Cell Signaling GPCRs

Question 1

Signal Transduction steps

Answer 1

1. Extracellular signal binds to receptor (on cell surface) - Reception
2. Cascade of intracellular reactions that triggers the cell’s response tot he original signal - Transduction
3. Intracellular Signaling proteins alter the activity of an effector protein(s) - Response

Question 2

Signal Transduction of Insulin (signaling cascade)

Answer 2

Activation of effector -> binding of glucose transporters to surface

Question 3

Second Messengers

Answer 3

Chemicals (Ions, water soluble, lipid soluble, etc.) that are generated in large amounts (in short time) in response to receptor activation and diffuse away from their source, spreading signal to other parts of the cell.

They are NOT intracellular proteins

Question 4

Examples of second messengers

Answer 4

Water soluble
IP3 (Inositol 1,4,5 phosphate), cAMP (cyclic AMP), and Ca2+

Lipid soluble
DAG (Diacylglycerol)

Question 5

Intercellular signaling

Answer 5

between cells
1. Contact dependent
2. Paracrine
3. Synaptic
4. Endocrine

Question 6

Contact dependent signaling

Answer 6

ex. Immune cells - ingulf and digest foreign antigen - requires close contact

Fast response

Question 7

Paracrine Signaling

Answer 7

More localized

ex. secretion of growth factors - bind to specific receptor on neighboring cells - fast, short lived response

Autocrine cells - cancer - secrete growth factors that act on themselves

Question 8

Synaptic Signaling

Answer 8

AP, propagate

NT release in synapse, because small area, fast response

Question 9

Endocrine Signaling

Answer 9

Travel long distance via blood circulation, bind to target with receptor

slower - traveling through blood

Question 10

Types of receptors extracellular signals can bind to

Answer 10

Cell surface receptors
Intracellular receptors

Question 11

Cell surface receptors

Answer 11

Bind to hydrophilic signals (Growth factors, peptide hormones, neurotransmitters)

Question 12

Intracellular receptors

Answer 12

Hydrophobic signals (Steroids) diffuse across plasma membrane and bind to receptor proteins inside the target cell

Question 13

How does a cell integrate all of the signaling information in order to make a decision

Answer 13

An individual cell - controls receptors at the surface and intracellular pathways - regulates signals and response - specificity

Is many/combo of signals - not just one signal - integrates them

Question 14

How does a signaling molecule have different effects on different target cells

Answer 14

Small number of types of signals (A) and receptors - Different response (B/C) - different intracellular pathway activated (D-also same signal can bind to different receptors)

Binding of ligand to R promotes signaling of predetermined state

Question 15

GPCRs are the largest family of ____

_____ % of known drugs work through them.

They are ____ proteins with ___ helices

Answer 15

cell surface receptors (>800 in humans)

Almost 50%

multipass TM, GTPases
alfa helices - more flexible conformation changes/sliding - receptor activation, Has a binding pocket

Question 16

G Protein subunits

Answer 16

have 3 subunits (Alfa, betta, gamma)
Are bound/associated with membrane

Question 17

Inactive G Protein

Answer 17

alfa-GDP - inactive
alfa subunit is a GTPase which becomes inactive when it hydrolyzes GTP to GDP

Question 18

Signaling steps through GPCRs

Answer 18

1. Extracellular signal binds to GPCR
2. Receptor undergoes conformational change, allowing it to bind to the G protein
3. Receptor acts like a GEF, inducing the alfa subunit to release GDP, allowing GTP to take its place
4. Conformational change in alfa subunit disassociates from receptor, resulting in activated alfa subunit and activated betta gamma subunit - both lead to other changes/response

Question 19

Alfa subunit is inactivated with it

Answer 19

hydrolyzes GTP to GDP

This is greatly enhanced by binding of specific regulators of G protein signaling (RGS) to alfa subunit

Turn off signal

Question 20

RGS

Answer 20

similar to GAP
enhances ability of alfa subunit to hydrolyze its own GTP

Question 21

cAMP

Timing -

Synthesized from -
Destroyed by -

Answer 21

An extracellular signal can increases cAMP concentrations more than 20 fold in seconds (a lot in a short period)

Is synthesized from ATP by adenylyl cyclase (adenylate cyclase) and destroyed by cAMP phosphodiesterase

Both adenylyl cyclase and phosphodiesterase balance/regulate each other

Question 22

How do Extracellular signals influence concentrations of cAMP

Answer 22

Activates GPCR that are coupled to stimulatory G protein (Gs) - binds to and activates adenylyl cyclase - increases cAMP

different GPCR (other signals) reduces cAMP - activating a inhibitory G protein (Gi)

Question 23

How does cholera toxin affect cAMP production?

Answer 23

Cholera toxin is an enzyme that catalyzes the transfer of ADP ribose from intracellular NAD+ to the alfa subunit of Gs (Gsα is covalently modified (ADP ribosylation)), it can no longer hydrolyze its bound GTP, causing it to remain in an active state, stimulating AC indefinitely. Prolonged elevation in cAMP causes an efflux of Cl- and water into the gut, causing severe diarrhea.

Question 24

Most animal cells, cAMP exerts its effects mainly by

Answer 24

activating PKA

Question 25

PKA activation

Answer 25

inactive state, PKA consists of a complex of two catalytic subunits and two regulatory subunits. binding of cAMP to the regulatory subunits alters their conformation, causing them to dissociate from the complex. released catalytic subunits are thereby activated to phosphorylate specific target proteins

Question 26

G Proteins Signaling by Phospholipids

Answer 26

G protein activates the plasma-membrane-bound enzyme phospholipase C-β (PLCβ) (by activated alfa betta and gamma subunits) which acts on/cleaves [PI(4,5)P2] into inositol 1,4,5-trisphosphate (IP3) and diacylglycerol. IP3 - water soluble - leaves PM and diffuses through cytosol to ER, binds to IP3 R - conf. change - exposes a high Ca2+ affinity binding site- cytosolic Ca2+ binds - these two (IP3 and Ca2+) causes the opening of the Ca2+ channel. Ca2+ stored in ER is released and binds to other IP3 receptors, causing more to open. Concentration of cytosolic Ca2+ rises. this alters the PKC so that it translocates from the cytosol to the cytoplasmic face of the plasma membrane. Diacylglycerol (remains embedded in PM) and Ca2+ and and neg charged membrane phospholipid phosphatidylserine (attracts PKC) - activates PKC PKC then phosphorylates target proteins that vary depending on the cell type.

Question 27

What is the term for the protein that is an α-subunit-specific GTPase-activating protein (GAP)?

Answer 27

Regulator of G protein signaling (RGS)

Question 28

PKA is turned off when

Answer 28

cAMP is converted to AMP by cAMP phosphodiesterase.