Lecture 25

Question 1

describe how a mutation in Hfe causes a signal transduction problem and leads to iron overloading in Hereditary Hemochromatosis

Answer 1

HFE is a membrane bound protein, found in the liver, that binds to one of the 2 transferrin receptors (Tfr1 or Tfr2)

Hepicidin signaling complexes are formed when there is sufficient iron in the cell

if Hfe is mutated, it cannot move from Tfr1 (low iron transferrin receptor) to bind to Tfr2(high iron transferrin receptor that turns on hepcidin expression when bound to Hfe) and iron uptake cannot be stopped via the expression of hepcidin

Question 2

How is protein kinase C activated?

Answer 2

Activated Gq subunit triggers phospholipase C to cut PIP2 into DAG (stays in the membrane) and IP3 (floats to the ER)

IP3 triggers the release of Ca2+ from the ER by binding to IP3 -gated Ca2+ receptors

Ca2+ and DAG bind to Protein kinase C which causes a conformational change that activates it

Question 3

What is the role of Grb2-SOS-Ras-Raf in a receptor tyrosine kinase pathway?

Answer 3

Grb2: protein that has an SH2 domain that binds with the dimerized RTK

SOS: it’s prolines are bound to the SH3 domain of Grb2. It is a GEF that adds GTP to Ras

Ras: binds to Raf after it receives a GTP from SOS. this initiates the MAP kinase cascade

Raf: is MAP kinase kinase kinase that is the start of the MAP kinase cascade

Question 4

Describe signal transduction by G-protein coupled receptors (GPCR’s)

Answer 4

GPCR’s are activated by ligand binding

Activated GPCR binds to a trimeric G protein and acts as a GEF (adds GTP where GDP was)

G alpha (one of 3 trimeric subunits) dissociates and goes to bind with the effector enzyme

G alpha hydrolyzes it’s GTP to GDP when activating the effector enzyme (returns to trimeric, inactivated state)

activated enzyme makes 2nd messenger, which elicits a biological response

Question 5

Describe receptor tyrosine kinases signal transduction

Answer 5

growth factor signal binds to a RTK monomers and causes a conformational change to a dimer

autophosphorylation then occurs, which allows the RTK dimer to be a “scaffold” for a complex of other proteins

RTK only binds to SH2 of Grb2, the rest complexes off of Grb2

SOS (part of the complex) is a GEF that adds a GTP to Ras

Ras binds to Raf and initiates the MAP kinase pathway

Question 6

describe JAK-STAT signal transduction

Answer 6

cytokines (ligand) binds to receptors (which dimerize) and then bind to JAKs

JAKs phosphorylate each other AND the receptor

The Receptor binds to and phosphorylates STATs

STATs the dissociate from the receptors (and JAKs on them), dimerize, and then translocate to the nucleus

the dimerized STATs then impact transcription, once they are in the nucleus

Question 7

describe Smad signal transduction

Answer 7

Serine-threonine receptors, activated by phosphorylation, dimerize

this complex then binds to R-Smad (receptor specific Smad) AND phosphorylates it

after phosphorylation, R-Smad then binds to Co-Smad (common Smad) and leaves the dimerized receptors

The R-Smad and Co-Smad complex then moves to the nucleus to impact the transcription of target genes

Question 8

What does Erythropoeitin employ to initiate signaling?

Answer 8

JAK-STAT signal transduction

Question 9

what oncogene plays a crucial role in cell division and is a frequent mutation in cancer? (first discovered human oncogene)

Answer 9

Ras

Question 10

Describe the series of MAPs involved in the MAP kinase cascade that occurs after Ras binds to Raf. (be sure to include what MAY occur at the end of the cascade.

