S1B5 - Signal Transduction

Question 1

α1 adrenergic stimulation is an example of a G(q) protein second messenger pathway response. Which of the following is an accurate step in this type of pathway signaling?

A) Phospholipid cleaves phospholipase C

B) Gq proteins activate phospholopids

C) IP₃ binds receptors on the endoplasmic reticulum

D) DAG is released into cytosol

E) DAG is cleaved into PIP₂ and IP₃

Answer 1

IP₃ binds receptors on the endoplasmic reticulum

The Gq pathway is as follows: Stimulation of Gq activates phospholipase C (PLC). PLC then cleaves intralipid membrane phospholipids- in the process, phosphatidylinositol (PIP2) is cleaved into diacylglycerol (DAG) and inositol triphosphate (IP3). DAG remains bound to the membrane and activates protein kinase C (PKC) and IP3 is released into the cytosol and binds to Ca channels in endoplasmic reticulum causing Ca release. The increase in cytosolic calcium concentration causes a cascade of intracellular activity.

Question 2

Which 5 autonomic receptors transmit signal through G(s)?

Answer 2

G(s) = β1, β2, H2, V2, D1 (need to memorize, but if you memorize G(q), this will be easy)

Question 3

Stimulation of G(s) results in what pathway? Which molecules are involved?

Answer 3

G(s) activates adenylyl cyclase. This increases cAMP production, which increases protein kinase A (PKA) activity.

Question 4

The sympathetic effects of the heart muscle is stimulated through activation of G (s) protein, 2nd messenger pathways. Which of the following is an accurate step in this type of pathway signaling?

A) cAMP phosphorylates several downstream targets

B) G(s) directly stimulates the production of cAMP from ATP

C) IP₃ is released by protein kinase A

D) G(s) activates a catalytic enzyme which produces cAMP

E) Adenylyl cyclase activates protein kinase A

Answer 4

G(s) activates a catalytic enzyme which produces cAMP

GTP displaces GDP on the alpha subunit, which is now activated and diffuses away from the membrane to activate adenylyl cyclase. Adenylyl cyclase converts ATP into cAMP, which activates PKA. PKA then phosphorylates downstream targets. When the GTP on the alpha subunit is hydrolyzed to GDP, the alpha subunit is inactivated and the alpha, beta, and gamma subunits reassociate with a G protein coupled receptor.

Question 5

Which 5 autonomic receptors transmit signal through G(q)?

Answer 5

G(q) = α1, H1, V1, M1, M3 (all the 1’s except β1 and D1)

• “HAVe 1 M&M” is a popular mnemonic used to remember G(q)

Question 6

Which 3 autonomic receptors transmit signal through G(i)?

Answer 6

G(i) = α2*, M2, D2

• Remember α2 is inhibitory → G(i) (rarely will M2 or D2 be tested)
• You can remember these inhibitory receptors as “MAD 2s inhibit”

Question 7

Through stimulation of G(q), what are the 2 byproducts of PIP2 metabolism?

Answer 7

PIP2 is metabolized to IP3 (inositol triphosphate) and DAG (diacylglycerol).

Question 8

In the G(q) stimulation pathway, which cellular compartment is the site of action for IP3?

Answer 8

IP3 is released into cytoplasm, where it binds to endoplasmic reticulum and increases intracytoplasmic Ca2+ concentration.

Question 9

Acetylcholine exhibits a parasympathetic effect on the heart through the M2 receptor. Activation of M2 receptors results in a down stream second messenger cascade. Which of the following receptors uses the same second messanger cascade as M2 receptors?

A) α2 and D2

B) D2

C) B2

D) α2

Answer 9

α2 and D2

Acetylcholine exerts a parasympathetic effect on the heart through the M2 receptor type. The M2 receptor is conducted through the G(i) protein. These proteins signaling pathways are also used in α2 and D2. β2 is also found in the heart, but it functions through the G(s) pathway.

Question 10

Which types of G protein coupled receptor transmits signals from D1 receptors?

