molecules that bind with high affinity to the opioid receptor (a membrane protein). Based on our discussion of
the thermodynamics of binding, answer the following questions (10 points)
A. Reto titrated a candidate compound C (fluorescently tagged) against the purified receptor and obtained
the isotherm below. Reto concluded that he found a high affinity specific binder. Is Reto's conclusion
correct? Why or Why not? What is missing from the experiment? (2)
1/Fluorescence polarization
450
400
350
300
250
200
150
100
50
0
0
10
20
30
40
[Ligand] (n)
50
60
70/nB. After performing the required controls, Reto obtained the following binding isotherm (left) and the
associated Scratchard plot (right). What is the value of K, for the interaction between C and the
receptor? What is the total protein concentration used in this experiment? (2)
Binding isotherm
Scratchard plot
1/Fluorescence anisotropy
300
250
200
150
100
50
0
0
10
20
30
40
[Ligand] (nM)
50
60
[bound ligand]/[free ligand]
0.6
0.5
0.4
0.3
0.2
0.1
0
1
2
3
4
bound ligand [L]bound (nM)
in
6/nC. Next, Reto performed the affinity measurements on mammalian cells that express the opioid receptor.
He is surprised to find that the K, from these measurements is over 10-fold higher than what he
measured with the purified protein. Can you speculate a possible reason that could explain this
observation?
D. Given the poor affinity of the compound C, Reto modified the library, rescreened and found a candidate
molecule R that binds the opioid receptor with a K, of 2nM. A high throughput measurement of binding
isotherms of R and other receptors revealed that R also binds to the glucagon receptor with a K₁ of 5
μM. Based on this data, what should be the concentration of R that Reto should recommend for further
testing of the compound R in vivo (i.e. in a living human body)? (2)/nE. In an in vitro experiment with R and the opioid receptor, what is the fractional saturation of the receptor,
when the concentration of R is 200 nM? (2)
Fig: 1
Fig: 2
Fig: 3
Fig: 4