Q: Which equation best describes the fractional saturation ($$\theta) of a $$protein by a ligand?

$$\theta = \dfrac{[L]}{K_d + [L]}$$

Q: If the $$K_d$$ for a protein-ligand interaction is $$1 \times 10$$^{-9}$$ \ M$$, what is the association constant $$K_a$$?

$$1 \times 10$$^{9}$$ \ M$$^{-1}$$

Question 1

Which equation best describes the fractional saturation ($$	heta) of a $$protein by a ligand?

Accepted Answer

$$	heta = \dfrac{[L]}{K_d + [L]}$$

Question 2

A protein has a $$K_d of$$ $$2 \ \mu M$$ for its ligand. At what ligand concentration will 50% of the binding sites be occupied?

Accepted Answer

$$2 \ \mu M$$

Question 3

If Protein X has a $$K_d of$$ $$1 	imes 10$$^{-8}$$ \ M$$ and Protein Y has a $$K_d of$$ $$1 	imes 10$$^{-6}$$ \ M$$ for the same ligand, which protein has the higher affinity for the ligand?

Accepted Answer

Protein X

Question 4

Which of the following statements about the dissociation constant ($$K_d) is $$correct?

Accepted Answer

A lower $$K_d$$ value indicates a higher affinity of the protein for its ligand.

Question 5

Given the equation $$	heta = \dfrac{[L]}{K_d + [L]}$$, what is the value of $$	heta$$ when $$[L] = 0$$?

Accepted Answer

$$0$$

Question 6

A sample contains $$10 \ mM$$ total protein-receptor concentration. If $$25\% of $$the receptors are occupied when the free ligand concentration is $$15 \ mM$$, what is the $$K_d$$ for the receptor-ligand interaction?

Accepted Answer

$$45 \ mM$$

Question 7

Which of the following is true regarding the relationship between $$K_d$$ and the rate constants $$k_{on}$$ and $$k_{off}$$?

Accepted Answer

$$K_d = \dfrac{k_{off}}{k_{on}}$$

Question 8

A protein-ligand binding curve is similar in shape to which other biochemical plot?

Accepted Answer

Michaelis-Menten enzyme kinetics plot

Question 9

If a protein has a $$K_d of$$ $$5 \ \mu M$$ and you want $$80\% of $$the binding sites occupied, what ligand concentration is required? Use $$	heta = \dfrac{[L]}{K_d + [L]}.$$

Accepted Answer

$$20 \ \mu M$$

Question 10

Which statement best explains why $$K_d is $$often used to compare the affinities of different proteins for the same ligand?

Accepted Answer

$$K_d is $$independent of protein concentration and reflects intrinsic binding affinity.

Question 11

A researcher observes that increasing salt concentration decreases the binding of a ligand to a protein. What does this suggest about the nature of the interaction?

Accepted Answer

Electrostatic interactions are important for binding.

Question 12

Which of the following curves on a protein-ligand binding plot would correspond to the protein with the lowest $$K_d$$?

Accepted Answer

The curve that reaches saturation at the lowest ligand concentration.

Question 13

If the $$K_d$$ for a protein-ligand interaction is $$1 	imes 10$$^{-9}$$ \ M$$, what is the association constant $$K_a$$?

Accepted Answer

$$1 	imes 10$$^{9}$$ \ M$$^{-1}$$

Question 14

A patient’s antibody binds to an antigen with a $$K_d of$$ $$2 	imes 10$$^{-7}$$ \ M. If $$the antigen concentration is $$2 	imes 10$$^{-7}$$ \ M$$, what is the fractional saturation ($$	heta$$)?

Accepted Answer

$$0.5$$

Question 15

Which of the following is NOT a factor that can affect protein-ligand binding affinity?

Accepted Answer

The color of the ligand

Question 16

If a protein has a $$K_d of$$ $$10 \ \mu M$$ and the ligand concentration is $$30 \ \mu M$$, what is the fractional saturation ($$	heta$$)?

Accepted Answer

$$0.75$$

Question 17

Which of the following best describes the meaning of $$K_d in a $$protein-ligand interaction?

Accepted Answer

The ligand concentration at which half of the protein binding sites are occupied.

Question 18

A protein has a $$K_d of$$ $$4 \ \mu M. If $$you want $$90\% of $$the binding sites occupied, what ligand concentration is required?

Accepted Answer

$$36 \ \mu M$$

Question 19

Which of the following statements is correct regarding the effect of $$K_d on $$the speed of binding?

Accepted Answer

A lower $$K_d$$ generally means faster binding if $$k_{on} is $$high.

Protein-Ligand Binding: Fractional Saturation and Affinity

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