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.)

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:

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.