Question 1

Which of the following statements best explains why glycine is not chiral, while all other standard amino acids are?

Accepted Answer

Glycine's side chain is a hydrogen atom, making two of its substituents on the alpha carbon identical.

Question 2

At physiological pH ($$pH = 7.4$$), what is the predominant form of a generic amino acid?

Accepted Answer

Zwitterion, with a deprotonated carboxyl group ($$-COO^-) $$and a protonated amino group ($$-NH_3$^+)$$

Question 3

Which amino acid contains a side chain that can form a disulfide bond, and what is the name of the resulting covalent linkage?

Accepted Answer

Cysteine; disulfide bond (cystine)

Question 4

A solution contains only alanine at $$pH = 2.0. $$Which of the following best describes the charge state of alanine at this pH?

Accepted Answer

Both the amino and carboxyl groups are protonated, resulting in a net positive charge.

Question 5

Which of the following amino acids is most likely to be found at the active site of an enzyme that catalyzes acid-base reactions near neutral pH, and why?

Accepted Answer

Histidine, because its imidazole side chain has a $$pK_a$$ near physiological pH and can act as both a proton donor and acceptor.

Question 6

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

Accepted Answer

$$pI is $$the pH at which the amino acid has no net charge.

Question 7

A patient with phenylketonuria (PKU) cannot metabolize phenylalanine properly. Which of the following is a likely consequence if phenylalanine is not restricted in the diet?

Accepted Answer

Accumulation of phenylalanine and its toxic metabolites, leading to intellectual disability.

Question 8

Which of the following amino acids absorbs ultraviolet light most strongly at 280 nm, and why is this property useful?

Accepted Answer

Tryptophan; it allows estimation of protein concentration in solution.

Question 9

Which of the following amino acids is classified as essential in the human diet?

Accepted Answer

Valine

Question 10

A peptide contains the sequence Lys-Glu-His. At $$pH = 7.4$$, what is the net charge of this tripeptide? (Assume N- and C-termini are not modified.)

Accepted Answer

Net charge = +1

Question 11

Which of the following best explains why L-amino acids are used in proteins rather than D-amino acids?

Accepted Answer

The ribosomal machinery specifically recognizes and incorporates only L-amino acids during protein synthesis.

Question 12

Which amino acid has a side chain that is an amide derivative of aspartate?

Accepted Answer

Asparagine

Question 13

A solution of glutamic acid is titrated with NaOH. At which pH range will the side chain carboxyl group lose its proton?

Accepted Answer

$$pH \approx 4.1$$

Question 14

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

Accepted Answer

Leucine

Question 15

Which of the following modifications occurs after protein synthesis and is important for collagen stability?

Accepted Answer

Hydroxylation of proline to form hydroxyproline

Question 16

Which of the following best describes a zwitterion?

Accepted Answer

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

Question 17

Which amino acid is most likely to be phosphorylated by a kinase during cell signaling?

Accepted Answer

Serine

Question 18

Which of the following statements about the side chain of proline is correct?

Accepted Answer

It forms a cyclic structure by bonding to the amino group, restricting backbone flexibility.

Question 19

Which of the following amino acids has a side chain with a guanidinium group, making it highly basic?

Accepted Answer

Arginine

Question 20

A mutation replaces a glutamate residue with a lysine residue in a protein. Which of the following changes is most likely to occur at physiological pH?

Accepted Answer

The local charge at that position will change from negative to positive.

Amino Acids: Structure, Properties, and Classification

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