Question 1

Which of the following statements best describes the zwitterionic form of an amino acid at physiological pH?

Accepted Answer

The amino group is protonated and the carboxyl group is deprotonated, resulting in both positive and negative charges with a net zero charge.

Question 2

Which amino acid is NOT chiral and why?

Accepted Answer

Glycine, because its side chain is a hydrogen atom, making the alpha carbon attached to two identical groups.

Question 3

Which of the following amino acids contains a sulfur atom in its side chain and is classified as hydrophobic?

Accepted Answer

Methionine

Question 4

A protein sample absorbs strongly at 280 nm. Which amino acids are most likely responsible for this absorbance?

Accepted Answer

Phenylalanine, Tyrosine, and Tryptophan

Question 5

Which of the following amino acids is most likely to form a disulfide bond in an oxidizing environment?

Accepted Answer

Cysteine

Question 6

Given the following pKa values for glycine: $$pK_a$$^{COOH}$$ = 2.4$$, $$pK_a$$^{NH_3^+}$$ = 9.8$$, calculate the isoelectric point ($$pI) of $$glycine.

Accepted Answer

$$pI = \frac{2.4 + 9.8}{2} = 6.1$$

Question 7

Which amino acid is most likely to act as a nucleophile at physiological pH due to its imidazole side chain?

Accepted Answer

Histidine

Question 8

Which of the following modifications increases the negative charge of an amino acid side chain in a protein?

Accepted Answer

Phosphorylation of serine

Question 9

Which amino acid is most likely to be found in the interior of a globular protein due to its hydrophobic side chain?

Accepted Answer

Leucine

Question 10

Which of the following best explains why proline is often found in cis peptide bonds in proteins?

Accepted Answer

Its cyclic side chain restricts rotation around the peptide bond, making the cis conformation more favorable.

Question 11

Which amino acid has a side chain that can bind reactive metals due to its carboxyl group and is negatively charged at neutral pH?

Accepted Answer

Aspartate

Question 12

Which of the following amino acids is classified as aromatic and can participate in electron resonance?

Accepted Answer

Tryptophan

Question 13

A mutation replaces a glutamate residue with lysine in a protein. What is the most likely effect on the protein's net charge at pH 7?

Accepted Answer

The net charge will increase by +2, as glutamate is negatively charged and lysine is positively charged at pH 7.

Question 14

Which of the following amino acids is most likely to be involved in hydrogen bonding due to its amide side chain?

Accepted Answer

Asparagine

Question 15

Which property distinguishes L-amino acids from D-amino acids in biological systems?

Accepted Answer

L-amino acids are the predominant enantiomer incorporated into proteins in living organisms.

Question 16

Which of the following amino acids is most likely to act as a nucleophile only when its side chain is deprotonated at high pH?

Accepted Answer

Serine

Question 17

Which equation correctly represents the Beer-Lambert law used to quantify protein concentration using absorbance at 280 nm?

Accepted Answer

$$A = \varepsilon c l$$

Question 18

Which amino acid is most likely to be flexible in protein structures due to the absence of a side chain?

Accepted Answer

Glycine

Question 19

Which of the following amino acids is classified as basic and has the highest side chain pKa?

Accepted Answer

Arginine

Question 20

A solution contains an amino acid with three ionizable groups: $$pK_a^1$ = 2.0$$, $$pK_a^2$ = 6.0$$, and $$pK_a^3$ = 10.0. $$What is the $$pI of $$this amino acid if the side chain is neutral at physiological pH?

Accepted Answer

$$pI = \frac{2.0 + 10.0}{2} = 6.0$$

Structure and Properties of Amino Acids: Foundations for Biochemistry

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