Answer 10

Alternatively, use the mnemonic: “QISS and QIQ till you’re SIQ of SQS”
α1 – Q
α2 – I
β1 – S
β2 – S

M1 – Q
M2 – I
M3 – Q

D1 – S
D2 – I
H1 – Q

H2 – S
V1 – Q
V2 – S

Question 11

In the G(q) stimulation pathway, where is the site of action for DAG?

Answer 11

DAG stays bound to cell membrane and, together with Ca2+ liberated from the endoplasmic reticulum due to IP3, activates protein kinase C (PKC)

Question 12

Describe the chain of enzymes/molecules affected when a G(i) protein is stimulated.

Answer 12

G(i) inhibits adenylyl cyclase. This inhibits cAMP production, which decreases protein kinase A (PKA) activity.

Question 13

Stimulation of G(q) results in activation of what enzyme?

Answer 13

G(q) stimulation activates phospholipase C (PLC), which stimulates the hydrolysis of PIP2 (phosphatidylinositol bisphosphonate).

Question 14

What kind(s) of receptors do neurotransmitters usually act on?

Answer 14

Neurotransmitters bind with receptors that also act as ion channels or they interact with G-proteins to stimulate effector enzymes to produce “second messengers”.

Question 15

What do most hormone receptors interact with?

Answer 15

Most hormone receptors interact with G-proteins to alter “second messenger” levels.

Question 16

Where are steroid hormone receptors located?

Answer 16

Steroid hormone receptors are cytosolic receptors which travel to the nucleus to alter mRNA synthesis.

Question 17

What kind of activity do most growth factor receptors have?

Answer 17

Most growth factor receptors have intrinsic tyrosine kinase activity and cause phosphorylation of tyrosine residues on specific proteins.

Question 18

What are the most important sympathetic neurotransmitters?

Answer 18

Catecholamines

• Epinephrine
• Norepinephrine
• Dopamine

Question 19

What is the most important parasympathetic neurotransmitter? What family of neurotransmitters is it in?

Answer 19

Acetylcholine is a parasympathetic neurotransmitter in the ester family.

Question 20

What are the indolamine neurotransmitters?

Answer 20

Indolamines

• Histamine
• Seratonin (5-HT)

Question 21

What are the peptide neurotransmitters?

Answer 21

Peptides

• Substance P
• Neuropeptide Y (NPY)
• Encephalins
• Somatostatin
• VIP

Question 22

What are the excitatory amino acid neurotransmitter?

Answer 22

Excitatory amino acid neurotranmitters

• Glutamate (E)
• Aspartate (D)

Question 23

What are the inhibitory amino acid transmitters?

Answer 23

Inhibitory

• gamma-aminobutyric acid (GABA)
• Glycine (G)

Question 24

What are the other, non-classified neurotransmitters?

Answer 24

Other

• Nitric Oxide
• Arachidonic acid
• Carbon Monoxide
• PAF
• Zinc

Question 25

Describe the general cycle of events in G-protein cycling.

Answer 25

1. Occupation of the receptor protein causes it to associate with the G-protein. 2. The receptor/G-protein complex allows a GTP to displace a GDP from the α-subunit. 3. Binding of GTP causes dissociation into free receptor, free (GTP) α-subunit, and free beta-gamma complex. 4. The (GTP) - α-subunit interacts with an effector protein to produce second messenger or occasionally to open ion channels. 5. beta-gamma-complexes were initially thought to be inert. This is probably not so, but their exact role is various signaling processes remains to be fully defined. 6. The α-subunit has intrinsic GTPase activity so the attached GTP is turned into GDP. This inactivates the α-subunit and causes it to rebind the beta-gamma-complex.

Question 26

What is cAMP made by? What substrate and cofactor are needed to make cAMP?

Answer 26

cAMP is made by a family to membrane spanning enzymes called collectively **Adenylate Cyclase**. * Currently types ADCY1 through ADCY10 have been well characterized. * An **ATP-Mg++ complex** is the substrate and free **Mg++** is a necessary cofactor.

