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 side chain is a hydrogen atom, so its alpha carbon is not attached to four different groups.

Question 2

At neutral pH, what is the predominant form of an α-amino acid?

Accepted Answer

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

Question 3

Which amino acids are primarily responsible for the strong UV absorbance of proteins at 280 nm?

Accepted Answer

Tyrosine and tryptophan

Question 4

A peptide bond forms between two amino acids through which type of reaction?

Accepted Answer

Condensation (dehydration synthesis)

Question 5

Why is peptide bond formation not thermodynamically favorable under standard conditions?

Accepted Answer

It is coupled to ATP hydrolysis in cells to drive the reaction forward.

Question 6

Which of the following best describes the structure of the peptide bond?

Accepted Answer

It is planar and stable due to electron delocalization.

Question 7

A solution contains a tetrapeptide. As the pH is increased, what happens to the overall charge of the peptide?

Accepted Answer

It becomes more negative.

Question 8

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

Accepted Answer

N-acetylation of lysine

Question 9

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), what is the isoelectric point ($pI$) if the side chain is neutral at $pH = 6.0$?

Accepted Answer

$pI = \dfrac{2.0 + 9.0}{2} = 5.5$

Question 10

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

Accepted Answer

Trypsin

Question 11

Which of the following best describes the difference between oligopeptides and polypeptides?

Accepted Answer

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

Question 12

Which of the following posttranslational modifications is important for stabilizing the structure of collagen?

Accepted Answer

Hydroxylation of proline

Question 13

If a protein solution is analyzed by UV spectroscopy, which wavelength would you use to specifically quantify protein concentration, and why?

Accepted Answer

280 nm, because aromatic amino acids absorb strongly here

Question 14

Which of the following statements about peptide bond hydrolysis is correct?

Accepted Answer

Peptide bonds are stable unless exposed to strong acid at high temperature or a catalyst.

Question 15

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

Accepted Answer

α-Amino group

Question 16

A peptide has the following sequence: Ala-Gly-Lys-Glu. At pH 7, what is the net charge on the peptide?

Accepted Answer

+1

Question 17

Which of the following modifications can block the N-terminus of a peptide?

Accepted Answer

N-formyl group

Question 18

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

Accepted Answer

Its planarity and partial double-bond character

Question 19

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

Accepted Answer

The overall charge becomes more positive.

Question 20

Which of the following is true regarding L- and D-alanine?

Accepted Answer

They are enantiomers and mirror images of each other.

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

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