Question 1

Which of the following best describes the chemical reaction that forms a peptide bond between two amino acids?

Accepted Answer

A condensation reaction between the α-carboxyl group of one amino acid and the α-amino group of another, releasing water

Question 2

Why is the peptide bond planar?

Accepted Answer

Because of the partial double-bond character due to resonance between the carbonyl oxygen and the amide nitrogen

Question 3

Given the tetrapeptide sequence Glu-Gly-Ala-Lys (EGAK), which of the following is the correct order for naming the peptide using the 'yl' convention?

Accepted Answer

Glutamylglycylalanyllysine

Question 4

Which of the following statements about the cis and trans conformations of the peptide bond is TRUE?

Accepted Answer

The trans conformation is generally favored, except for peptide bonds involving proline

Question 5

A peptide has the sequence Ser-Ala-Val. Which end of the peptide is the N-terminus?

Accepted Answer

The serine residue

Question 6

Which of the following best explains why proline is more likely to be found in the cis conformation of a peptide bond compared to other amino acids?

Accepted Answer

Proline's side chain forms a ring with the backbone nitrogen, reducing the energy difference between cis and trans forms

Question 7

Which of the following is the correct formula to calculate the isoelectric point ($pI$) of a peptide with $n$ net positive charges at $pH = 0$ and a ranked list of $pK_a$ values?

Accepted Answer

$pI = \frac{pK_{a,n} + pK_{a,n+1}}{2}$

Question 8

At a pH above its isoelectric point ($pI$), what is the net charge of a typical peptide?

Accepted Answer

Net negative

Question 9

Which of the following groups in a peptide is most likely to be deprotonated at pH 9.0?

Accepted Answer

The side chain carboxyl group of aspartic acid

Question 10

A polypeptide has the following $pK_a$ values for its ionizable groups: 2.0, 4.0, 7.0, and 10.0. If the net charge at $pH = 0$ is +2, what is the $pI$ of the peptide?

Accepted Answer

$pI = \frac{4.0 + 7.0}{2} = 5.5$

Question 11

Which of the following best describes the titration curve of a polypeptide with multiple ionizable groups?

Accepted Answer

It displays multiple buffering regions corresponding to the $pK_a$ values of the ionizable groups

Question 12

If a peptide is placed in a solution at a pH much lower than its $pI$, what will be its net charge?

Accepted Answer

Net positive

Question 13

Which of the following is NOT a correct feature of the peptide bond?

Accepted Answer

It allows free rotation around the C-N bond

Question 14

Which of the following best explains why the peptide bond has restricted rotation?

Accepted Answer

The resonance between the carbonyl oxygen and the amide nitrogen creates partial double-bond character

Question 15

A researcher is analyzing a peptide at pH 7.0. The peptide contains lysine, glutamic acid, and serine. Which residue is most likely to carry a positive charge at this pH?

Accepted Answer

Lysine

Question 16

Which of the following is the correct structure for a peptide bond?

Accepted Answer

$\mathrm{C=O-NH}$

Question 17

Which of the following steps is NOT required when calculating the $pI$ of a peptide?

Accepted Answer

Averaging all $pK_a$ values in the peptide

Question 18

Which of the following best describes the directionality of peptide synthesis in cells?

Accepted Answer

From the N-terminus to the C-terminus

Question 19

A peptide has a net charge of zero at pH 6.0. What is the best description of this pH?

Accepted Answer

It is the isoelectric point ($pI$) of the peptide

Question 20

Which of the following is a correct statement about the titration curve of a peptide?

Accepted Answer

The curve will have as many inflection points as there are ionizable groups

Structure and Properties of Peptides

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