Lecture 8 - G Protein Signaling Flashcards Preview

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Flashcards in Lecture 8 - G Protein Signaling Deck (72)
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1

What are 2 main intracellular signaling pathways? How can both be removed?

1. Phosphorylation
2. GTP-binding protein

Hydrolysis can remove P in both

2

What is the primary messenger of a GPCRs?

The ligand: hormone, NT, etc.

3

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

4

How are G proteins classified?

By the identity of the alpha subunit

5

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

9 genes encoding 12 proteins

6

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

1. Guanine nucleotide binding site
2. GTPase activity

7

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

Very high affinity

8

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

9

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

10

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

11

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

Specific to different tissues: olfactory system and gustatory system

12

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

13

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

14

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

15

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

16

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

17

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

When bound by G alpha s bound to GTP

18

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

Cyclic nucleotide phosphodiesterase: cAMP => 5'-AMP

19

How is cAMP formed?

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

20

What is the full name of cAMP?

Adenosine 3'-5'-cyclic monophosphate

21

Why is cAMP a good second messenger?

Very easily degradable

22

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

23

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

Nope

24

Usual # of transmembrane domains in GPCRs?

7

25

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

26

Describe the extracellular domains of transmembrane proteins.

Glycosylated and have disulfide bonds

27

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

NOPE because of reducing environment

28

Where is the ligand binding site of transmembrane receptors?

Within the membrane

29

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

2nd and 3rd cytosolic loops

30

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

C-terminal domain