Exam 2 Lecture 14

Question 1

What is the largest family of membrane receptors in the human genome?

Answer 1

G-protein coupled receptors (GPCRs)

Question 2

In what ways do GPCRs have a high degree of structural similarity?

Answer 2

They have 7 𝛼-helical transmembrane regions that span the membrane, they have an extracellular N terminus and an intracellular C terminus, and they undergo a conformational change upon receiving a signal

Question 3

GPCRs signal through what?

Answer 3

Heterotrimeric G-proteins

Question 4

Why do GPCRs have no enzyme activity and no scaffolding like RTKs?

Answer 4

The ligand binds on the outside and changes the receptor conformation

Question 5

What are some key events in the discovery of receptors?

Answer 5

1878: Langley concluded that there is some substance in nerve endings in which pilocarpine and atropine will interact with
1905: Langley proposed that nicotine and curare reacted with a reactive substance
1907: Enrich proposed the existence of chemoreceptor for drugs
1948: Anquist discovered that there are receptors for epinephrine that act on 𝛽 and 𝛼 receptors
1965: Propranolol is the first clinically active 𝛽 blocker

Question 6

GPCRs are also known as what?

Answer 6

7 transmembrane (7 TM) receptors

Question 7

GPCRs all do what?

Answer 7

They all couple to G proteins to initiate downstream signal transduction

Question 8

What is the key to initiating signaling?

Answer 8

The structure will change!

Question 9

How is rhodopsin a bit different?

Answer 9

It is constantly on and then turned off unlike other receptors that are constantly off and then turned on

Question 10

GPCRs represent what?

Answer 10

They represent the largest known class of biologically important transmembrane proteins conserved throughout evolution.

Question 11

How many GPCRs are there?

Answer 11

There are 865 in humans but there are only 363 GPCRs (not counting olfactory GPCRs) that are targeted by general ligands in which about 120 GPCRs are orphans in which the endogenous ligand is not known and the physiological function is unknown but then only 46 GPCRs are targeted by drugs

Question 12

What percent of drugs on the pharmaceutical market do GPCRs represent?

Answer 12

40%

Question 13

In its active state, which subunit has GTPase activity?

Answer 13

G𝛼

Question 14

How many subtypes are there of each subunit?

Answer 14

G𝛼 has 20 distinct subtypes. G𝛽 has 5 subtypes. G𝛾 has 12 subtypes.

Question 15

What happens when GDP is released for GTP to bind?

Answer 15

When GTP is bound, the 𝛼 and 𝛽 subunits dissociate from one another.

Question 16

What happens during the activation of the G protein cycle?

Answer 16

GDP is released so GTP can bind

Question 17

What happens during deactivation of the G protein cycle?

Answer 17

Hydrolysis of GTP to GDP

Question 18

The main takeaway of the G protein cycle is:

Answer 18

If bound to GDP → is resting, the pathway is dead

If bound to GTP → have an activated GPCR

Question 19

In its inactive state, where is GDP is bound in the G protein?

Answer 19

GDP is bound in the middle

Question 20

A G protein consists of what 2 domains?

Answer 20

A Ras-like domain and an 𝛼-helical domain

Question 21

What is the mechanism of G protein activation?

Answer 21

The 𝛼-helix moves and the helix from the Ras-like domain goes into the G protein → basically the 𝛼-helical domain swings out 120 degrees so that GDP gets released → pac-man

Question 22

What is one of the ways that GPCR signal terminates?

Answer 22

Receptor desensitization/internalization:

1. Phosphorylation of the C terminus of GPCRs by GRKs
2. Binding of 𝛽-arrestin which leads to desensitization
3. Recruitment to clathrin coated pits which leads to internalization
4. Has 2 choices: recycling or degradation

Question 23

How does internalization via clathrin coated pits work?

Answer 23

Goes into the pit and form a residue that pulls the receptor into the cytosol so that it can no longer be activated

Question 24

𝛽-arrestin can also do what?

Answer 24

It can also initiate signaling pathways on its own

Question 25

What is another way to terminate the GPCR signal on the G protein level?

Answer 25

GAP turns off the signal in which intrinsic GTPase activity converts the active/on form (GTP bound) to the inactive/off form (GDP bound)

Question 26

What does the i and s stand for in the G𝛼 subunit?

Answer 26

i = inhibitory
s = stimulatory

Question 27

What is the general scheme of the signaling pathway for GPCRs?

Answer 27

GPCR → G-protein → effector → 2nd messenger → kinases and channels

Question 28

What is the signaling pathway of Gs and Gi?

Answer 28

Gs will stimulate and Gi (G-protein) will inhibit adenylate cyclase (effector) → cAMP (2nd messenger) → PKA (kinase)

Question 29

What is the Gq signaling pathway?

Answer 29

Gq (G-protein) → phospholipase C-𝛽 (effector) → both diacylglycerol (DAG) and IP3 (both 2nd messengers) → DAG activates PKC (kinase) and IP3 activates IP3 receptor which then activates Ca2+

Question 30

What is adenylate cyclase?

Answer 30

It is an integral membrane protein with many transmembrane segment and a cytoplasmic domain that can catalyze the conversion of ATP to cAMP. Is stimulated by G𝛼s and inhibited by G𝛼i

Question 31

What is cAMP?

Answer 31

It is a second messenger that has a major role in activating PKA (protein kinase A) and is broken down by phosphodiesterases (PDE) from cAMP to 5’-AMP

Question 32

What is an example of a phosphodiesterase inhibitor?

Answer 32

Viagra

Question 33

What is the structure of adenylate cyclase?

Answer 33

Has a large transmembrane region (𝛼-helical) and has 2 catalytic domains (C1a and C2a) where G𝛼s would bind

Question 34

What is PKA?

Answer 34

Is also called cAMP-dependent protein kinase and it mediates most cAMP signaling. It consists of 2 regulatory subunits (R-subunits) and 2 catalytic subunits (C-subunits)

Question 35

The binding of cAMP to R-subunits results in what?

Answer 35

Results in the dissociation of C-subunits from R-subunits → now there are free catalytic subunits of PKA that can activate substrates downstream

Question 36

What are some cellular substrates of PKA?

Answer 36

Enzymes in glycolysis and glycogenolysis, ion channels, and CREB (cAMP response element binding)

Question 37

What is CREB?

Answer 37

CREB is a transcription factor and its phosphorylated form by PKA binds CRE and rapidly changes transcription levels → initiates transcription on a number of target cells

Question 38

What is the most important activator of PKA?

Answer 38

cAMP