Question 1

Which of the following statements best describes the zwitterionic form of an amino acid at physiological pH (7.4)?

Accepted Answer

The amino group is protonated ($$\mathrm{NH_3$^+}) $$and the carboxyl group is deprotonated ($$\mathrm{COO^-}$$), resulting in a net neutral charge.

Question 2

Which amino acid is optically inactive due to its side chain, and why?

Accepted Answer

Glycine, because its R group is a hydrogen atom, making the alpha carbon non-chiral.

Question 3

A protein contains a high proportion of cysteine residues. Which structural feature is most likely stabilized by these residues?

Accepted Answer

Disulfide bonds ($$\mathrm{S-S}) $$formed by oxidation of cysteine residues stabilize tertiary and quaternary structures.

Question 4

Which amino acid has a side chain that can act as a buffer near physiological pH due to its $$pK_a$$ value?

Accepted Answer

Histidine, because its imidazole side chain has a $$pK_a$$ around 6.0.

Question 5

Which of the following amino acids is essential in humans and must be obtained from the diet?

Accepted Answer

Leucine

Question 6

What is the net charge of glycine at a pH below its $$pK_1 (2.34$$)?

Accepted Answer

Net charge is +1, because both the amino group ($$\mathrm{NH_3$^+}) $$and carboxyl group ($$\mathrm{COOH}) $$are protonated.

Question 7

Which amino acid is most likely to be found in the interior of a globular protein due to its hydrophobic side chain?

Accepted Answer

Valine

Question 8

A mutation replaces a glutamate residue with lysine in a protein. What is the most significant change in the protein’s properties?

Accepted Answer

A negatively charged side chain is replaced by a positively charged side chain, potentially altering ionic interactions.

Question 9

Which of the following best describes the formation of a peptide bond?

Accepted Answer

A condensation reaction between the carboxyl group of one amino acid and the amino group of another, releasing $$\mathrm{H_2O}.$$

Question 10

Which amino acid is most likely to participate in hydrogen bonding due to its side chain?

Accepted Answer

Serine, because its side chain contains a hydroxyl group ($$\mathrm{CH_2OH}).$$

Question 11

Which of the following amino acids is classified as aromatic?

Accepted Answer

Tryptophan

Question 12

What is the effect of increasing pH on the net charge of an amino acid?

Accepted Answer

The net charge becomes more negative as protons are removed from the amino and carboxyl groups.

Question 13

Which amino acid is a precursor for the biosynthesis of other amino acids and acts as a major entry point for $$\mathrm{NH_4$^+} in $$metabolism?

Accepted Answer

Glutamate

Question 14

Which of the following amino acids contains a sulfhydryl group and can form disulfide bonds?

Accepted Answer

Cysteine

Question 15

Which structural level of protein is directly determined by the sequence of amino acids?

Accepted Answer

Primary structure

Question 16

Which amino acid is most likely to be found at the active site of an enzyme due to its ability to act as both an acid and a base?

Accepted Answer

Histidine

Question 17

Which of the following amino acids is classified as nonessential in humans?

Accepted Answer

Asparagine

Question 18

Which amino acid is most likely to be involved in the formation of collagen due to its post-translational modification?

Accepted Answer

4-Hydroxyproline

Question 19

Which of the following statements about the isoelectric point ($$pI) of an $$amino acid is correct?

Accepted Answer

At $$pI$$, the amino acid has a net charge of zero and is least soluble in water.

Amino Acids: Structure, Classification, and Biochemical Roles

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