Lecture 8 - G Protein Signaling Flashcards Preview

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Flashcards in Lecture 8 - G Protein Signaling Deck (72)
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What are 2 main intracellular signaling pathways? How can both be removed?

1. Phosphorylation
2. GTP-binding protein

Hydrolysis can remove P in both


What is the primary messenger of a GPCRs?

The ligand: hormone, NT, etc.


What are 3 characteristics of G proteins?

1. Need both the ligand and GTP for the trigger the production of a second messenger
2. Receptor agonists stimulate a low Km GTPase
3. GDP and GTP modulate the affinity of a receptor to agonists, but not to antagonists


How are G proteins classified?

By the identity of the alpha subunit


How many G protein alpha subunit genes do we have? how many proteins do they encore for?

9 genes encoding 12 proteins


What are the 2 parts of the G protein alpha subunit?

1. Guanine nucleotide binding site
2. GTPase activity


Describe the binding of GTP to the G protein alpha subunit?

Very high affinity


What do the kinetics of the GTPase activity of the G protein alpha subunit depend on? Explain how this works.

Presence or absence of the activated receptor: without the activated receptor kcat is 10 times faster than the dissociation rate of GDP = the rate of the GTPase is limited by the dissociation rate of GDP


In the absence of an activated receptor, what form is the G protein in? What is the purpose of this?

Alpha subunit bound to GDP

Purpose = default "OFF" state


Would GDP be able to dissociate from the alpha subunit of the G protein in the absence of an activated receptor?

Yes, but low probability event


What are the different types of G proteins that activate adenylyl cyclase?

Specific to different tissues: olfactory system and gustatory system


What are the 6 proteins that can be directly activated by G proteins?

1. AC
2. K+ channels
3. GC
4. cGMP phosphodiesterase
5. PLC-beta
6. Rho family of GTPases


4 steps of G alpha s cycle of activation/deactivation?

1. Gs is bound to GDP = OFF
2. Gs comes in contact with an activated receptor which causes displacement of bound GDP by GTP
3. Alpha subunit bound to GTP dissociates from beta and gamma subunits and activates AC. This also USUALLY causes the ligand to dissociate from the receptor
4. Galpha s hydrolyzes GTP thereby turning itself off and reassociates with the beta-gamma dimer


How does the beta-gamma dimer affect the alpha subunit of the G protein?

WHEN IN THE HETEROTRIMER form, the dimer stabilizes the OFF state by increasing the affinity of the alpha subunit for GDP


How does the dissociation of the alpha subunit from the G protein heterotrimeric form affect the receptor? What is this called?

It inhibits its binding to the ligand by decreasing its affinity for it = heterotropic allosteric inhibition of the receptor


What is the built in OFF mechanism of the G alpha GPCRs?

They have low affinity for the ligand when the G alpha subunit dissociates


When is the enzyme targeted by the alpha subunit of the G protein active?

When bound by G alpha s bound to GTP


How is cAMP degraded after being activated by AC? Equation.

Cyclic nucleotide phosphodiesterase: cAMP => 5'-AMP


How is cAMP formed?

Adenylyl cyclase: ATP => cAMP + PPi (3' OH attacks alpha P)


What is the full name of cAMP?

Adenosine 3'-5'-cyclic monophosphate


Why is cAMP a good second messenger?

Very easily degradable


Will a GPCR agonist affect the affinity of the receptor to the ligand?

Yes! Because through activation of G alpha s, the affinity of the receptor to the ligand will decrease


Will a G alpha GCPR antagonist affect the affinity of the receptor to the ligand?



Usual # of transmembrane domains in GPCRs?



How does desensitization of GPCRs work? 2 ways (what do we call the first one?)

1. Removing the receptor off the cell surface = homologous desensitization
2. Phosphorylation of the receptor inhibiting its ability to interact with a G protein


Describe the extracellular domains of transmembrane proteins.

Glycosylated and have disulfide bonds


Are disulfide bonds and carb groups ever found in the cytosolic domains of transmembrane proteins? Why?

NOPE because of reducing environment


Where is the ligand binding site of transmembrane receptors?

Within the membrane


What parts of the GPCRs interact with the G protein alpha subunit?

2nd and 3rd cytosolic loops


What parts of the GPCRs interact with the G protein beta-gamma subunit?

C-terminal domain