Q: A protein binds to a ligand with a $$K_d of$$ $$2.5 \times 10$$^{-5}$$ M. Another protein binds to the same ligand with a K_d$ of 7.5$$ \times $$10^{-7} M. $$Which protein has a higher affinity for the ligand?

The protein with $$K_d = 7.5 \times 10$$^{-7}$$ M

Question 1

Which statement best explains why most ligands are not perfectly complementary to their binding sites until AFTER they bind to a protein?

Accepted Answer

Ligands and proteins undergo conformational changes upon binding, which increases the complementarity between them.

Question 2

A protein binds to a ligand with a $$K_d of$$ $$2.5 	imes 10$$^{-5}$$ M. Another protein binds to the same ligand with a K_d$ of 7.5$$ 	imes $$10^{-7} M. $$Which protein has a higher affinity for the ligand?

Accepted Answer

The protein with $$K_d = 7.5 	imes 10$$^{-7}$$ M

Question 3

Given the equilibrium $$P + L ightleftharpoons PL$$, and initial concentrations $$[P]_0 = 1.0 	imes 10$$^{-6}$$ M, $$[L]_0 = 1.0 	imes $$10^{-5} M$, and K_d = 4.0$$ 	imes $$10^{-6} M$$, what is the equilibrium concentration of $$PL$?

Accepted Answer

$$2.0 	imes 10$$^{-6}$$ M

Question 4

What is the fraction of protein bound to ligand ($$Y) $$when $$[L] = K_d$$?

Accepted Answer

$$Y = 0.5$$

Question 5

Which of the following best describes the effect of binding oxygen to hemoglobin?

Accepted Answer

It initiates a conformational change from the T-state to the R-state, increasing oxygen affinity.

Question 6

Why does fetal hemoglobin have a higher affinity for oxygen than adult hemoglobin?

Accepted Answer

Fetal hemoglobin binds oxygen more tightly, allowing oxygen transfer from mother to fetus.

Question 7

Plasmodium falciparum, the malaria parasite, slightly decreases the pH of red blood cells. What effect does this have on hemoglobin's oxygen affinity?

Accepted Answer

It decreases hemoglobin's affinity for oxygen, promoting oxygen release to tissues.

Question 8

Which amino acid change in hemoglobin leads to sickle cell anemia?

Accepted Answer

A glutamate to valine substitution at position 6 of the beta chain

Question 9

Which of the following best explains why the artificial sweetener aspartame is sweet?

Accepted Answer

It binds to sweet-sensing receptors on the tongue, triggering a signaling cascade.

Question 10

In the context of protein-ligand binding, what does a low $$K_d$$ value indicate?

Accepted Answer

High affinity between the protein and ligand

Question 11

Which of the following is a consequence of the salt bridge between CysHC3 and LysFG1 in hemoglobin?

Accepted Answer

It stabilizes the T-state (deoxy form) of hemoglobin in the absence of oxygen.

Question 12

Which of the following best describes cooperative binding in hemoglobin?

Accepted Answer

Binding of one oxygen molecule increases the affinity for subsequent oxygen molecules.

Question 13

Which of the following is a disadvantage of an oxygen carrier that is nearly saturated in the tissues (as shown by curve A in the oxygen dissociation graph)?

Accepted Answer

It cannot efficiently release oxygen to the tissues.

Question 14

Which of the following is a disadvantage of an oxygen carrier that is never close to being saturated, even in the lungs (as shown by curve B in the oxygen dissociation graph)?

Accepted Answer

It cannot carry very much oxygen to the tissues.

Question 15

Which of the following amino acids is most likely to form a hydrogen bond with galactose in the binding site of a protein?

Accepted Answer

Asparagine

Question 16

If a protein is 20% saturated with ligand, what is the fraction of protein bound to ligand ($$Y$$)?

Accepted Answer

$$Y = 0.2$$

Question 17

Which of the following best describes the role of the $$K_d in $$protein-ligand binding?

Accepted Answer

$$K_d is $$the concentration of ligand at which half the protein is bound to ligand.

Question 18

Which of the following changes would most likely increase the sweetness of a peptide-based artificial sweetener?

Accepted Answer

Replacing a non-polar amino acid with a polar amino acid that interacts with the sweet receptor

Question 19

Which of the following statements about the binding of galactose to its protein binding site is correct?

Accepted Answer

The binding site is composed of amino acids from widely separated regions of the primary sequence.

Protein-Ligand Binding, Hemoglobin Function, and Carbohydrate Recognition: Biochemistry Study Notes

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