Receptor Theory

Question 1

What is dissociation constant equal to at equilibirum?

Answer 1

rate of dissociation / rate of association

At equilibrium: Concentration of ligand required to bind 50% of available binding sites

Question 2

What is KD

Answer 2

Dissociation constant
a measure of AFFINITY
(concentration of ligand that binds to half the receptors)

Question 3

What is a hot ligand?

Answer 3

a labelled ligand (usually labelled radioactively)

Question 4

In a competitive ligand binding assay what has fixed concentrations?

Answer 4

the receptor and the hot ligand (radioligand)

Question 5

In a competitive ligand binding assay what has increasing concentrations?

Answer 5

the competing ligand

Question 6

How do you Determine ability of competitor to displace binding (Ki) in a competitive ligand binding assay?

Answer 6

The competing ligand will displace the radio ligand from the receptor reducing the amount of bound radioligand and will form an unlabelled complex (RX) thus the amount reduction in RL* observed at the end of the experiment tells you the amount of competition for the receptor.

The ability of the competing ligand to do this gives an estimate of affinity for the competing ligand Ki

Question 7

Does lower or higher Ki mean higher affinity?

Answer 7

lower Ki

Question 8

What does Ki measure?

Answer 8

It is the concentration of competing ligand required to displace 50% of the specific binding of radioligand

Thus it gives an estimate of affinity

Not the same as the KD

Assay conditions affect Ki values

The Ki value is dependent upon the concentration of radioligand

The Ki value for a competing ligand (x) is dependent on the radioligand (L*) used