Question 1

Which of the following statements best describes the structure of an α-amino acid at neutral pH?

Accepted Answer

The amino group is protonated and the carboxylic acid group is deprotonated, forming a zwitterion.

Question 2

Which amino acid does NOT have a chiral α-carbon?

Accepted Answer

Glycine

Question 3

What is the relationship between L-alanine and D-alanine?

Accepted Answer

They are enantiomers.

Question 4

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

Accepted Answer

Methionine

Question 5

At which wavelength do proteins typically show maximum absorbance due to aromatic amino acids?

Accepted Answer

$280\ \mathrm{nm}$

Question 6

Which amino acid side chain is most likely to be involved in calcium binding due to post-translational modification?

Accepted Answer

γ-carboxyglutamate

Question 7

Which of the following equations correctly represents the formation of a peptide bond between two amino acids?

Accepted Answer

$\mathrm{Amino\ acid}_1 + \mathrm{Amino\ acid}_2 \rightarrow \mathrm{Dipeptide} + \mathrm{H}_2\mathrm{O}$

Question 8

Which property of the peptide bond contributes most to its planarity and stability?

Accepted Answer

Delocalization of electrons creating partial double-bond character

Question 9

Which enzyme would you use to specifically cleave a peptide bond on the carboxyl side of lysine residues?

Accepted Answer

Trypsin

Question 10

A tetrapeptide has the sequence Glu-Phe-Lys-Tyr. As the pH increases, what happens to the overall charge of the peptide?

Accepted Answer

It becomes more negative.

Question 11

Which of the following modifications is commonly found at the N-terminus of proteins to block degradation?

Accepted Answer

N-acetylation

Question 12

Which of the following amino acids is classified as a nonpolar aromatic amino acid?

Accepted Answer

Phenylalanine

Question 13

What is the typical $pK_a$ range for the side chain of lysine?

Accepted Answer

$9.8-10.4$

Question 14

Which of the following best explains why peptide bond hydrolysis is thermodynamically favorable but peptides are stable under physiological conditions?

Accepted Answer

The activation energy for hydrolysis is high, requiring strong acid or a catalyst.

Question 15

Which amino acid would you expect to absorb the most UV light at 280 nm?

Accepted Answer

Tryptophan

Question 16

A peptide is described as a polyampholyte. What does this mean?

Accepted Answer

It can act as both an acid and a base due to multiple ionizable groups.

Question 17

Which of the following is a common post-translational modification that regulates gene expression?

Accepted Answer

N-acetylation of lysine

Question 18

If a peptide is boiled in 6N HCl for 24 hours, what is the most likely outcome?

Accepted Answer

The peptide bonds will be hydrolyzed, breaking the peptide into free amino acids.

Question 19

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

Accepted Answer

Histidine

Question 20

Which of the following best describes the difference between a peptide and a polypeptide?

Accepted Answer

A peptide contains usually only 3 residues, while a polypeptide contains more than 15 residues.

Amino Acids: Structure, Stereochemistry, Classification, and Properties

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