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4. Reto is an aspiring medicinal chemist whose project involves screening a library of compounds to find

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

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50

0

0

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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

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0

0

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[Ligand] (nM)

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[bound ligand]/[free ligand]

0.6

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0.2

0.1

0

1

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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)

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