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 has no net overall charge.

Question 2

Given glycine has $$pK_{a1} = 2.34 ($$carboxyl) and $$pK_{a2} = 9.60 ($$amino), what is its isoelectric point ($$pI$$)?

Accepted Answer

$$pI = \dfrac{2.34 + 9.60}{2} = 5.97$$

Question 3

At a pH below the $$pK_{a1} of $$glycine, what is the predominant form of glycine?

Accepted Answer

$$NH_3$$^+ - $$CH_2 - $$COOH$$ (net charge = +1)

Question 4

Which of the following statements about buffer regions of amino acids is correct?

Accepted Answer

Buffering is strongest near the $$pK_a$$ values of ionizable groups.

Question 5

Which type of bond is primarily responsible for stabilizing the primary structure of proteins?

Accepted Answer

Peptide (covalent) bonds

Question 6

A protein has a molecular weight of 33,000 Da. Approximately how many amino acid residues does it contain?

Accepted Answer

$$\dfrac{33,000}{110} = 300$$ residues

Question 7

Which amino acid is capable of forming disulfide bonds, and what is the significance of these bonds?

Accepted Answer

Cysteine; they stabilize protein tertiary and quaternary structure.

Question 8

Which of the following best explains why hydrophobic amino acids tend to be buried inside soluble proteins?

Accepted Answer

They minimize unfavorable ordering of water molecules by clustering together.

Question 9

Which protein purification method separates proteins based on their specific binding affinity to a ligand?

Accepted Answer

Affinity chromatography

Question 10

During SDS-PAGE, proteins are separated primarily based on which property?

Accepted Answer

Molecular weight

Question 11

Which of the following interactions is the strongest under physiological conditions?

Accepted Answer

Covalent bonds

Question 12

A protein is found to have four distinct polypeptide chains assembled together. This is an example of which level of protein structure?

Accepted Answer

Quaternary structure

Question 13

Which of the following best describes the role of histones in the cell?

Accepted Answer

They bind DNA via ionic interactions with the phosphate backbone.

Question 14

Which of the following amino acids would be most likely to be found in the interior of a globular protein?

Accepted Answer

Leucine

Question 15

Which method would you use to detect a specific protein and its post-translational modifications in a cell extract?

Accepted Answer

Western blotting

Question 16

If you add a strong base to a glycine solution at pH 2.0, what is the first group to lose a proton?

Accepted Answer

The carboxyl group

Question 17

Which of the following equations correctly represents the calculation of the isoelectric point ($$pI) $$for an amino acid with two ionizable groups?

Accepted Answer

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

Question 18

Which of the following best describes a zwitterion?

Accepted Answer

A molecule with both positive and negative charges but a net charge of zero.

Question 19

Which of the following is NOT a noncovalent interaction important for protein folding?

Accepted Answer

Disulfide bonds

Question 20

Which purification method would be most effective for separating a protein based on its size?

Accepted Answer

Size-exclusion chromatography

Amino Acid Chemistry, Protein Structure, and Protein Methods

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