Question 1

Which of the following statements best explains why the peptide bond in proteins is described as 'planar and rigid'?

Accepted Answer

Because the peptide bond has partial double-bond character due to resonance, restricting rotation.

Question 2

A mutation replaces a glycine residue with a bulky tryptophan in the middle of an α-helix. What is the most likely effect on the protein structure?

Accepted Answer

The α-helix will be destabilized due to steric hindrance from the bulky side chain.

Question 3

Which of the following best describes the hydrogen bonding pattern in an α-helix?

Accepted Answer

Hydrogen bonds form between the carbonyl oxygen of residue $$i$$ and the amide hydrogen of residue $$i+4.$$

Question 4

Which of the following amino acids is most likely to disrupt an α-helix when present in the middle of the helix?

Accepted Answer

Proline

Question 5

A protein is denatured by treatment with urea and β-mercaptoethanol. What is the primary effect of β-mercaptoethanol in this process?

Accepted Answer

It breaks disulfide bonds.

Question 6

Which of the following best explains why globular proteins have a hydrophobic core and a hydrophilic surface?

Accepted Answer

Hydrophobic residues cluster in the interior to minimize contact with water, while hydrophilic residues interact with the aqueous environment.

Question 7

Which of the following is a key difference between parallel and antiparallel β-sheets?

Accepted Answer

Antiparallel β-sheets have linear hydrogen bonds, making them more stable than parallel β-sheets.

Question 8

A Ramachandran plot shows a cluster of points at $$\phi = -140^\circ$$ and $$\psi = +135^\circ. $$Which secondary structure is most likely present?

Accepted Answer

Antiparallel β-sheet

Question 9

Which of the following statements about collagen is FALSE?

Accepted Answer

Collagen is stabilized primarily by hydrogen bonds between backbone carbonyl and amide groups within a single chain.

Question 10

Which of the following best describes the driving force for spontaneous protein folding in aqueous solution?

Accepted Answer

Hydrophobic effect, which increases solvent entropy by burying nonpolar residues in the protein core

Question 11

Which of the following is a correct statement about quaternary structure?

Accepted Answer

It involves the association of two or more folded polypeptide chains into a functional complex.

Question 12

Which of the following is a real-life consequence of a mutation in the collagen gene that replaces glycine with another amino acid?

Accepted Answer

Development of osteogenesis imperfecta (brittle bone disease)

Question 13

Which of the following is the main stabilizing force in the tertiary structure of globular proteins?

Accepted Answer

Hydrophobic interactions among nonpolar side chains

Question 14

Which of the following best describes the amphipathic nature of an α-helix?

Accepted Answer

Hydrophobic and hydrophilic side chains are arranged such that one side of the helix is hydrophobic and the other is hydrophilic.

Question 15

Which of the following is a plausible reason why proline is often found in β-turns but not in α-helices?

Accepted Answer

Proline's rigid ring structure introduces a kink, which is compatible with the tight turn but disrupts the regular α-helix.

Question 16

Which of the following best explains why silk fibroin is strong but inelastic?

Accepted Answer

It is composed of antiparallel β-sheets with extensive hydrogen bonding and close packing of small side chains.

Question 17

Which of the following is a correct statement about the denaturation of proteins?

Accepted Answer

Denaturation disrupts non-covalent interactions, leading to loss of secondary, tertiary, and quaternary structure.

Question 18

Which of the following is a key advantage of quaternary structure in proteins such as hemoglobin?

Accepted Answer

It allows for cooperative binding of ligands, enhancing functional regulation.

Question 19

Which of the following best describes the outcome of the Anfinsen experiment with ribonuclease A?

Accepted Answer

The protein spontaneously refolded to its native structure after removal of denaturants, demonstrating that all folding information is encoded in the primary sequence.

Question 20

A patient with cystic fibrosis has a deletion of phenylalanine at position 508 in the CFTR protein. What is the molecular consequence of this mutation?

Accepted Answer

The protein is misfolded and retained in the endoplasmic reticulum, reducing chloride transport at the plasma membrane.

Three-Dimensional Structure of Proteins: Secondary, Tertiary, and Quaternary Organization

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