Drug Receptors (Ch 2)

Question 1

list receptor type based on molecular structure

Answer 1

+ seven transmembrane (7TM) receptors
ex) GPCRs; 2/3 of all non-abx drugs bind to GPCRs
+ ligand-gated channels
+ ion channels
+ catalytic receptors
+ nuclear receptors
+ transporters
+ enzymes

Question 2

describe the cell signaling process

Answer 2

1. neuron releases neurotransmitters (signaling molecule – drug or endogenous ligand) from signaling cell
2. signaling molecule (1st messenger) binds to receptor
3. signal transduction proteins produce 2nd messengers
4. 2nd messengers target effector protein to elicit effect (ex. cellular metabolism, function, movement etc.)

Question 3

what types of drug processes will cause a lag period when regulating gene expression?

Answer 3

any processes that requires transcription/translation (protein production/mRNA)

Question 4

how long is a lag period usually

Answer 4

30 min - several hours

Question 5

what is drug persistence?

Answer 5

any drug that can be given once, but will remain in body for a long time

protein degradation pathways vary

Question 6

how long can drug persistence be?

Answer 6

hours to days

Question 7

what are the steps of a generic phosphorylation cascade?

Answer 7

1. drug binds to receptor
2. receptor confirmation change occurs
3. inactive protein (kinase) becomes activated
4. activated protein kinase 1 phosphorylates and takes protein kinase 2 and phosphorylates it (taking phosphate group from ATP, now making it ADP), thus becoming active
5. effector activated > elicits effect

Question 8

what is a kinase

Answer 8

any enzyme that attaches phosphate group to protein

Question 9

Name 4 transmembrane signaling methods by which drug-receptor interactions exert their effects

Answer 9

1. inTRAcellular receptors (lipid soluble/uncharged)
2. ion channels
3. catalytic receptors
   + receptor can become the enzyme itself inside the cell
   + activate a protein that activates subsequent enzymes (phosphorylation cascades)
4. GPCRs
   + receptor COUPLED to a second receptor (g-protein)

Question 10

GPCR characteristics

Answer 10

G protein = guanine nucleotide (GTP) binding protein, which sets up the signaling process

activated receptor can activate 100s of g-proteins (multiplied response)

2/3 of all non-abx drugs

500 are identified, 500 orphans

fast response – metabotropic ion channels (also have rapid desensitization)
OR slow response – transcription factor activation (lag)

Question 11

what is an orphan receptor?

Answer 11

no known endogenous ligand; roughly half of all receptors are identified

Question 12

GPCR RECEPTOR (non-g protein) structure

Answer 12

receptor:
7TM (seven transmembrane) alpha helices (crosses membrane 7 times)

active site on outside of cell

inside the cell, site present to interact with g-protein

pleiotropy: several downstream effects possible

amino (NH3+) and carboxy (COO-) terminal ends

Question 13

GPCR G-PROTEIN structure

Answer 13

trimeric: 3 subunits – amino acid strands (alpha, beta, gamma)
+ alpha most important (binds to GDP or GTP)
+ beta/gamma for binding to the receptor
GDP (inactive) turns into GTP (active)

Question 14

list the steps for a generic GPCR signaling process

Answer 14

1. drug binds to GPCR
2. confirmation change occurs
3. inactive GDP bound to alpha subunit gets replaced by activated GTP
4. GTP + alpha subunit detach off g-protein and signal 2nd messengers
5. second messengers activate effector protein to elicit effects
6. alpha subunit + inactive GDP reattach to GPCR

Question 15

what are some examples of second messengers

Answer 15

cGMP
calcium (released from cell storage)
DAG

most common:
cAMP
IP3

Question 16

give an example of a g-protein and its second messenger and the effects it causes

Answer 16

G alpha-stimulatory class > adenylyl cyclase (effector) > cAMP increased > effector (B-adrenergic receptor (to increase HR/contractility)

G alpha-inhibitory class > adenylyl cyclase K+ channel (effector) > cAMP (decreased) > alpha adrenergic receptor/muscarinic ACh receptors (to decrease HR)

Question 17

GPCR + cAMP

Answer 17

1. 1st messenger attaches to GPCR
2. GTP replaces GDP on alpha subunit
3. GTP+Alpha sub attach to effector protein
4. adenylyl cyclase converts ATP to cAMP and cAMP levels increase
5. cAMP becomes 2nd messenger and activates protein kinase A
6. protein kinase A starts phosphorylation cascade of other proteins
7. cellular responses are elicited

Question 18

describe the structure of RTKs

Answer 18

outer membrane bound receptor site where ligand binds and activates

in the cytoplasmic catalytic domain, receptor has:
+ tyrosine kinase (amino acid enzyme that attaches phosphate groups)
+ once dimerization occurs (two monomers fuse), tyrosine kinase becomes phosphorylated

Question 19

what do second messengers do?

Answer 19

second messengers signal the target protein to elicit downstream effects, which in turn will elicit cellular responses

Question 20

define desensitization in GPCRs

Answer 20

if an agonist is bound for a long period of time (i.e. covalently bonded), the GPCR can mute the prolonged drug response over time by using a process called desensitization

Question 21

define beta arrestin’s role in GPCR desensitization

Answer 21

1. GPCR with ligand bound goes from closed conformation to open conformation
2. the open conformation of the receptor has 3 -OH groups at the terminal end inside the cytoplasm which get phosphorylated and the alpha subunit binds becomes activated by binding with GTP and starts the signaling cascade
3. if the receptor remains in its open conformation for a long period of time (which may be due to the drug being covalently bound), beta arrestin binds to the terminal end of the receptor with the now phosphorylated groups
4. beta arrestin ARRESTS the signaling cascade, then interacts with the clatherin coated pit, which is involved in endocytosis
5. the clatherin coated pit endocytoses the receptor, ligand, and beta arrestin

two possible outcomes may follow:

a. the drug may become unbound from the receptor and the receptor wil be dephosphorylated by proteases and can be “recycled” back into the cell membrane

b. a lysosome and the endosome merge and break down the receptor/drug

Question 22

what is tyrosine phophatase?

Answer 22

an enzyme that strips phosphate from a protein

Question 23

list the steps of receptor tyrosine kinase signal transduction

Answer 23

1. two monomer RTKs bind to their respective ligands
2. dimerization occurs
3. phosphorylation of RTKs occurs via ATP
4. effector proteins signaled and downstream effects occur

Question 24

what is the difference between ligand gated vs voltage gated ion channels?

Answer 24

ligand gated:
+more common
+ionotropic
+ligand binding site and channel on the same protein
+nACh receptor

voltage gated:
+found in excitable cells (neurons, muscle, endocrine cells)
+closed at resting membrane potential (the baseline charge on membrane until the threshold is met)
+types based on ion selectivity (Ca2+, K+, Na+)
+regulated by physical “gates”

Question 25

ionotropic ligand gated ion channels

Answer 25

more common
ligand binding site and channel on same protein
ex) nicotinic/ACh receptor found on surface of skeletal muscle

Question 26

metabotropic ligand gated ion channel

Answer 26

more complicated
ligand activates GPCR
second messenger activity opens the channel

ex) odorant receptor

Question 27

list the types of ligands that can bind to receptors INSIDE the cell

Answer 27

gasses (NO)
lipid soluble agents (steroids)