Biochemistry—Protein Structure and Function Which of the following statements about common protein examples is INCORRECT? (Assume standard textbook definitions for Protein G, fatty-acid binding protein, and hemoglobin.)
-
AFatty acid binding protein is composed largely of β-sheet (β-barrel) structure.
-
BHemoglobin is a tetramer containing four polypeptide subunits.
-
CProtein G contains only α-helix and no β-sheet.
-
DProtein G contains both α-helix and β-sheet motifs.
-
EAll of the above are correct.
Answer
Correct Answer: Protein G contains only α-helix and no β-sheet.
Explanation
Introduction / Context:Proteins exhibit hierarchical structure (primary to quaternary) and recurring secondary motifs like α-helices and β-sheets. This question checks factual knowledge of well-characterized proteins: Protein G, fatty acid binding protein (FABP), and hemoglobin, and asks you to spot the single incorrect statement.
Given Data / Assumptions:
- Protein G (GB1 domain) is a small fold that includes both α-helix and β-strands.
- FABPs are classic β-barrel proteins, dominated by β-sheets.
- Hemoglobin is a tetramer (α2β2 in adult humans).
Concept / Approach:The strategy is to recall hallmark structural facts: (1) GB1 fold is βαβββ, not all-helix; (2) FABP is β-rich; (3) hemoglobin has four subunits and binds oxygen cooperatively via heme groups.
Step-by-Step Solution:
Step 1: Evaluate the FABP claim. FABP forms a β-barrel; thus “largely β-sheet” is correct.Step 2: Evaluate the hemoglobin claim. Adult Hb (HbA) contains four subunits; this is correct.Step 3: Evaluate the Protein G claims. Protein G (GB1) has one α-helix plus several β-strands; therefore the statement “only α-helix” is false, while “contains both α-helix and β-sheet” is true.Verification / Alternative check:Standard structural biology references and PDB entries for GB1, FABP, and Hb reinforce these canonical facts about their folds and quaternary states.
Why Other Options Are Wrong:
- FABP largely β-sheet: correct description of a β-barrel fatty-acid binder.
- Hemoglobin tetramer: correct for adult human hemoglobin.
- Protein G has both α and β: correct for GB1 fold.
- “All of the above are correct”: not true because one statement is incorrect.
Common Pitfalls:Confusing “only α-helix” motifs with mixed βα folds; assuming FABP is α-helical because many membrane proteins are helical (FABP is soluble and β-rich).
Final Answer:Protein G contains only α-helix and no β-sheet.