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?

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

Study Guide - Practice Questions

Study Guide - Flashcards