Chapter 16: Cell Signaling Part 2

Question 1

G-protein

Answer 1

• membrane bound GTP-binding protein involved in intracellular signaling
• composed of three subunits
• intermediary usually activated by the binding of a hormone or other ligand to a transmembrane receptor

Question 2

GPCR

Answer 2

G-Protein Coupled Receptors

Question 3

G-Protein Coupled Receptors

Answer 3

• largest super family of cell surface receptors
• mediated most responses from external world(senses of sight, smell, taste)
• can be activated by a single ligand
  (adrenaline, acetylcholine, serotonin)

Question 4

Describe an activated GCPR

Answer 4

• an acitvated GPCR activates G proteins by encouraging the alpha subunit to expel its GDP and pick up GTP
• signal molecule binds to active receptor, changes conformation of receptor
• alteration of alpha subunit of G protein allows it to exchange its GDP for GTP, creates additional change to activate alpha and beta-y complex

Question 5

What does the alpha and beta-y complex provide activated GPCR/G proteins once activated?

Answer 5

• two activated parts can then interact directly with target proteins in plasma membrane, may relay signal to other destinations as well
• the longer these target proteins remain bound to an activated alpha subunit, or a beta-y complex, more prolonged the relayed signal will be

Question 6

Describe the binding of GPCRs to trimeric G-proteins

Answer 6

• trimeric G proteins have 3 subunits
• when the GPCR is activated by a signlaing molecule, it activates the alpha subunit of the G protein
• causes it to release GDP and bind to GTP
• now the alpha subunit dissociates from the other subunits and both are activated
• the signal is shut off when the alpha subunit hydrolyzes the GTP to GDP
• then the alpha subunit binds to the other subunits again and the signals are shut down

Question 7

What can some membrane-bound enzymes, activated by G proteins, do?

Answer 7

• produce small messenger molecules
• once activated enzymes produce small molecules(second messengers) rapidly, molecules rapidly diffuse away from the source, amplifies and spreads to intracellular signal
• messenger molecules bind to specific signaling proteins in cell and influence activity

Question 8

Two most frequent small messenger molecules produced by membrane-bound enzymes

Answer 8

• adenylyl cyclase
• phospholipase C

Question 9

Adenylyl cyclase

Answer 9

• produces small molecule cyclic cyclase
• enzymes that catalyzes formation of cyclic AMP from ATP, important in intracellular signaling pathways

Question 10

phospholipase C

Answer 10

• produces small molecule inositol triphosphate and diacylglycerol
• enzyme associated with the plasma membrane that generates two smaller messenger molecules in response to activation

Question 11

Describe how enzyme adenylyl cyclase is activated by GPCR

Answer 11

• adenylyl cyclase makes cyclic AMP
• many activated GPCR affect adenylyl cylcase, alters intracellular concentration of 2nd messenger cyclic AMP
• adenylyl cyclase is activated by the alpha subunit of the trimeric G protein
• creates dramatic increase in synthesis of cyclic AMP to ATP
• to terminate the signal, 2nd enzyme called cyclic AMP phosphodiesterase rapidly converts cyclic AMP to ordinary AMP

Question 12

cyclic AMP

Answer 12

• small intracellular signaling molecule generated from ATP in response to hormonal stimulation of cell surface receptors
• synthesized by adenylyl cyclase and degraded by cyclic AMP phosphodiesterase
• formed from ATP by cyclization reaction that removes two phosphate groups from ATP and joins “free” end of remaining phosphate group to the sugar part of the AMP molecule
• degradation reaction breaks this new bond, forming AMP

Question 13

Describe one type of cAMP pathway, Epinephrine

Answer 13

• epinephrine stimulates glycogen breakdown in skeletal muscle cells
• hormone activates a GPCR, which turns on G protein that activates adenylyl cyclase to boost production of cyclic AMP
• adrenalin released, binds to GPCR, activates adneylyl cyclase, makes cAMP
• then cAMP activates protein kinase A(PKA): it phosphorylates another kinase, which phosphorylates another enzyme involved in breakdown from glycogen to glucose

Question 14

PKA

Answer 14

• protein kinase A
• activated by rise in intracellular cyclic AMP

Question 15

Describe how GPCR can activated cAMP pathways that regulate transcription

Answer 15

• rise intracellular cyclic AMP can activate gene transcription
• PKA, activated by intracellular cyclic AMP, can enter nucleus and phosphorylates specific transcription regulators
• phosphorylated transcription regulator proteins stimulate transcription of whole set of target genes
• examples: hormone synthesis in endocrine cells and production of proteins involved in long term memory in brain

