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 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 structure of the peptide bond?

Accepted Answer

It is planar and stable due to electron delocalization.

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), what is the isoelectric point ($$pI) of $$this amino acid?

Accepted Answer

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

Question 7

Which of the following modifications is most likely to play a role in stabilizing the structure of collagen?

Accepted Answer

4-Hydroxyproline

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, high temperature, 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-Ala-Lys-Gly. At pH 7, what is the overall charge on this peptide? (Assume N-terminus and C-terminus are free, and side chain $$pK_a$$ values: Glu = 4.2, Lys = 10.5)

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 posttranslational modification is most likely to be involved in gene expression or suppression?

Accepted Answer

N-acetyllysine

Question 14

Which of the following is a correct statement about the stereochemistry of amino acids found in proteins?

Accepted Answer

All amino acids in proteins are L-enantiomers except glycine, which is achiral.

Question 15

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

Accepted Answer

α-Amino group

Question 16

A protein sample is contaminated with nucleic acids. Which wavelength would best distinguish protein from nucleic acid contamination using UV spectroscopy?

Accepted Answer

280 nm

Question 17

Which of the following modifications would most likely block the N-terminus of a peptide?

Accepted Answer

N-formyl group

Question 18

Which of the following best explains why peptide bond formation requires coupling to ATP hydrolysis in cells?

Accepted Answer

Peptide bond formation is thermodynamically unfavorable and requires energy input.

Question 19

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

Accepted Answer

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

Amino Acids and Peptides: Structure, Properties, and Reactions

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