Question 1

Which of the following statements best describes the behavior of glycine during titration as pH increases from 1 to 12?

Accepted Answer

The α-carboxylate group deprotonates first, followed by the α-amine group, resulting in a zwitterion at intermediate pH.

Question 2

What is the net charge of glycine at pH 6, given its pKa values ($$pK_{a,\,\alpha	ext{-COOH}} = 2.3$$, $$pK_{a,\,\alpha	ext{-NH}_3$$^+} = 9.6$$)?

Accepted Answer

Net charge is 0 (zwitterion)

Question 3

Calculate the isoelectric point ($$pI) of $$glycine using its pKa values ($$pK_{a,\,\alpha	ext{-COOH}} = 2.3$$, $$pK_{a,\,\alpha	ext{-NH}_3$$^+} = 9.6$$).

Accepted Answer

$$pI = \frac{2.3 + 9.6}{2} = 5.95$$

Question 4

Aspartic acid has three ionizable groups with pKa values of $$2.1$$, $$3.9$$, and $$9.8. $$What is the isoelectric point ($$pI) $$for aspartic acid?

Accepted Answer

$$pI = \frac{2.1 + 3.9}{2} = 3.0$$

Question 5

Which equation is used to relate pH, pKa, and the ratio of deprotonated to protonated forms in acid-base titrations?

Accepted Answer

$$pH = pK_a + \log \left( \frac{[A^-]}{[HA]} \right)$$

Question 6

A protein is most soluble in water when:

Accepted Answer

The protein has a net charge (pH above or below its $$pI)$$

Question 7

Which amino acid is NOT chiral?

Accepted Answer

Glycine

Question 8

Why are L-amino acids used almost exclusively in natural proteins?

Accepted Answer

L-amino acids allow proteins to adopt a well-defined active structure and promote specific molecular recognition

Question 9

Which amino acid is classified as a nonpolar, aromatic amino acid?

Accepted Answer

Phenylalanine

Question 10

Which amino acid side chain is most likely to form disulfide bonds, stabilizing protein structure?

Accepted Answer

Cysteine

Question 11

A mutation in the beta-globin gene replaces a polar glutamic acid with a nonpolar valine. What is the most likely consequence for hemoglobin function?

Accepted Answer

A hydrophobic sticky patch forms, leading to aggregation and sickle-cell anemia

Question 12

Which of the following best describes the peptide bond between amino acids?

Accepted Answer

It is an amide bond with partial double-bond character, restricting rotation

Question 13

Why is peptide bond formation energetically unfavorable, and why does hydrolysis not occur spontaneously under physiological conditions?

Accepted Answer

Peptide bond formation requires ATP input, and hydrolysis is kinetically unfavorable due to high activation energy

Question 14

Which of the following amino acids is most likely to be found on the surface of a protein, interacting with water?

Accepted Answer

Serine

Question 15

Which post-translational modification is critical for cell signaling and involves the addition of a phosphate group?

Accepted Answer

Phosphorylation

Question 16

Which rare amino acid is encoded by the UGA stop codon and is important in oxidative and calcium homeostasis?

Accepted Answer

Selenocysteine

Question 17

Which of the following best explains why proteins with similar amino acid sequences in different species can have the same biological function?

Accepted Answer

Their sequence similarity is sufficient to preserve the structure and function despite evolutionary divergence

Question 18

Which amino acid is classified as a basic amino acid and is often involved in proton transfers during enzymatic catalysis due to its side chain pKa near physiological pH?

Accepted Answer

Histidine

Question 19

Which of the following is a consequence of the use of only L-amino acids in protein synthesis?

Accepted Answer

Protein surfaces are asymmetric, enabling highly specific molecular recognition

Introduction to Proteins: The Primary Level of Protein Structure

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