Question 27

What breaks down cAMP and terminates its activity? Can this process be controlled?

Answer 27

**Phosphodiesterases** break down cAMP and terminate its activity. Phosphodiesterase activity **is also subject to control.**

Question 28

Highlight the key steps in the guanylate cyclase system (using cDMP and NO as second messengers).

Answer 28

1. Membrane bound **guanylate cyclase** appears to be directly coupled to receptors and **forms cGMP from GTP** when receptors become occupied. 2. **NO** is formed when the amino acid **arginine** is broken down into **NO and citrulline** by nitric oxide synthetase. **NO synthetase** is activated by **Ca/calmodulin** complex. 3. **NO** exerts its effect by activating a soluble, cytosolic type of guanylate cyclase and thus **increasing cGMP**. 4. **NO** is a unique second messenger because it **is membrane soluble**. This allows it to diffuse to **nearby cells** and **increase cGMP** levels in those cells as well. Such a phenomena occurs between vascular endothelial cells and nearby smooth muscle cells 5. **cGMP** exerts its effect in a variety of ways: direct binding to ion channels, **altering** the activity of some **phosphodiesterases**, and by **activating** a **cGMP-dependent protein kinase**. 6. **Phosphodiesterases** **break down cGMP** and terminate its action.

Question 29

What are the two broad classes of acetylcholine receptors?

Answer 29

Acetylcholine is a neurotransmitter in both the central and peripheral nervous systems. It binds to two broad classes of receptors: **nicotinic receptors** and **muscarinic receptors.**

Question 30

How many subunits do nicotinic receptors have? How are they arranged?

Answer 30

Nicotinic receptors are composed of various combinations of **5 subunits**: α, β, γ, delta, and ε. The subunits are arranged to form a **central cavity that extends across the membrane.**

Question 31

How are cations able to cross membranes through nicotinic receptors?

Answer 31

The core of the nicotinic receptor protein is normally too small for ion passage. When **ACh binds** to the receptor recognition sites on the **α-subunits** a **conformational change** allows cations to cross the membrane through the central receptor core.

Question 32

What's different between nicotinic receptors on skeletal muscles and those on autonomic ganglia?

Answer 32

Nicotinic receptors on skeletal muscle (designated **N1**, or **Nm**) have different α and β subunits than nicotinic receptors on autonomic ganglia (designated **N2** or **Ng**). N1 and N2 are each receptor gene product families, not unique receptor types.

Question 33

What are muscarinic receptors activated by and what do they do?

Answer 33

**Muscarinic receptors**, in contrast, **are not channels** despite being **activated by ACh**. Muscarinic receptors operate through **G-proteins** to **alter the activity** of a number **of second messenger systems**.

Question 34

Agrenergic substances bind to what three families of receptors?

Answer 34

Adrenergic substances bind to three families of receptors: **β-receptors**, **α1-receptors**, and **α2-receptors**. Note that α1-receptors and α2-receptors represent different families; whereas, all β-receptor subtypes (β1, β2, β3) belong to a single family. There are currently three known subtypes of α1-receptors and three known subtypes of α2-receptors.

Question 35

All adrenergic receptors are coupled through what proteins?

Answer 35

All adrenergic receptors are coupled through **G-proteins.**

Question 36

What does occupation of β-receptors stimulate?

Answer 36

Occupation of β-receptors stimulates **adenylate cyclase** to produce cAMP and activate PKA. This is **Gs**-mediated.

Question 37

What does occupation of α1-receptors lead to?

Answer 37

Occupation of **α1-receptors** increases IP3 and DAG for some subtypes (α1B), and activates Ca++-channels for other subtypes (α1A). Presumably these effects are **Gq**-mediated and involve **PLC activation**.

Question 38

What does occupation of α2-receptors lead to?

Answer 38

Occupation of **α2-receptors** leads to **inhibition of adenylate cyclase**. Presumably these effects are **Gi**-mediated.