GPCR AND RTKs

Question 1

Common drugs that act at GPCR receptors include

Answer 1

antidepressive and antipsychotic drugs (clozapine, fluvoxamine and haloperidol) as well as aplaviroc and maraviroc for HIV AIDs.

Question 2

Many other drugs that act at other “metabotropic” receptors include

Answer 2

atropine and scopolamine (muscarinic receptors).

Question 3

The GPCR drug class consists of receptors of the common “7 transmembrane domain” structure.
True or false

Answer 3

True

Question 4

They contain an amino terminal, 7 transmembrane domains and an intracellular heteromeric G-protein binding domain.

True or false

Answer 4

True

Question 5

GPCRs have a complex activation pathway. The simple description would be that binding of the ligand to the extracellular domain at TM 3, 5, 6 and 7. This produces several conformational changes

True or false

Answer 5

True

Question 6

a. Intermolecular interactions in TM6 are stabilized leading to a movement of this region of TM6 toward the inside of the membrane (Toggle switch)

True or false

Answer 6

True

Question 7

The ionic lock that is formed from an interaction between arginine of the E/DRY domain and a glutamate at the bottom of TM6 is disrupted. This allows TM6 to move toward the cytoplasmic interface to create a binding site for the heteromeric ________

Answer 7

G protein.

Question 8

c. The NPXXY domain bends inward toward ______ to stabilize the receptor in a G-protein binding conformation

Answer 8

TM6

Question 9

The development of the G-protein binding site allows binding of _______ (with GDP bound)

Answer 9

G- proteins

Question 10

Heteromeric G-proteins are composed of three primary subunits The first is a _______ which exists in a number of isoforms (Gs, Gi/Go, Gq/G11, G12/G13). The other component is the _________ dimer.

Answer 10

Ga, Gβ/Gλ

Question 11

In its resting, inactive state GDP is bound to ______ and Gα is bound to Gβ/Gλ. When GTP is bound Gα and Gα dissociates from Gβ/Gλ and binds to the effector.

Answer 11

Ga, Gβ/Gλ

Question 12

ACTIVATION OF GPCR

3 STEPS

Answer 12

Ligand Binds and a conformational change produces a G-protein binding site on the receptor (see objective 3 above)

b. Binding of the G-protein to the GPCR allows bound GDP to be displaced by GTP.
c. GTP bound G-protein dissociates from the GPCR and binds to the effector (adenylate cyclase or phospholipase C).

Question 13

DEACTIVATION OF GPCR SIGNALING

Answer 13

​Deactivation:
d. GTP is hydrolyzed to GDP and
the G-protein dissociates from the effector. The GDP bound G-protein is now ready to bind to another activated GPCR.

e. Binding to the effector activates protein kinases (Protein Kinase A/ Protein Kinase C).

Question 14

DESENSITIZATION OF GPCR SIGNALING

Answer 14

f. The GPCR receptor is phosphorylated.

g. Arrestin Binds

Question 15

TRAFFICKING STEP OF GPCR SIGNALING

Answer 15

h. Arrestin bound receptors are endocytosis via clathrin coated pits and targeted for recycling to the surface or degradation.

Question 16

cAMP signaling pathway and principle enzymes involved

Answer 16

cAMP steps:

a. G protein activates adenylyl cyclase
b. Adenylyl cyclase converts ATP to cAMP
c. cAMP binds and activates Protein Kinase A
d. Protein Kinase A phosphorylates proteins, typically other protein kinases as well.

Question 17

Phosphatidylinositol signaling pathway and principe enzymes.

Answer 17

PIP pathway:
a. G protein activates Phospholipase C.
b. Phospholipase C cleaves PIP2 into DAC and IP3
c. IP3 binds to intracellular IP3 Receptors to release Calcium
d. Calcium activates cell surface calcium channels
• Calcium activates Protein Kinase C
• DAC activates Protein Kinase C
Note: Calcium can also activate other kinases via Ca/calmodulin dependent protein kinases.
e. Protein Kinase C phosphorylates intracellular proteins.

Question 18

Describe the fundamental structure of a Receptor Tyrosine Kinase.
RTKs contain four distinct domains:

Answer 18

a. Ectodomain – extracellular ligand binding domain.
b. Transmembrane domain
c. Kinase domain – kinase catalytic site
d. Juxtamembrane domain – contains phosphorylation sites and adapter protein binding site (Binds to SH domains)

Question 19

Describe the activation pathway for Receptor Tyrosine Kinase

Answer 19

Activation of RTKs begins with binding of the ligand to RTK monomers. The monomers dimerize to form an activated receptor (kinase function is activated). The activated receptor dimer autophosphylates the receptor creating binding sites for adapter(or relay) proteins. Effector proteins bind to the adapter to initiate downstream signaling.

Question 20

Identify therapeutic application so of drugs targeting receptor tyrosine kinases.

Answer 20

Most current applications of RTKs are for cancer treatment due to their involvement in cell growth and proliferation. Most of these drugs inhibit either binding of the endogenous ligand, dimerization or phosphorylation (kinase action).