Exam 2 Lecture 11

Question 1

What is the main idea behind ligand gated ion channels?

Answer 1

1. First messenger binds to the channel (on the outside of the cell)
2. Channel opens to allow ions (Na+, Ca2+, K+) to cross

Question 2

Why are many ligand gated ion channels involved in neuronal signaling?

Answer 2

The neurotransmitter binding to the channel receptor induces ion flow which leads to the postsynaptic signal of depolarization (excitatory cell response) or hyperpolarization (inhibitory cell response)

Question 3

What are the 2 domains of ligand gated ion channels?

Answer 3

1. Extracellular domain that binds the first messenger

2. Transmembrane domain that is the pore in which the ions can flow through

Question 4

What are some examples of first messengers of ligand gated ion channels?

Answer 4

Serotonin, nicotinic acetylcholine, glutamate, glycine, and GABA

Question 5

What is the clinical relevance of ligand gated ion channels?

Answer 5

They are thought to be where anaesthetic agents act and are involved in BP regulation and cardiovascular diseases

Question 6

What is the mechanism of the nicotinic acetylcholine receptor?

Answer 6

1. Acetylcholine binding to the extracellular domain triggers a structural change in the transmembrane helices to widen the channel
2. The channel opens to allow Na+ ions flow through down the gradient and depolarize the post-synaptic membrane

Question 7

What is special about the nicotinic acetylcholine receptor?

Answer 7

It is a pentameric receptor

Question 8

What is the mechanism of receptor tyrosine kinases?

Answer 8

1. Ligands induce dimerization of the extra and intracellular domains that will then activate intracellular catalytic activity
2. Activated cytoplasmic domains phosphorylate itself on tyrosine (autophosphorylate) to form binding sites specific for a downstream protein
3. Sequential phosphorylation of other substrate proteins/downstream signaling proteins
4. Signal termination mostly by receptor internalization with ligand dissociation and degradation

Question 9

What are some examples of receptor tyrosine kinases?

Answer 9

Insulin receptor and growth factor receptors like epidermal GF or platelet-derived GF

Question 10

How big is the family of receptor tyrosine kinases?

Answer 10

Have numerous families with various extracellular domains that bind the messenger and many are associated with cell growth → dysfunction could result in cancer

Question 11

Example of the activation of Ras/MAP kinase signal cascade

Answer 11

Growth factor receptor binds the growth factor ligand and initiates the MAP kinase cascade: activated Ras binds and activates MAP3K → MAP2K → MAPK → trigger of the cascade by the phosphorylation of multiple protein substrates

Question 12

What is the importance of G-protein Ras activation?

Answer 12

There is a molecular switch to activate cell proliferation, migration, transformation, and survival

Question 13

What happens when there is an oncogenic K-Ras mutation?

Answer 13

1. Permanent activation of the Ras enzyme
2. Present in the majority of pancreatic ductal adenocarcinoma and other human cancers
3. The diagnosis and chemotherapeutic agents are lacking

Question 14

What is Ras?

Answer 14

Is a membrane bound, monomeric G protein that is critical in cell proliferation, differentiation, adhesion, migration, and apoptosis and is normally activated by growth factors that bind to RTKs or by T cell receptors

Question 15

What are the 3 Ras genes that are the most common oncogenes in humans and permanent activation can lead to pancreatic cancers?

Answer 15

HRas, KRas, NRas

Question 16

What is the switch for the activation and inactivation of Ras?

Answer 16

The conformation of the 2 loops differs between the GTP and GDP bound states so the binding of GTP switches from the off to the on state and in the activated GTP bound state, Ras can transiently bind to specific effectors and activate multiple different pathways. Ras can turn itself off by hydrolyzing GTP

Question 17

What are some examples of Ras effectors?

Answer 17

PLCε, RalGDS, Raf, and PI3K

Question 18

With receptor tyrosine kinases, what has to happen in order to activate the receptor?

Answer 18

The receptors have to dimerize! If they don’t dimerize, the receptor cannot be activated

Question 19

In what way do GPCRs have the same structure and in what ways do they structurally differ from one another?

Answer 19

All GPCRs have 7 transmembrane domains (7 places where it crosses the membrane) but the difference between them is where the ligand binds and loops

Question 20

What are GPCRs and how do they work?

Answer 20

They are a common target for pharmaceuticals and are many hormone and neurotransmitter receptors. They undergo a conformational change upon receiving a signal and they signal through heterotrimeric G proteins (so if there is no G protein, the GPCR won’t do anything)

Question 21

How are GPCRs different than RTKs?

Answer 21

GPCRs don’t have any enzymatic activity and do not have scaffolding

Question 22

What are some examples of ligands/first messengers for GPCRs?

Answer 22

Ca2+, odorants, pheromones, small molecules like amino acids, amines, nucleotides, peptides, prostaglandins, and proteins such as TSH, LH, FSH, interleukins, and chemokines

Question 23

What happens when the G protein is activated?

Answer 23

The alpha, beta, and gamma subunits break up

Question 24

What is the G protein cycle (trimeric G protein)?

Answer 24

• The release of GDP is slow
• The transient association with an agonist bound GPCR increases GDP release from G𝛼 → GEF activity
• Big amplification occurs in which one agonist bound GPCR can activate 10-100 G proteins
• GTPase activity converts the active/on form (GTP bound) to the inactive/off form (GDP bound) → can be facilitated by RGS proteins

Question 25

Why is GPCR referred to as the GEF component?

Answer 25

Overtime, GTP will by hydrolyzed to GDP but RGS proteins can accelerate this process

Question 26

Heterotrimeric G-proteins that couple to GPCRs have how many subunits?

Answer 26

3 subunits: G𝛼, G𝛽, and G𝛾 in which G𝛼 has GTPase activity

Question 27

What is the importance of heterotrimeric G proteins?

Answer 27

The association with an agonist bound GPCR induces the dissociation of GDP and GTP binding. The activated G𝛼 dissociates from the 𝛽𝛾 complex and can stimulate or repress a particular effector protein

Question 28

What is the big takeaway of ligand gated ion channels?

Answer 28

1. oligomeric with an extracellular domain (ligand binding) and transmembrane helices (channel)
2. Involved in endocrine signaling
3. Ligands are neurotransmitters → gating leads to depolarization of neuronal membranes

Question 29

What is the big takeaway of RTK pathways?

Answer 29

1. RTKs autophosphorylate intracellular domains for docking in initial steps of signal pathways
2. Function: growth, metabolism → important for anticancer targets
3. Example is the Ras/MAP kinase cascade

Question 30

Why are GPCRs referred to GEFs for hetero-trimeric G proteins?

Answer 30

1. Trimeric G proteins cycle through inactive GDP bound forms and active GTP bound forms
2. Different types of G proteins have different effectors (like adenylate cyclase or phospholipase C)

Question 31

What is the big takeaway about signaling pathways?

Answer 31

They are a network of protein-protein interactions and are highly regulated through multiple phosphorylations and GTPase activities. Interconnecting!!!