Question 1

Which of the following best describes the isoelectric point ($$pI) of an $$amino acid?

Accepted Answer

The pH at which the amino acid carries no net electrical charge.

Question 2

Given the following $$pK_a$$ values for valine: $$pK_{a1} = 2.32 ($$carboxyl group) and $$pK_{a2} = 9.62 ($$amino group), calculate the isoelectric point ($$pI) of $$valine.

Accepted Answer

$$pI = \frac{2.32 + 9.62}{2} = 5.97$$

Question 3

A student is asked to determine the $$pI of $$leucine, given $$pK_{a1} = 2.4$$ and $$pK_{a2} = 9.6. $$What is the correct calculation?

Accepted Answer

Question 4

Why is the isoelectric point ($$pI) of an $$amino acid important in protein purification techniques such as isoelectric focusing?

Accepted Answer

It allows separation of proteins based on their net charge at different pH values.

Question 5

If a molecule has $$pK_a$$ values of 3.8 and 9.4, what is its isoelectric point ($$pI$$)?

Accepted Answer

$$pI = \frac{3.8 + 9.4}{2} = 6.6$$

Question 6

A protein is placed in a buffer at its isoelectric point. What will be the net charge of the protein?

Accepted Answer

Zero

Question 7

Which property of amino acids allows their isoelectric point ($$pI) to be $$calculated using $$pK_a$$ values?

Accepted Answer

They have both acidic and basic functional groups.

Question 8

A patient’s blood protein is found to precipitate at pH 5.5. What does this suggest about the protein’s isoelectric point ($$pI$$)?

Accepted Answer

The $$pI is $$approximately 5.5.

Question 9

Which of the following statements about the isoelectric point ($$pI) is $$FALSE?

Accepted Answer

At $$pI$$, the molecule is most likely to migrate in an electric field.

Question 10

Which equation is used to calculate the isoelectric point ($$pI) of a $$simple amino acid with two ionizable groups?

Accepted Answer

$$pI = \frac{pK_{a1} + pK_{a2}}{2}$$

Question 11

A researcher is analyzing a mixture of amino acids using isoelectric focusing. Which amino acid will migrate the least at its $$pI$$?

Accepted Answer

Any amino acid at its $$pI.$$

Question 12

Which of the following is a correct statement about the relationship between $$pK_a$$ values and the isoelectric point ($$pI) of an $$amino acid?

Accepted Answer

The $$pI is $$calculated using the $$pK_a$$ values of the ionizable groups that lose or gain protons near neutrality.

Question 13

A solution contains a mixture of glycine and glutamic acid. At pH 6.0, which amino acid will have a net negative charge?

Accepted Answer

Glutamic acid

Question 14

Which of the following would increase the solubility of a protein in water?

Accepted Answer

Adjusting the pH away from the protein’s isoelectric point ($$pI)$$

Question 15

A biochemist is studying a peptide with three ionizable groups: $$pK_{a1} = 2.0$$, $$pK_{a2} = 7.0$$, and $$pK_{a3} = 10.0. $$Which two $$pK_a$$ values should be used to calculate the $$pI$$?

Accepted Answer

$$pK_{a2}$$ and $$pK_{a3}$$

Question 16

During isoelectric focusing, a protein stops migrating when:

Accepted Answer

It reaches a region where the pH equals its $$pI.$$

Question 17

Which of the following is a consequence of a protein being at its isoelectric point ($$pI) in $$solution?

Accepted Answer

The protein has no net charge and is prone to precipitation.

Question 18

A solution of an amino acid is adjusted to a pH below its isoelectric point ($$pI). $$What is the net charge on the amino acid?

Accepted Answer

Positive

Question 19

Which of the following best explains why the isoelectric point ($$pI) $$can be used to separate proteins in a mixture?

Accepted Answer

Proteins have different $$pI$$ values due to their unique amino acid compositions.

Amino Acids: Isoelectric Point and Structure Determination

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