Question 1

Which of the following statements best describes the difference between L- and D- amino acids in biological systems?

Accepted Answer

L-amino acids are the predominant form in proteins, whereas D-amino acids are rare and typically found in some bacterial cell walls and antibiotics.

Question 2

Which amino acid is NOT chiral and why?

Accepted Answer

Glycine, because its side chain is a hydrogen atom, making the alpha carbon attached to two identical groups.

Question 3

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

Accepted Answer

The amino group is protonated ($$\mathrm{NH_3$^+}) $$and the carboxyl group is deprotonated ($$\mathrm{COO^-}$$), forming a zwitterion.

Question 4

Which amino acid contains a sulfur atom in its side chain and is classified as hydrophobic?

Accepted Answer

Methionine (Met, M)

Question 5

Which of the following amino acids is most likely to form disulfide bonds in proteins, and under what cellular conditions does this typically occur?

Accepted Answer

Cysteine, in the oxidizing environment of the endoplasmic reticulum or extracellular space.

Question 6

Which amino acid side chain is most likely to act as a nucleophile at physiological pH due to its imidazole group?

Accepted Answer

Histidine (His, H)

Question 7

Given the following pKa values for glycine: $$pK_a$$^{	ext{COOH}$$} = 2.4$$, $$pK_a$$^{	ext{NH}$$_3^+} = 9.8$$, calculate the isoelectric point ($$pI) of $$glycine.

Accepted Answer

$$pI = \frac{2.4 + 9.8}{2} = 6.1$$

Question 8

Which amino acid is most likely to absorb UV light at 280 nm and why is this property useful in protein quantification?

Accepted Answer

Tryptophan (Trp, W), because its aromatic ring has delocalized pi electrons that strongly absorb at 280 nm.

Question 9

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

Accepted Answer

Valine (Val, V)

Question 10

A protein is found to contain a high proportion of proline residues. Which structural property is most likely affected by this?

Accepted Answer

The protein's secondary structure, as proline induces kinks and disrupts alpha helices.

Question 11

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

Accepted Answer

Glutamine (Gln, Q)

Question 12

Which amino acid is most likely to bind reactive metals due to its negatively charged carboxylate side chain at neutral pH?

Accepted Answer

Aspartate (Asp, D)

Question 13

Which of the following modifications increases the number of side chain functional groups in proteins and can regulate protein activity?

Accepted Answer

Phosphorylation of serine residues

Question 14

Which amino acid is most likely to act as a proton donor and acceptor at physiological pH due to its side chain pKa near 6.5?

Accepted Answer

Histidine (His, H)

Question 15

Which of the following best describes the CORN mnemonic for identifying the L-enantiomer of amino acids?

Accepted Answer

When viewing the molecule from the hydrogen atom, the groups COOH, R, and NH2 are arranged clockwise.

Question 16

Which amino acid is most likely to be involved in nucleophilic catalysis in enzyme active sites due to its side chain amine group?

Accepted Answer

Lysine (Lys, K)

Question 17

A solution contains a mixture of amino acids at pH 2.0. Which group is most likely to be protonated?

Accepted Answer

The carboxyl group ($$\mathrm{COOH})$$

Question 18

Which amino acid is most likely to be found at the surface of a soluble protein, interacting with water?

Accepted Answer

Serine (Ser, S)

Question 19

Which amino acid is classified as aromatic and nonreactive, and what is its three-letter code?

Accepted Answer

Phenylalanine, Phe

Structure and Properties of Amino Acids: Foundations for Biochemistry

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