Question 16

Describe activation of Phospholipase C

Answer 16

• GPCRs coupled to PLC which leads to increased intracellualr mediator Ca++ (inositol pathway triggers an increase)
• Ca++ activates protein kinase C (PKC)
• IP diffuses through cytosol, triggers release of Ca2+ from ER by binding and opening Ca2+ channels in ER membrane
• large electrochemical gradient for Ca2+ across membrane causes Ca2+ to rush out of ER, intro cytosol
• diacylglycerol remains in plasma membrane, uses Ca2+ alongside it to activate enzyme protein kinase(PKC)
• PKC recruited from cytosol to cytosolic face of plasma membrane, phosphorylates own set of intracellular proteins, propagating signal

Question 17

Explain how GPCRs are involved in vision

Answer 17

• GPCR based visual signaling pathways analyzed through rod photoreceptor cells in eye, responsible for noncolor vision in dim light
• Rhodopsin is GPCR

Question 18

Rhodopsin

Answer 18

• G protein, activates G protein transducin when stimulated by light
• connection to rod photoreceptors, cell from retina is quite sensitive to light

Question 19

Describe Rhodopsin process

Answer 19

• rod stimualted by light, signal is relayed from the rhodopsin molecules in the discs, through the cytosol, to cation channel in plasma membrane of outer segment
• cation channels close in response to cytosolic signal
• produces change in membrane potential
• change in memrbane potential alters rate of neurotransmitter release from synaptic region of the cell
• released neurotransmitters act on retinal nerve cells that pass signal to brain

Question 20

Adaptation

Answer 20

• adjustment of sensitivity following repeated stimulation
• allows a cell or organism to register small changes in a signal despite a high background level of stimulation
• depends on negative feedback

Question 21

negative feedback

Answer 21

• an intense response in the photoreceptor cell decreases cytosolic Ca2+ concentration, inhibiting enzymes, responsible for signal amplification

Question 22

Describe the process of light adaptation

Answer 22

• in absence of light signal, 2nd messenger molecule cyclic GMP continued to produce by guanylyl cyclase in cytosol of photoreceptor cell
• cyclic GMP binds to cation channels of plasma membrane to keep them open
• rhodopsin activated, activates alpha subunit of transducin
• transducin turns on enzyme cyclic GMP phosphodiesterase

Question 23

Describe cyclic GMP to GMP

Answer 23

• GMP phosphodiesterase breaks down cyclic GMP to GMP
• decrease in concentration of cylic GMP reduces cyclic GMP bounds to cation channels, they close
• closed channels decrease NA+ influx into cytosol, slow neurotransmitter release and changing membrane potential/voltage channels

Question 24

Describe shutting down of GPCRs

Answer 24

• if active for a long time, GRK phosphorylates the receptor on the cytoplsamic side
• then arrestin binds, this recruits clathrin and the receptor is internalized by endocytosis
• the receptor will be degraded in lysosomes

Question 25

What does cross-talk mean between signaling pathways

Answer 25

• certain pathways described in this chapter can overlap each other and work across and in between each other in order to happen, can cycle around through all the steps

Question 26

Nanoparticles

Answer 26

• various types, such as: lipsome, albumin-based, micelle, polymer-based, and Gold
• examples above being used to deliver drugs in clinical trials
• have drugs and antibodies on outside; antibodies take out the “bad,” drugs treat cell

Question 27

What do radio waves do to iron oxide nanoparticles

Answer 27

• heat them up!
• example using insulin
• antibodies bring a nanoparticle to the body
• TRPVI is gate type structure in membrane that responds to heat by opening
• amino acid his tag binds particle to TRPVI
• opened channel brings more Ca2+ into cell, activates phosphatase, creates NFAT
• NFAT is a trasncription factor that will activate transcription of insulin, has P blocking/masking NLS

Question 28

Can the nanoparticles bind to cells expressing the TRPV1 channel with the his tag?

Answer 28

• Yes!
• nano particles binds to TRP channels on the surface of cells

Question 29

Can radio frequency cause an increase in Ca2+ in the cytoplasm?

Answer 29

Yes!

Question 30

Can the radio frequency cause NFAT to move in to the nucleus? Does the release of insulin depend on calcineurin?

Answer 30

• experiments in cell culture prove their idea will work in animal model
• NFAT moves into the nucleus when cells are exposed to radio frequency
• insulin release needs calcineurin activation