Question 1

Which of the following statements about amino acid chirality is CORRECT?

Accepted Answer

All amino acids except glycine are chiral and most have L-stereochemistry in proteins.

Question 2

Amino acids are considered zwitterions at physiological pH. Which of the following best describes a zwitterion?

Accepted Answer

A molecule with both a positively charged group and a negatively charged group, but overall neutral.

Question 3

Which amino acid does NOT have a chiral center?

Accepted Answer

Glycine

Question 4

At pH 7.4, what is the net charge of the amino acid glutamate (Glu, E)? (Assume pK$_a$ values: $\alpha$-COOH = 2.0, $\alpha$-NH$_3^+$ = 9.5, side chain COOH = 4.2)

Accepted Answer

$-1$

Question 5

Which of the following amino acids contains a sulfur atom in its side chain?

Accepted Answer

Methionine

Question 6

Which property of the peptide bond is MOST responsible for its planarity?

Accepted Answer

Partial double-bond character due to resonance between the carbonyl and amide nitrogen

Question 7

A protein is run on SDS-PAGE after treatment with SDS and a reducing agent. Which property will determine its migration through the gel?

Accepted Answer

Molecular weight (size) only

Question 8

Which of the following best explains why proline disrupts regular secondary structures in proteins?

Accepted Answer

Its side chain forms a ring with the backbone nitrogen, restricting rotation.

Question 9

Which technique would be most effective for separating a protein with a pI of 4.5 from a protein with a pI of 9.5 at pH 7.0?

Accepted Answer

Ion exchange chromatography

Question 10

A solution contains a mixture of three proteins: 15 kDa, 30 kDa, and 75 kDa. Which protein will elute FIRST from a size exclusion chromatography column?

Accepted Answer

75 kDa protein

Question 11

Which of the following amino acids is most likely to be found on the surface of a soluble, globular protein?

Accepted Answer

Aspartate

Question 12

Which of the following statements about the isoelectric point (pI) of a protein is TRUE?

Accepted Answer

At its pI, a protein has no net charge and is least soluble in water.

Question 13

Which amino acid side chain can form a covalent disulfide bond with another of the same type?

Accepted Answer

Cysteine

Question 14

A peptide has the sequence Lys-Glu-His. What is its net charge at pH 7.0? (Assume pK$_a$ values: Lys side chain = 10.5, Glu side chain = 4.2, His side chain = 6.0, N-term = 9.5, C-term = 2.0)

Accepted Answer

$+1$

Question 15

Which of the following best describes the function of SDS in SDS-PAGE?

Accepted Answer

It denatures proteins and imparts a uniform negative charge.

Question 16

Which amino acid is most likely to absorb UV light at 280 nm due to its aromatic side chain?

Accepted Answer

Tryptophan

Question 17

A protein is purified using affinity chromatography with a nickel column. Which tag is most likely fused to the protein?

Accepted Answer

Polyhistidine (His-tag)

Question 18

Which of the following modifications is commonly used to regulate protein function by adding a phosphate group to serine, threonine, or tyrosine residues?

Accepted Answer

Phosphorylation

Question 19

Which of the following best explains why D-amino acids are rare in human proteins but found in bacterial cell walls?

Accepted Answer

Ribosomal protein synthesis in humans uses only L-amino acids, while some bacteria incorporate D-amino acids for structural purposes.

Question 20

Which of the following amino acids has a side chain with a pK$_a$ closest to physiological pH (7.4), making it an important buffer in enzyme active sites?

Accepted Answer

Histidine

Amino Acids and Protein Primary Structure

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