AP Biology: Unit 1, Macromolecules & Protein Structure (Drill 1)

Question 1. Based on the table, which of the following proteins has quaternary structure?

• A) Protein W only, because it is stabilized by hydrophobic interactions.
• B) Protein Y only, because it contains both alpha-helices and beta-pleated sheets.
• C) Proteins X and Z, because both consist of more than one polypeptide subunit. ✓
• D) All four proteins, because all proteins with secondary structure also have quaternary structure.

Explanation: The passage defines quaternary structure as applying to proteins with more than one polypeptide subunit. The table shows Protein X has 4 subunits and Protein Z has 2 subunits -- both qualify. Proteins W and Y each have only 1 polypeptide subunit and therefore lack quaternary structure. A and B incorrectly base quaternary structure on stabilizing interactions or secondary structure content rather than subunit number. D is incorrect -- quaternary structure is not a universal feature of all proteins; it requires multiple polypeptide subunits.

Question 2. Protein W functions as a membrane transport protein. Based on its predominant secondary structure and key stabilizing interactions, which of the following best explains how Protein W is suited for its function?

• A) Alpha-helices are rigid structures that prevent the protein from changing shape, making it ideal for structural support in membranes.
• B) Hydrophobic interactions between R groups allow Protein W to associate stably with the hydrophobic interior of the phospholipid bilayer, while alpha-helices are a common structural feature of transmembrane domains. ✓
• C) Disulfide bridges anchor Protein W to the extracellular surface of the membrane, which prevents the transport protein from diffusing laterally within the bilayer.
• D) Beta-pleated sheets can form pore-like structures in some membrane proteins, allowing solutes to pass through membrane channels formed by the protein.

Explanation: Many transmembrane proteins contain alpha-helical domains that span the hydrophobic core of the lipid bilayer. Hydrophobic R group interactions stabilize the protein within the bilayer interior. The passage and table both support this -- Protein W has alpha-helices as its predominant secondary structure and hydrophobic interactions as its key tertiary stabilizing force, both consistent with membrane embedding. A incorrectly describes alpha-helices as rigid and immutable. C describes disulfide bridges, which are not listed for Protein W. D does not apply to Protein W, which is described as alpha-helical and stabilized by hydrophobic interactions.

Question 3. A student argues that two proteins with identical primary structures must have identical functions. A researcher responds by describing a scenario in which the same polypeptide sequence folds differently depending on its cellular environment. Which of the following best evaluates the student's claim in light of the researcher's response?

• A) The student's claim is correct because the primary structure of a protein fully and rigidly determines its tertiary structure under all cellular conditions.
• B) The student's claim is correct because function depends only on the number of polypeptide subunits, not on folding.
• C) The student's claim is flawed because if the same primary structure can produce different tertiary structures in different environments, the resulting proteins may have different shapes and therefore different functions. ✓
• D) The student's claim is flawed because identical primary structures always produce different tertiary structures due to random variation in folding.

Explanation: Primary structure largely determines the potential folding of a protein, but environmental conditions -- temperature, pH, ionic concentration, chaperone proteins -- can influence how folding proceeds. If the same sequence folds into different conformations, the active site geometry or binding surfaces may differ, altering function. The researcher's scenario directly illustrates this possibility. A overstates the determinism of primary structure by ignoring environmental influences on folding. B incorrectly bases function on subunit number rather than three-dimensional shape. D overstates the claim in the opposite direction -- identical sequences do not always fold differently; the point is that they can under different conditions.

Question 4. A mutation changes a single amino acid in Protein Y, replacing a hydrophobic R group in the protein's interior with a hydrophilic R group. Which of the following best predicts the consequence of this mutation?

• A) The mutation will have no effect on the protein because single amino acid changes never alter the overall folding or function of any protein, even when the substituted amino acid differs in charge, size, polarity, or position within an important functional region for this protein.
• B) The mutation will increase the stability of Protein Y by adding a new hydrogen-bonding opportunity to the protein's interior.
• C) The mutation is likely to disrupt the tertiary structure of Protein Y because a hydrophobic R group in the interior is typically thermodynamically favorable, so replacing it with a hydrophilic R group disrupts stabilizing interactions and may cause misfolding or loss of function. ✓
• D) The mutation will convert Protein Y from an enzyme into a structural protein by altering its secondary structure.

Explanation: Protein folding is driven in part by the hydrophobic effect -- hydrophobic R groups are typically buried in the protein's interior away from water, where they form stabilizing hydrophobic interactions with other nonpolar residues. Replacing such a residue with a hydrophilic R group introduces a polar side chain into a nonpolar environment, disrupting those interactions and destabilizing the local fold, potentially causing misfolding or loss of enzymatic function. A is incorrect -- single amino acid changes can have profound effects on protein structure and function. B oversimplifies -- introducing a hydrophilic residue into a hydrophobic interior is more likely to destabilize than stabilize the structure. D incorrectly predicts a change in functional class based on a single amino acid substitution.

Question 5. The researcher argues that the diversity of protein functions seen in the table -- transport, structural support, enzyme activity, and hormone signaling -- is ultimately traceable to differences in amino acid sequence. Which of the following best supports this argument?

• A) All four proteins contain peptide bonds, and these peptide bonds directly determine each protein's function regardless of the amino acid sequence.
• B) Different amino acid sequences produce different three-dimensional shapes that determine how proteins interact with other molecules, accounting for their distinct functions. ✓
• C) Proteins with more polypeptide subunits are always more functionally complex than those with fewer subunits.
• D) The type of secondary structure -- alpha-helix vs. beta-pleated sheet -- directly determines protein function independent of amino acid sequence.

Explanation: The passage establishes the hierarchical relationship: primary structure (amino acid sequence) determines secondary and tertiary folding, and the resulting three-dimensional shape determines function. Different sequences produce different three-dimensional shapes that determine how proteins interact with substrates, membranes, receptors, or structural partners -- accounting for the functional diversity seen in the table. A is incorrect -- peptide bonds are universal to all proteins and do not differentiate function. C is incorrect -- subunit number does not determine functional complexity; many highly complex enzymes are monomers. D is incorrect -- secondary structure type is a consequence of sequence, and function depends on overall three-dimensional shape, not secondary structure type alone.