Answer 10

(in order)

Raf: MAP kinase kinase kinase

Mek: MAP kinase kinase

Erk: MAP kinase

Nucleus, where gene transcription is increased (possibly causing cancer)

Question 11

what is signal transduction? briefly list the steps that occur during this beginning with the ligand and ending in altered gene expression

Answer 11

the process that a cell uses to get a message from the outside of a cell to the inside of a cell

ligand to receptor to intracellular signal proteins to effector proteins to altered gene expression

Question 12

Describe how adenylyl cyclase, cAMP, and PKA are all related

Answer 12

Adenylyl cyclase generates cAMP from ATP

cAMP interacts with the 2”regulatory subunits” on the inactive form of PKA in order to dissociate them from the 2 catalytic subunits

The inactive form of PKA becomes active once the 2 regulatory PKA units dissociate from the 2 catalytic PKA subunits

Question 13

When a GPCR attaches ONLY to G alpha q OR G alpha o, what is the effector enzyme?

Answer 13

phospholipase C

NOT adenylyl cyclase

Question 14

what does phospholipase do once it is activated?

Answer 14

it cleaves the membrane protein PIP2 into IP3 and DAG

IP3 is diffusable

DAG is membrane bound

Question 15

Ca2+ binds to PKC, along with DAG, to activate it. What is the other role that Ca2+ has?

Answer 15

it can serve as a 2nd messenger when it binds to calmodulin

Question 16

What is the main signal that RTK’s respond to?

Answer 16

RTK conduct signal transduction in response to growth factor signals

Question 17

RTK’s signal through cytoplasmic domains. what do these domains contain?

Answer 17

tyrosine kinase

Question 18

When activating proteins via protein phsophorylation, RTK’s phosphorylate what specific portion of the protein?

Answer 18

tyrosine amino acid residues

Question 19

out of RTK, JAK-STAT, and Smad signal transduction, which of them are “more direct routes” of signaling?

Answer 19

JAK-STAT and Smad

Question 20

Name an example of serine threonine Smad signal transduction in the human body.

Answer 20

the hfe protein and it’s control over the expression of hepcidin (which then controls ferrotportin expression, NOT hfe)

this pathway regulates iron metabolism

Question 21

hereditary chromatoisis is an ___ ____ disease that is common in _____. Hereditary chormatosis results in genetic ______ ____.

Answer 21

autosomal recessive

men

iron overloading

Question 22

what is the function of hepcidin?

Answer 22

ferroportin is the receptor for hepcidin

hepcidin binds to ferroportin, which causes internalization and destruction of ferroportin via proteolysis

Question 23

What happens when iron replete is occurring? what about when iron is depleted?

Answer 23

replete: hepcidin expression is up and ferroportin levels are down

Depleted: hepcidin expression is down and ferroporting levels are up

Question 24

What happens when iron replete is occurring? what about when iron is depleted?

Answer 24

replete: hepcidin expression is up and ferroportin levels are down

Depleted: hepcidin expression is down and ferroportin levels are up

Question 25

Hfe works through the _____ pathway to induce hepacin expression

Answer 25

Smad

Question 26

Receptors of all kinds are _____

Answer 26

specific

Question 27

List the 5 kinds of G proteins

Answer 27

G alpha s

G alpha i

G alpha o

G alpha q

G alpha olf

Question 28

name 2 “targets” of GPCR’s

Answer 28

adenyly cyclase

phospholipase C

Question 29

List 4 second messenger molecules

Answer 29

cAMP

DAG

IP3

Ca2+

Question 30

List 3 “downstream effectors”

Answer 30

PKA

PKC

gated-ion channels

Question 31

DMT1 and Ferroportin are active in what part of the body? explain where hepcidin is made and where it conducts it’s activity

Answer 31

the intestinal enterocytes

hepcidin is made in the liver but has action in the intestine (on ferroportin levels)

Question 32

What is the mutation in the Hfe gene called?

Answer 32

C282Y

Question 33

If there is a lot of iron around a cell, do you want more or less hepcidin? why?

Answer 33

you want more, so that you have less ferroportin (which brings iron in, and thats what you dont want to happen)