How drugs act?

Question 1

What are the 5 major protein targets for drug action?

Answer 1

1. Receptors
2. Enzymes
3. Specific Circulating Plasma Proteins
4. Carrier Molecules (Transporters)
5. Ion Channels

Question 2

 Protein molecule which function 
to recognize and respond to 
endogenous chemical signals
• recognize/bind specific endogenous 
ligands
• may also recognize/bind xenobiotics
 Classified based on ligands
 Grouped into 4 major 
superfamilies

Answer 2

Receptors

Question 3

What are the 4 superfamilies of receptors?

Answer 3

• Ligand-Gated Ion Channels
• G-Protein Coupled Receptors
• Kinase-Linked & Related Receptors
• Nuclear Receptors

Question 4

How many subunits are found in ligand-gated ion channels?

Answer 4

4-5 subunits

Question 5

Which subfamily of receptors form dimers?

Answer 5

Kinase-Linked

Question 6

variation may arise from ______:

• single gene can give rise to more than one receptor isoform
• splicing can result in inclusion or deletion of one or more mRNA coding regions giving rise to short or long forms of protein
• big role in G-protein coupled receptors

Answer 6

alternative mRNA splicing

Question 7

• share structural features with voltage-
gated ion channels
• “ionotropic” receptors

Answer 7

 Ligand-Gated Ion Channels

Question 8

— nicotinic acetylcholine receptor (nAChR)
— gamma-aminobutyric acid type A receptor
(GABAA)
* inhibitory neurotransmitter
— glutamate receptors [N-methyl-D-aspartate
(NMDA), a-amino-3-hydroxy-5-methylisoxazole-4-propionic
acid (AMPA), and kainate types]
* excitatory neurotransmitter

are examples of what subfamily of receptor?

Answer 8

Ligand-gated ion channels

Question 9

— best characterized of all cell-surface receptors
— pentamer: four different polypeptide
subunits
* 2 a, 1 b, 1 g, and 1 d, MW from 43K-50K each
* each subunit crosses plasma membrane 4 times
— acetylcholine binds sites on a subunits
— conformational change occurs
— transient opening of central aqueous channel
— Na+ flow from outside to inside cell
* down electro-chemical gradient
— cell depolarizes
— all occurs in milliseconds

Answer 9

• nAChR

Question 10

• largest superfamily of receptors
• “metabotropic” receptors
• 7 transmembrane spanning domains

Answer 10

 G-Protein Coupled Receptors

Question 11

• The following are examples of _____ receptors:
— muscarinic acetylcholine receptor (mAChR)
— opioid receptors (m, k, d)
— gamma-aminobutyric acid type B receptor (GABAB)
— serotonergic receptors (5-hydroxytryptamine or 5-HT, 1-7 types)
— adrenergic receptors (a and b types)
— angiotensin II receptors (1, 2, 3, 4 types)
— endothelin receptors (A, B, C types)
— histamine receptors (1, 2, 3 types)
— photon receptors (retinal rod and cone)

Answer 11

 G-Protein Coupled Receptors

Question 12

______ of Opioid Receptors
— heroin, morphine, oxycodone,
hydrocodone

Answer 12

Agonists

Question 13

______ of Opioid
Receptors
— naloxone, naltrexone

Answer 13

• Competitive Antagonists

Question 14

— activation of GPCR is normally a result of diffusible ligand in solution acting on a receptor
— but GPCR receptor activation can occur as a result of protease activation
* protease cleaves off part of N-terminal domain of receptor
* “tethered agonist”: remaining attached domain is free to interact with ligand-binding domain
— examples:
* thrombin: a protease involved in blood clotting activates PAR
* PAR-2 is activated by a protease released from mast cells following degranulation

Answer 14

• Protease Activated Receptors (PAR)

Question 15

— applicable to all GPCRs
— occurs via 2 main mechanisms
* receptor phosphorylation
* receptor internalization
— example:
* b-adrenergic receptors
* b-arrestin phosphorylates receptor 
reducing receptor affinity for G-proteins
* receptor can then be internalized
* all is rapidly reversible

Answer 15

• Desensitization

Question 16

What are the 3 ways a GPCR can be desensitized?

Answer 16

Phosphorylation
Internalization in lysosome
Endocytosis

Question 17

— involved mainly in events controlling cell growth and differentiation
— act indirectly by regulating gene transcription
— signal transduction generally involves dimerization of two receptor molecules followed by autophosphorylation of tyrosine residues
— all have large extracellular ligand-binding domain connected via single membrane spanning domain to an intracellular domain which has enzymatic activity

Answer 17

Kinase-Linked & Related Receptors

Question 18

— all have large extracellular ligand-binding domain connected via single membrane spanning domain to an intracellular domain which has enzymatic activity

Answer 18

Kinase-Linked & Related Receptors

Question 19

What are the three major families of Kinase-Linked & Related Receptors?

