Question 1

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

Accepted Answer

Glycine's alpha carbon is attached to two hydrogen atoms, so it lacks four different groups.

Question 2

At neutral pH, the structure of an α-amino acid is best described as:

Accepted Answer

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

Question 3

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

Accepted Answer

Tryptophan; it allows for quantification of proteins.

Question 4

A peptide bond forms between two amino acids through which type of reaction, and what is released as a byproduct?

Accepted Answer

Condensation; water is released.

Question 5

Which of the following best describes the geometry and stability of the peptide bond?

Accepted Answer

It is planar and stable due to delocalization of electrons.

Question 6

Given the following $pK_a$ values for a hypothetical amino acid: $pK_{a,1} = 2.0$ (carboxyl), $pK_{a,2} = 9.0$ (amino), and $pK_{a,3} = 6.0$ (side chain), calculate the isoelectric point ($pI$) for this amino acid if the side chain is neutral at $pH = 6.0$.

Accepted Answer

$pI = \dfrac{2.0 + 9.0}{2} = 5.5$, because the side chain is neutral at $pH = 6.0$

Question 7

Which of the following modifications is most likely to play a role in gene expression or suppression?

Accepted Answer

N-acetylation of lysine

Question 8

Why is peptide bond hydrolysis thermodynamically favorable, yet peptides are stable under physiological conditions?

Accepted Answer

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

Question 9

Which enzyme would you use to selectively cleave a peptide at the carboxyl side of lysine and arginine residues?

Accepted Answer

Trypsin

Question 10

A tetrapeptide has the sequence Glu-Gly-Ala-Lys. At pH 7, what is the overall charge on this peptide?

Accepted Answer

−1

Question 11

Which of the following statements about oligopeptides and polypeptides is correct?

Accepted Answer

Oligopeptides contain 3–15 residues, while polypeptides contain more than 15 residues.

Question 12

Which of the following best describes the effect of increasing pH on the overall charge of a peptide?

Accepted Answer

The overall charge becomes more negative.

Question 13

Which of the following is a posttranslational modification that stabilizes collagen structure?

Accepted Answer

Hydroxylation of proline

Question 14

Which of the following groups has a typical $pK_a$ range of $8.0-9.0$ in proteins?

Accepted Answer

α-Amino group

Question 15

A researcher wants to block the N-terminus of a peptide to prevent further elongation during synthesis. Which modification should they use?

Accepted Answer

N-formyl group

Question 16

Which of the following best explains why L-alanine and D-alanine are considered enantiomers?

Accepted Answer

They are mirror images of each other and are non-superimposable.

Question 17

Which of the following amino acids would you expect to be positively charged at physiological pH?

Accepted Answer

Lysine

Question 18

Which property of the peptide bond is most important for the formation of secondary protein structures such as α-helices and β-sheets?

Accepted Answer

Its planarity and partial double-bond character.

Question 19

Which of the following best describes the role of ATP hydrolysis in peptide bond formation during protein biosynthesis?

Accepted Answer

ATP hydrolysis provides the energy to make peptide bond formation thermodynamically favorable.

Question 20

Which of the following statements about the titration curve of histidine is correct?

Accepted Answer

The $pI$ of histidine is the pH at which the net charge is zero.

Amino Acids and Peptides: Structure, Properties, and Biochemical Significance

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