Flashcards in Lecture 8 - G Protein Signaling Deck (72)
What are 2 main intracellular signaling pathways? How can both be removed?
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?
2. K+ channels
4. cGMP phosphodiesterase
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