L2 - How drugs bind to their targets

Question 1

A _________ receptor translates an external signal into a cellular process

Answer 1

A transmembrane receptor translates an external signal into a cellular process

Question 2

What are 3 key concepts about intracellular receptors?

Answer 2

1. Receptors are inside the cell - this means the drug needs to be able to cross the plasma membrane
2. Drugs that target these receptors must be lipid soluble (or have some other transport mechanism to cross the pm)
3. Effects typically have a slow onsset and are long-lasting (not rapidly reversible

Question 3

What is a classic example of a ligand that binds to intracellular receptors?

What is their mode of action?

Answer 3

• eg steroid hormones
• Mode of action is to bind to the lipid binding domain (LBD) of a steroid hormone receptor, leading to a displacement of HSP (heat shock protein) or other chaperone.
  
  • exposes DNA recognition domain and leads to activation of transcription of target genes

Question 4

Activation of G-Protein coupled receptors promotes _______ exchange of the _________ G-protein

Answer 4

Activation of G-Protein coupled receptors promotes GDP-GTP exchange of the Heterotrimeric G-protein

Question 5

Which subunit of the GCPR has GTPase activity and what is the result of this activity?

Answer 5

The G-alpha subunit has GTPase activity that acts like a molecular ‘timer’ or ‘countdown’ to terminate the signal

• After GTP is hydrolyzed, the inactive G-protein complex reassembles and the system can reset

Question 6

While active, the G-proteins can influence ______ that alter cellular activity

Answer 6

While active, the G-proteins can influence effector proteins that alter cellular activity

Question 7

GPCR’s are usually categorized based on the subtype of _______ that is associated

Answer 7

GPCR’s are usually categorized based on the subtype of G-alpha that is associated

Question 8

Which Galpha subtype activates Adenylate cyclase (AC)?

Answer 8

Gs (G alpha s)

Question 9

Which Galpha subunit activates Phospholipase C (PLC)?

Answer 9

Gq activates PLC

Question 10

Which G_alpha subunit inhibits Adenylate cyclase (AC)

Answer 10

G_i inhibits Adenylate Cyclase

Question 11

What is the result of activating receptors coupled to Gs

Answer 11

• Receptors coupled to Gs trigger increased activity of AC
• leading to production of cyclic AMP (cAMP)
• Activates cAMP dependent protein kinases
• These responses typically happen very quickly and mediate moment to moment control of physiological functions

Question 12

What happens when a receptor coupled to Gq is activated?

Answer 12

• Trigger increased activity of PLC (phospholipase C)
  
  • leads to production of inositol triphosphate (IP3) from the breakdown of PIP2
  • IP3 triggers release of ER Ca2+
    
    • influence a variety of signaling pathways
  • Other products of PIP2 breakdown (DAG) lead to activation of protein kinase C (PK-C) and target substrates

Question 13

What are Tyrosine kinase receptors and how do Tyrosine Kinase Receptors (TKR) translate extracellular ligands into changes in cellular signaling?

Answer 13

• Activation of TKR’s is driven by dimerization of receptors in the presence of a ligand causing the receptors to auto-phophorylate and become activated
• Drugs that inhibit or stimulate the activation of these receptors will influence downstream signaling mechanisms

Question 14

Compare the following properties between Intracellular receptors and Cell-Surface Receptors:

1. Membrane permeable drugs?
2. Circulating form of drugs
3. Speed of response
4. Breakdown/termination of signal

Answer 14

Compare the following properties between Intracellular receptors (Steroid hormone receptor) and Cell-Surface Receptors (GPCR/TKR):

1. Membrane permeable drugs?
   
   • Intracellular: Yes - often steroid drugs (must cross PM)
   • Cell-Surface: Not necessary
2. Circulating form of drugs
   
   • IC: Bound to carrier globulin because of low solubility in plasma
   • EC: Variable
3. Speed of response
   
   • IC: SLOW - req DNA binding and activation of target genes
   • EC: FAST - rapid transduction via protein conformational change and intracellular signaling cascades
4. Breakdown/termination of signal
   
   • IC: SLOW - large fraction of hormone is bound = usually a large reserve (Effects are also slow to reverse)
   • EC: FAST signal can rapidly terminate due to rapid GTPase cycle

Question 15

What is the fastest mechanism of signaling in the body?

Answer 15

Ion channels

• allow ions to cross PM very rapidly leading to changes in membrane voltage
• Can be ligand-binding or voltage-gated

Question 16

A substance/drug that binds to a receptor and generates an effect is called a(n):

Answer 16

Agonist

Question 17

How is agonism depicted?

Answer 17

Usually with a Concentration-Response Curve

(Concentration = x-axis)

(Drug Effect = y-axis)

Question 18

What is the EC50 and the Emax of a Concentration-response curve?

Answer 18

• EC50
  
  • Effective Concentration 50
  • refers to the concentration of drug that yields a 50% maximal effect
• Emax
  
  • This is the maximal biological effect observed with the drug

Question 19

What does Efficacy refer to?

Answer 19

Refers to the maximal drug effect (Emax)

Question 20

What does Potency refer to?

Answer 20

Potency refers to the concentration dependence (EC50)

• A drug with a low EC50 has a strong potency (a small concentration elicits a large effect)

Question 21

• A __________ can generate the maximal observed effect.
• A _________ can generate a fractional effect
• ________ cannot generate a biological efffect on their own
• _________ cause suppression of basal activity (ie reduce activity)

Answer 21

• A Full Agonist can generate the maximal observed effect.
• A Partial Agonist can generate a fractional effect
• ANTAGONISTS cannot generate a biological efffect on their own
• Inverse Agonists cause suppression of basal activity (ie reduce activity)

Question 22

What equation is used to calculate drug effect?

Answer 22

E = (Emax x [Drug]) / ([drug] + EC50)