Endocrine 2 - Cell Signaling

Question 1

What are the two types of cell receptors?

Answer 1

1. Surface receptors

2. Intracellular receptors

Question 2

What three things determine cellular response to ligand binding?

Answer 2

1. Receptor type
2. 2nd messenger signals
3. Cellular state

Question 3

Is downregulation important (turning off receptor signaling)?

Answer 3

Yeah

Question 4

How do intracellular receptors work?

Answer 4

1. Inactive intracellular receptors are bound to inhibitory proteins.
2. Hormone binds to receptor binding side (separate from where the inhibitory protein is).
3. Conformational change –> inhibitory protein leaves, exposes the DNA binding site.
4. Receptor binds DNA, regulates transcription.

Question 5

What are the three types of surface receptors?

Answer 5

1. Ion-channel linked
2. G-protein linked
3. Enzyme-linked

Question 6

How do heterotrimeric G-protein receptors work?

Answer 6

1. Receptor (separate from G-protein) binds ligand –> conformational change of the receptor opens a binding site for G-protein.
2. G-protein (beta+gamma+alpha subunits) is bound to GDP (inactive) and binds to the new site on the receptor.
3. G-protein conformational change –> GTP displaces GDP.
4. alpha G-protein subunit + GTP dissociates from everything and goes to bind adenylyl cyclase.
5. The alpha-GP-adenylyl cyclase thing converts ATP–>cAMP + PPi
6. GTP hydrolysis –> alpha re-associates with beta gamma and it starts over.

Question 7

How does cholera toxin work?

Answer 7

It crosses the plasma membrane in enterocytes and its enzymatic activity binds an ADP ribose to G-alpha S, blocking its GTPase activity –> always on –> Cl- channel is always open –> Cl- flows into GI lumen, water follows –> diarrhea.

Question 8

What enzyme makes cAMP? What enzyme degrades it?

Answer 8

adenylyl cyclase makes it from ATP.

cAMP phosphodiesterase turns cAMP into 5’-AMP

Question 9

How does cAMP work?

Answer 9

It binds to the regulatory subunits on protein kinase A (two regulatory subunits each with two cAMP sites per protein kinase A) –> regulatory subunit dissociation from the catalytic subunits –> catalytic subunits are active.

Question 10

How do protein kinases that go into the nucleus work?

Answer 10

They phosphorylate a cyclic AMP response element binding factor (CREB), a transcription factor –> binds to promoters on DNA

Question 11

What are the three mechanisms of signal inactivation?

Answer 11

1. cAMP phosphodiesterase cleavage of cAMP –> 5’-AMP
2. Protein kinase A can come back and phosphorylate the receptor –> desensitization.
3. Protein kinase A phosphorylates another kinase –> other kinase multi-phosphorylates the receptor –> receptor binds to B-arrestin –> signal shut down

Question 12

Is calcium concentration much higher outside the cell than inside the cell?

Answer 12

Yeah

Question 13

Which second messenger molecule is responsible for opening intracellular calcium stores?

Answer 13

IP3

Question 14

How does IP3 work?

Answer 14

Phosphatidyl inositol bisphosphate (a plasma membrane lipid) is cleaved by phospholipase C-beta –> DAG (still in plasma membrane) + IP3 (from the head group).

DAG activates protein kinase C
IP3 binds to receptors in the ER and mitochondria –> opens channels –> Ca2+ release from intracellular stores

Question 15

Ca2+ can bind _______, which regulates the activity of ~50 enzymes.

Answer 15

calmodulin

Question 16

What does calmodulin do in T cells? What is the clinical significance of this?

Answer 16

Once activated by Ca2+, calmodulin binds to calcineurin (a phosphatase), which goes and dephosphorylates a transcription factor called NfatC –> activation of T cells.

Clinical significance: immunosuppresive therapies deactivate calmodulin –> no T cell activation

Question 17

RAS is a type of monomeric ___ protein that act as molecular switches.

Answer 17

monomeric G protein

Question 18

When is RAS active? What does it activate?

Answer 18

When its bound to GTP it then binds MAP kinases –> transcription regulation

Question 19

How do integral tyrosine kinase receptors work?

Answer 19

1. Ligand binds
2. Dimerization activates
3. Autophosphorylation of the receptor
4. Downstream signaling molecules bind to the receptor dimer
5. Molecules get phosphorylated
6. Signal propagates

Question 20

The insulin receptor is a ________ ________ receptor that activates which second messenger molecule?

Answer 20

tyrosine kinase receptor that activates PI-3 kinase

Question 21

How do JAK/STAT receptors work?

Answer 21

1. Ligand binds, activates JAK tyrosine kinase
2. JAK phosphorylates STAT on a tyrosine residue (signal tranducers and activators of transcription)
3. STATs dimerize causing exposure of a nuclear translocation signal
4. STAT dimer goes to the nucleus, binds DNA, regulates transcription

Question 22

What did Eero Mantyranta have?

Answer 22

Autosomal dominant erythrocytosis. Messed up C-terminal region of EPO receptor –> receptor could not recruit a tyrosine phosphatase to turn off –> increased RBC synthesis.

Question 23

cAMP levels can be increased in cells by all of the following except:

a) stimulation of adenylyl cyclase activity
b) binding of epinephrine to its plasma membrane receptor
c) inhibition of phosphodiesterase activity
d) binding of insulin to its plasma membrane receptor
e) activation of G proteins

Answer 23

d) binding of insulin to its plasma membrane receptor

- its a tyrosine kinase and activates a PI-3 kinase, not cAMP (G-protein receptors do that)

Question 24

IP3:

a) is a membrane-bound activator of protein kinase C
b) activates adenylyl cyclase thereby increasing cAMP levels
c) is a water soluble inducer of Ca2+ release from the ER
d) is an intracellular Ca2+ binding protein
e) directly activates protein kinase G

Answer 24

c) is a water soluble inducer of Ca2+ release from the ER

Question 25

You have discovered a “new” hormone that is a protein with a molecular size of 180 AAs. Which statement is most likely to be correct about this hypothetical hormone?

a) It will probably have its action by entering the cell and binding a nuclear receptor
b) It will probably be degraded by sulfanation in the liver followed by excretion in the urine
c) It can be effectively administered orally
d) It will be stored in the cell in membrane-bound secretory vesicles

Answer 25

d) it will be stored in the cell in membrane bound secretory vesicles