Question 1

Which of the following statements best describes the structure of an $$ \alpha -$$amino acid at neutral pH?

Accepted Answer

The amino group is protonated and the carboxylic acid group is deprotonated, resulting in a zwitterion.

Question 2

Which amino acid does NOT have a chiral $$ \alpha -$$carbon?

Accepted Answer

Glycine

Question 3

L-alanine and D-alanine are examples of which type of stereoisomers?

Accepted Answer

Enantiomers

Question 4

Which of the following amino acids would absorb the most UV light at 280 nm?

Accepted Answer

Tryptophan

Question 5

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

Accepted Answer

Condensation (dehydration synthesis)

Question 6

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

Accepted Answer

It is coupled to ATP hydrolysis to proceed in cells.

Question 7

Which property of the peptide bond contributes most to its planarity and stability?

Accepted Answer

Delocalization of electrons (resonance) across the bond

Question 8

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 9

A solution contains a tetrapeptide with the sequence Glu-Gly-Ala-Lys. At pH 7, which of the following best describes its overall charge?

Accepted Answer

Neutral, as the charges of Glu and Lys cancel each other

Question 10

Which of the following modifications is commonly found at the N-terminus of peptides and can block further reactions at this end?

Accepted Answer

N-formyl group

Question 11

Which of the following statements about posttranslational modifications is TRUE?

Accepted Answer

They can regulate gene expression or suppression.

Question 12

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

Accepted Answer

Trypsin

Question 13

Given the following pK values for a hypothetical amino acid: $$ pK_1 = 2.0 $$ ($$ \alpha -$$COOH), $$ pK_2 = 9.0 $$ ($$ \alpha -NH$_2$$), and $$ pK_R = 4.0  ($$side chain), calculate the isoelectric point (pI) if the side chain is acidic.

Accepted Answer

$$ pI = \frac{2.0 + 4.0}{2} = 3.0 $$

Question 14

Which of the following best explains why peptide bonds are stable under physiological conditions?

Accepted Answer

They require strong acid and high temperature or a catalyst for hydrolysis.

Question 15

Which of the following amino acids is most likely to be involved in calcium binding due to posttranslational modification?

Accepted Answer

γ-carboxyglutamate

Question 16

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 17

Which of the following groups has a typical $$ pK $$ value in the range of 8.0–9.0?

Accepted Answer

α-Amino group

Question 18

A protein sample is quantified by measuring its UV absorbance. Which wavelength is most appropriate for this measurement, and which amino acids contribute most to the absorbance?

Accepted Answer

280 nm; tyrosine and tryptophan

Question 19

Which of the following best describes the effect of posttranslational modifications on protein function?

Accepted Answer

They can regulate protein activity, localization, and interactions.

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

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