high in the dark?

Answer 24

Can open a second-messenger operated ion channel which allows calcium and sodium to enter the cytoplasm.
Alters the signal output to the CNS.

Question 25

How is a GPCR activated?

Answer 25

An agonist binds to it

Question 26

What is the receptor desensitisation phenomenon?

Answer 26

While a GPCR is activated, protein kinases can phosphorylate the receptor and prevent it from activating further G-proteins

Question 27

How are G proteins involved in controlling heart inotropy?

Answer 27

β-adrenoreceptors (GPCRs) are activated by adrenaline.
They increase the open probability of voltage-opened calcium channels via Gs
Influx of Ca brings about a positive inotropic effect

Question 28

How does Gs act indirectly with VOCCs (in the context of inotropy)

Answer 28

cAMP ➡️ PKA ➡️ phosphorylation and activation of VOCCs.

Question 29

How are GPCRs involved in chronotropy of the heart?

Answer 29

M2 muscarinic choline receptors are activated.
This increases the open probability of V-G potassium channels via Gi
Increased plasma membrane permeability to K+ causes hyperpolarisation, slowing the intrinsic firing rate, resulting in a negative chronotropic effect.

Question 30

How are GPCRs involved in arteriolar vasoconstriction/smooth muscle contraction

Answer 30

Sympathetically released noradrenaline acts on α1-adrenoreceptors to stimulate phospholipase C and IP3 production via Gq.
IP3 releases ER calcium and initiates a contractile response.

Question 31

How does bronchoconstriction occur?

Answer 31

Parasympathetically released Ach interacts with M3 muscarinic receptors on bronchiolar smooth muscle

Question 32

How does modulation of neurotransmitter release occur?

Answer 32

Can be influenced by pre-synaptic GPCRs.
Presynaptic μ opioid receptors can be stimulated by endogenous opioids or by analgesics such as morphine.
Couples to Gα1 proteins
The G-βγ liberated fro. The heterotrimer interact with VOCCs to reduce calcium entry, reducing neurotransmitter release.