Answer 19

— Receptor Tyrosine Kinases (RTKs)
— Serine/Threonine Kinases
— Cytokine Receptors

Question 20

— insulin receptor
* activates PI3 kinase 
pathway
» turns on or off gene 
expression
» activates glycogen 
synthesis
* activates Mitogen-
Activated Protein (MAP) 
Kinase pathway
» turns on or off gene 
expression

Answer 20

• Receptor Tyrosine

Kinases (RTKs)

Question 21

— smaller group than RTKs
— structurally and functionally very similar 
to RTKs
— phosphorylate serine and threonine 
instead of tyrosine
— e.g. transforming growth factor (TGF)

Answer 21

• Serine/Threonine Kinases

Question 22

— interleukins, interferons, chemokines,
etc
— lack intrinsic enzymatic activity in
intracellular domains!!
— associate and activate other kinases
* binds and activates Janus Kinase (Jak)
* Jak binds and activates Signal Transducers
and Activators of Transcription
» a.k.a. the “Jak-STAT” pathway
* downstream turns on or off gene expression

Answer 22

• Cytokine Receptors

Question 23

— ligand-activated transcription factors
— ligand examples:
* estrogens, progestins, androgens, 
glucocorticoids, mineralocorticoids, vitamin D, 
vitamin A (retinoid receptors), fatty acids, etc.
— two main locations in the cell
* cytoplasmic
* nuclear
— ligand-binding and DNA-binding domains

Answer 23

Nuclear receptor

Question 24

Of all receptor superfamilies, which one must have ligand get into cell to activate receptor?

Answer 24

Nuclear receptor

Question 25

— cytoplasmic * most are bound to Heat Shock Proteins when no ligand is present * most form homodimers upon ligand binding (e.g. steroid receptors) * some form heterodimers with Retinoid X Receptor (e.g. thyroid hormone) * translocate to nucleus to regulate gene expression — nuclear (e.g. fatty acid receptors) * constitutively present in nucleus * form heterodimers with Retinoid X Receptor (RXR) — interact with hormone response elements on genes to regulate gene expression • Example: Androgen Receptors

Answer 25

• Nuclear Receptors (“Steroid Superfamily”)

Question 26

— ______ that are key rate-limiting steps in biochemical reactions are the best drug targets — strategy is most often to reduce this activity through drug inhibition

Answer 26

enzymes

Question 27

— competitively inhibit rate-limiting step in cholesterol biosynthesis in liver * liver upregulates LDL receptors thereby reducing plasma LDL concentrations

Answer 27

• HMG-CoA Reductase Inhibitors | “Statins”

Question 28

— monoclonal antibodies that recognizes TNF-a | — bind TNF-a removing it from circulation

Answer 28

• Infliximab and adalimumab

Question 29

* important in moving substances across lipid bilayer membranes * often good drug targets as they regulate key cellular events

Answer 29

Ion and Small Molecule Transporters

Question 30

* neurotransmitter uptake (norepinephrine, 5-HT, glutamate, etc.) * organic ion transporters (organic acids and bases) * p-glycoprotein (Multi-Drug Resistance)

Answer 30

Small molecule transporters

Question 31

— protective role in moving potential toxicants out of gastrointestinal epithelial cells back into lumen to prevent absorption — overexpressed in certain tumor cells leading to drug resistance — can be blocked by drugs * could increase absorption of some drugs * could potentially increase activity of anti-cancer drugs

Answer 31

• p-glycoprotein (Multi-Drug Resistance)

Question 32

________ important everywhere — establishes electrochemical gradient by moving Na+ out and K+ in against concentration gradient — requires energy (ATP) to function — key in all muscle contraction, nervous conduction, ion gradient establishment, etc — often provides the driving force for other ion transporters — can be inhibited by drugs (e.g. digoxin, a cardiac glycoside used for heart failure)

Answer 32

• Na+/K+ ATPase

Question 33

``` • structure — very similar in structure and function to ligand-gated ion channel receptors • Ca++ channels (L, T, N types) • Na+ channels (fast and slow types) • K+ channels (voltage- and ligand- gated types) — produce at least 9 different K+ currents in heart, vascular smooth muscle, and other tissues such as pancreas ```

Answer 33

Voltage-Gated Ion Channels

Question 34

``` — channels open or close depending upon the electrical gradient (voltage) across the plasma membrane * resting membrane potential ~ -90 mV * depolarized membrane potential ~ 0 mV — channels change opened/closed or activated/resting states as electrical potential changes from -90 mV to +10 mV (inside relative to outside) — channels often susceptible to binding by various compounds, including xenobiotics ```

Answer 34

• voltage-dependent gated ion channels

Question 35

— Ca++ Channel Blocker * binds to L-type Ca++ channels in heart and vascular smooth muscle * blocks movement of Ca++ from outside to inside * reduces cardiac contraction (negative inotropic effect) * slows cardiac conduction (negative chronotropic effect) * reduces vascular smooth muscle contraction * reduces blood pressure

Answer 35

Verapamil

Question 36

______ inhibits L type Ca channel reduces heart rate and contractility

Answer 36

Verapamil