Question 1

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

Accepted Answer

Phenylalanine

Question 5

At a pH below the pI (isoelectric point) of a peptide, what is the net charge on the peptide?

Accepted Answer

Net positive

Question 6

Which amino acid absorbs the most strongly in the near-ultraviolet region (around 280 nm)?

Accepted Answer

Tryptophan

Question 7

Which of the following equations represents the Henderson-Hasselbalch equation used to calculate the pH of a solution containing an ionizable amino acid group?

Accepted Answer

$$pH = pK_a + \log_{10} \left( \frac{[A^-]}{[HA]} \right)$$

Question 8

Which of the following modifications can occur at the N-terminus of a peptide and affect its function?

Accepted Answer

N-formylation

Question 9

During peptide bond formation, which molecule is released as a byproduct?

Accepted Answer

Water ($$H_2O)$$

Question 10

Which enzyme class is responsible for cleaving peptide bonds at specific amino acid sequences?

Accepted Answer

Proteases

Question 11

If a tetrapeptide contains the sequence Glu-Lys-Ser-Ala, what is the net charge at pH 7, given the side chain pK values: Glu ($$pK_a \approx 4.1$$), Lys ($$pK_a \approx 10.5$$), and the N- and C-termini ($$pK_a \approx 9.0$$ and $$pK_a \approx 2.0$$ respectively)?

Accepted Answer

+1

Question 12

Which of the following statements about peptide bonds is TRUE?

Accepted Answer

Peptide bonds are planar due to resonance stabilization.

Question 13

Which amino acid side chain is most likely to participate in calcium binding due to its carboxylate group?

Accepted Answer

Aspartic acid

Question 14

Which of the following amino acids is positively charged at physiological pH?

Accepted Answer

Arginine

Question 15

A protein sample is analyzed by UV spectroscopy at 280 nm. Which amino acid is primarily responsible for the absorbance at this wavelength?

Accepted Answer

Tryptophan

Question 16

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

Accepted Answer

Phosphorylation

Question 17

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

Accepted Answer

Methionine

Question 18

Which of the following best describes a zwitterion?

Accepted Answer

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

Question 19

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

Accepted Answer

Glutamine

Question 20

Which of the following is TRUE regarding the titration curve of histidine?

Accepted Answer

It shows three distinct buffering regions corresponding to the ionizable groups.

Amino Acids: Structure, Stereochemistry, Classification, and Properties

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