Question 1

Which statement best describes the chemical nature of the peptide bond formed between two amino acids?

Accepted Answer

It is a condensation reaction resulting in the loss of a water molecule and formation of a covalent bond between the carboxyl group of one amino acid and the amino group of another.

Question 2

What is the primary reason the peptide bond is planar?

Accepted Answer

The partial double-bond character due to resonance between the carbonyl oxygen and the amide nitrogen restricts rotation, keeping six atoms in the same plane.

Question 3

Given the amino acid sequence Ser-Ala-Val, which end of the peptide is the N-terminus?

Accepted Answer

The end with the free amino group.

Question 4

Which amino acid is most likely to be found in the cis conformation of a peptide bond?

Accepted Answer

Proline

Question 5

A peptide bond is formed between which two groups of adjacent amino acids?

Accepted Answer

The α-carboxyl group of one amino acid and the α-amino group of another.

Question 6

What is the net charge of the EGAK tetrapeptide at pH = 0, given all ionizable groups are fully protonated?

Accepted Answer

$+2$

Question 7

How do you calculate the isoelectric point ($pI$) of a polypeptide with multiple ionizable groups?

Accepted Answer

Average the pKa values that correspond to the transition from positive to neutral and neutral to negative net charge.

Question 8

Which of the following best explains why the trans conformation of the peptide bond is generally favored over the cis conformation?

Accepted Answer

Trans conformation minimizes steric clashes between adjacent side chains.

Question 9

If a polypeptide has a $pI$ of 6.0, what will be its net charge at pH 8.0?

Accepted Answer

Net negative charge

Question 10

Which of the following is true regarding the titration curve of a polypeptide?

Accepted Answer

Each ionizable group contributes a distinct inflection point at its $pK_a$.

Question 11

Which of the following amino acid side chains is most likely to be ionized at physiological pH ($	ext{pH} = 7.4$)?

Accepted Answer

Aspartic acid (pKa $	ext{side chain} 	ext{COOH} 	ext{~} 4.0$)

Question 12

A peptide has the sequence Glu-Gly-Ala-Lys (EGAK). If the pKa values of its ionizable groups are 1.8, 4.2, 7.8, and 10, what is its $pI$?

Accepted Answer

$pI = \frac{4.2 + 7.8}{2} = 6.0$

Question 13

Which of the following is NOT a correct step in drawing a peptide in two dimensions?

Accepted Answer

Draw the backbone with all atoms in a tetrahedral geometry.

Question 14

Which of the following best describes the nomenclature for a tripeptide composed of serine, alanine, and valine?

Accepted Answer

Seryl-alanyl-valine

Question 15

Which property of the peptide bond is most important for protein secondary structure formation?

Accepted Answer

Its planarity and partial double-bond character, restricting rotation.

Question 16

If a peptide has a net positive charge at pH 2 and a net negative charge at pH 10, what can be inferred about its $pI$?

Accepted Answer

The $pI$ is between 2 and 10.

Question 17

Which of the following statements about the resonance structures of the peptide bond is correct?

Accepted Answer

Resonance gives the peptide bond partial double-bond character, restricting rotation.

Question 18

Which of the following best describes a zwitterion in the context of polypeptides?

Accepted Answer

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

Question 19

Which of the following is a correct statement about the titration curve of a protein with multiple ionizable groups?

Accepted Answer

The isoelectric point ($pI$) is the pH at which the net charge of the protein is zero.

Structure and Properties of Peptides: Biochemistry Study Notes

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