Notesale: Turn your study into money

Document Preview

Extracts from the notes are below, to see the PDF you'll receive please use the links above

---

Blueprint Series

Complete Solution
Chapter 15

15
SIGNAL TRANSDUCTION
AND G PROTEIN-COUPLED
RECEPTORS

REVIEW THE CONCEPTS
1
...
Receptor proteins are present in
target cells
...
Intracellular signal transduction pathways are evolutionarily
highly conserved
...
Endocrine, paracrine, and autocrine signaling differ according to the distance over
which the signaling molecule acts
...
In animals, the signaling molecule is carried to target cells by the blood or other extracellular fluids
...
In autocrine signaling, the cell that releases the signaling molecule is also the
target cell
...

3
...

Because the Kd for receptor 2 (10−9 M) is lower than that for receptor 1 (10−7 M),
the ligand shows greater affinity for receptor 2 than for receptor 1
...
For receptor 1, the Kd is 10−7 M and the concentration of free ligand [L]
is 10−8 M
...
091, that is, only 9% of the receptors

61

62

CHAPTER 15: Signal Transduction and G Protein-Coupled Receptors

have bound ligand at a free ligand concentration of 10−8 M
...
91; 91% of the receptors have bound ligand
...
To purify a receptor by affinity chromatography, a ligand for the receptor is
chemically linked to beads used to form a column
...

The receptor will bind to the ligand attached to the beads and other proteins will
wash out
...
Analyzing its molecular weight by SDS-PAGE may be sufficient to identify the recovered receptor or provide protein for sequencing
...

To isolate active Gas with a pull-down assay, the adenylyl cyclase (AC) effector
enzyme to which only the active form of Gas binds is linked to beads either
directly by chemical linkage or indirectly through an anti-AC antibody bound to
protein A beads
...
The amount
of Gas pulled down can be quantified under different signaling conditions
...
Binding of the extracellular signal or, in the case of rhodopsin, absorption of a
photon by retinal, causes a conformational change in the orientation of the transmembrane helices that changes the conformation of the C3 loop and C4 segment
(and in some cases the C2 loop) on the cytosolic side of the receptor
...

6
...
Upon ligand binding to its receptor, the receptor undergoes a conformational change that affects the associated trimeric G protein
...
The
released Gas-GTP or Gbg complexes then activate downstream effector proteins
...
A mutant Ga subunit
with increased GTPase activity would be expected to hydrolyze GTP to GDP
at a faster rate and thus reduce the time that the Ga subunit remains in the active
state
...

7
...

In this case, association of Gas with adenylyl cyclase would yield an increase in
energy transfer and fluorescence at 527 nm when excited with 440 nm light
...
Steps at which a single active component activates multiple targets amplify the
signal
...
In contrast, each active Gas protein activates only one AC, and each
cAMP participates in the activation of only one PKA, neither of which directly

CHAPTER 15: Signal Transduction and G Protein-Coupled Receptors

amplifies the signal response
...

9
...
In the cytosol it
catalyzes a chemical modification of Ga proteins that prevents hydrolysis of
bound GTP to GDP
...
This causes
continuous activation of adenylyl cyclase even in the absence of hormonal stimulation
...

10
...
These receptors are coupled to an inhibitory
G protein
...
The muscle cell becomes
hyperpolarized, which reduces the frequency of muscle contraction
...
Rhodopsin contains a
light absorbing pigment, 11-cis-retinal, that is covalently linked to opsin
...
This activated
opsin then interacts and activates transducin, an associated G protein
...

The released catalytic subunits of the phosphodiesterase hydrolyzes cGMP to
5’-GMP
...
As with the cardiac muscle system, signal activation
ultimately results in hyperpolarization of the photoreceptor cells
...
Epinephrine binding to the b-adrenergic receptor causes an activation of adenylyl cyclase through the activation of Gas, a stimulatory G protein
...
An agonist acts like
the normal hormone, which in this case would be epinephrine
...

In contrast, an antagonist binds to the receptor but does not activate the receptor
...
In fact, it would reduce a normal epinephrine stimulated
response because it would prevent epinephrine from binding to the receptor
...
The epinephrine-cAMP signaling pathway—from binding of epinephrine to the
receptor to activation of PKA—is essentially the same in all the cells
...

13
...

The increase in cAMP levels as a result of ligand binding to the receptor leads
to an activation of protein kinase A
...


63

64

CHAPTER 15: Signal Transduction and G Protein-Coupled Receptors

Phosphorylated receptor can bind ligand but is reduced in its ability to
activateadenylyl cyclase
...
Phosphorylated receptors are resensitized by the
removal of phosphates byphosphatases
...

14
...
AKAP15 protein localizes PKA next to Ca2+
channels in heart muscle cells, which reduces the time that otherwise
would be required for diffusion of PKA catalytic subunits from their
sites of activation to the Ca2+- channel substrates
...
Localization of PKA at the nuclear
membrane facilitates rapid entry of some of the activated PKA catalytic
subunits into the nucleus, where they phosphorylate transcription
factors
...
Cleavage of PIP2 by phospholipase C generates IP3 and DAG
...
When ER stores of Ca2+ are
depleted, the IP3-gated Ca2+ channels bind to and open store-operated
Ca2+ channels in the plasma membrane,allowing an influx of Ca2+
...
The principal
function of DAG is to activate protein kinase C, which then
phosphorylates specific target proteins
...
Prior to stimulation of the IP3-DAG signaling pathway, Ca2+-ATPases
establisha resting level of Ca2+ in the cytosol of <10-7 M, at which level
few calmodulin sites have Ca2+ bound and the calmodulin is inactive
...
The Ca2+ binding affinity of
calmodu-lin is exquisitely tuned to bind and release Ca2+ in response to
the physiological changes of Ca2+ concentration in the cytoplasm used
for signaling
...
In rod cells, cGMP opens cation channels, whereas the primary activity
of cGMPin smooth muscle cells, like cAMP, is to activate a kinase

---