Flashcards in 1.0 Drug Receptor Interactions Deck (36)
What forces are involved in binding ligand to receptor? (stronger to weakest)
4) Van der Waals
What is an agonist?
Binds to a receptor and generates a response
What is an antagonist?
A drug which opposes the action of an agonist drug
Competitive = same binding site. No efficacy
- Different mechanisms
1) Binds to allosteric site → ↓ agonist binding
2) Irreversible binding to same site
3) A drug that activates another pathway → opposing the effect of agonist (functional non-competitive agonist)
Amount of receptor that is bound by a given concentration of drug
How tight the drug-receptor complex it
How readily the drug unbinds
Define affinity constant
Inverse of the concentration of drug needed to occupy half the receptors
What are the equations for affinity constant?
Relationship between the amount of receptor occupied by drug and the size of the response
The maximum response measured when all receptors are occupied with drug
Concentration of drug that leads to 50% of maximal response
Potency refers to the concentration of a drug causing a particular magnitude of response (e.g. EC₅₀)
Combination of affinity and efficacy
A drug is selective for a particular receptor if it has a high affinity for one particular receptor type and low affinity (if any) for others
Antagonist concentration needed to reduce agonist response by 50%
This is not constant (↑ [agonist] → ↑ IC₅₀)
Name an agonist and antagonist for muscarinic and nicotinic receptors:
Agonist = Muscarine
Antagonist = Atropine
Agonist = Nicotine
Antagonist = d-tubocurarine
What is Ka?
↑K₁ → ↑ affinity of drug to receptor
Units = M⁻¹
What is Kd?
Dissociation constant (K₁=1/Kd)
Units = M
Kd = concentration of drug that gives half the maximum occupancy
What is K₂?
Affinity constant for antagonist
What is pA₂?
The concentration of antagonist needed to shift EC₅₀ across 2 fold = 10^pA₂
What is the formula for fraction of receptors bound?
What makes up total binding of a drug?
1) Specific binding
- Binding to receptor
2) Non-specific binding
- Binding to sites away from receptor
- Linear and non-saturable
What is the spare receptor (receptor reserve) concept?
Only a fraction of the receptors are needed to produce a maximum response
In these situations Kd >> EC₅₀
What is the significance of receptor reserve?
1) Allows lower concentrations of drug/molecule to cause effect (this is useful as agonists can therefore be low affinity, meaning that their effect has rapid onset and offset)
2) Maximum response can still be achieved despite some loss of receptors through desensitisation/ antagonism
Give some reasons why EC₅₀ and Kd are not the same:
1) Receptor reserve
2) Response may occur before drug-receptor binding interaction reaches equilibrium
3) Response may occur due to >1 pathway
4) Concentration of drug bathing tissues may be different to concentration of drug reaching receptors
How long does nACh channel open for when agonist binds?
1ms (allowing 10,000 Na⁺ to enter)
What are the subunits of nACh?
α x 2
Each subunit has M1-M4 stretches. M2 from each unit lines the pore
What is receptor desensitization?
Prolonged exposure to an agonist → receptors are prevented from producing their normal effect (transduction mechanism is uncoupled)
Desensitisation may include a change in drug affinity or a change in number of receptors (or both)
What are the mechanisms of desensitization in β₂ adrenoreceptor?
Seconds → minutes
a) Heterologous desensitization (PKA)
b) Homologous desensitization (βARK)
Receptors taken up by endocytosis
Minutes → hours
PKA alter mRNA stability → ↓ receptors
How many membrane spanning segments do voltage gated ion channels have?
4 groups of 6 membrane spanning segments
(Na⁺ + Ca²⁺ channels = one polypeptide, K⁺ channel = 4 polypeptides)
What are the three Na⁺ channel conformations?
What fibres are preferentially targetted by LAs?