Competition Binding Flashcards
What is the expression for total receptor concentration?
B_max = [R] + [DR] + [IR]
This equation represents the total concentration of receptors in a binding assay.
What does K_D represent in the context of drug binding?
K_D = [D] · [R]/[DR]
K_D is the dissociation constant, indicating the affinity between a drug and its receptor.
What is the relationship between [R] and [DR] using K_D?
[R] = K_D · [DR]/[D]
This equation helps in calculating the unbound receptor concentration based on drug and bound receptor concentrations.
How is [IR] expressed in terms of K_i?
[IR] = [I] · [R]/K_i
Here, [IR] is the concentration of the inactive receptor complex.
What is the significance of competition binding assays?
They measure binding in the presence of different concentrations of a competing drug
This method helps evaluate the effectiveness of a drug against a known ligand.
What is the IC50 value for atropine in the competition binding curve?
IC50 = 2.34E-9 M
IC50 is the concentration of a drug that inhibits a biological process by 50%.
What is the Cheng-Prusoff equation used for?
To relate IC50 to K_i
This equation is crucial for understanding drug affinity in competitive binding assays.
What does the p notation represent?
pH = -log[H+], pK_D = -log K_D, pK_i = -log K_i
The p notation simplifies the representation of very small values of K_D and K_i.
What is considered high affinity for a drug?
High affinity is ≤ 1E-9 M
This indicates a strong interaction between the drug and its receptor.
What are the advantages of using p notation?
- Works logically: increases with affinity
- pK values are normally distributed
This makes comparisons of drug affinities easier and more intuitive.
What is a limitation of radioligand binding assays?
They only tell us about drug affinity, not about post-binding effects
Understanding the consequences of binding is crucial for drug efficacy.
What is IC50?
The concentration of a competing ligand that reduces radioligand binding to 50% of control
What do EC50 and ED50 represent?
The concentration of the dose that is required to give 50% of the maximal effect the drug can achieve.
These parameters are a measure of potency
What does efficacy mean?
The tendency of a drug to activate a receptor once it has bound to it
What is a full agonist?
A drug whose efficacy is the same as that of the natural agonist at the receptor
What is insurmountable antagonism?
Antagonism that cannot be overcome by increasing the agonist concentration
What is a orthosteric antagonist?
Another name for a competitive antagonist (binds to allosteric site and blocks receptor activation)
What are spare receptors?
When a system has more receptors than are needed to achieve maximum effect
What is tone in context of biological systems?
The underlying level of activation of a signalling system
What is the Gaddum equation?
Bound = Bmax X [D]
——————————
[D] + KD (1 + [I]/ Ki)
Where
Bmax = total number of binding sites
[D] = drug of which binding is being measured
KD = affinity of drug
[I] = competing ligand
Ki = affinity for I
What are competition assays?
- use low concentration of radioligand
- measure control binding
- measure binding in presence of different concentrations of competing drug
- expressed as percentage of control binding
How is the Gaddum equation tranformed to measure the affinity of drugs?
% control = 100 (1 - [I]. )
————————-
[I] + Ki (1 + [D] / KD)
Where
[D] = concentration of radioligand sometimes referred to as ‘L’
[I] = concentration of competing ligand
% control = binding of radioligand in presence of competing ligand as a % of specific control binding of D (binding of D in absence of competing ligand)
How is the Gaddum equation tranformed to plot data?
% control = 100 (1 - [I] )
—————
[I] + IC50
Where
IC50 = inhibitory conc. of ‘I’ that gives 50% of control binding
Benefit of competition assays
Just need a radioligand with a known affinity
Can test any non-radioactive drug that binds to the same site
