Protein Structure — Hemoglobin's quaternary structure: identify the correct composition of its polypeptide chains.

Introduction:
Hemoglobin is a classic example of quaternary structure, where multiple polypeptide subunits assemble into a functional protein complex. Understanding its subunit stoichiometry is foundational for physiology and biochemistry.

Given Data / Assumptions:

• Adult human hemoglobin (HbA) is the common reference.
• Alpha and beta chains each bind a heme group.
• Function depends on cooperative interactions among subunits.

Concept / Approach:
Recall that adult HbA is a tetramer with 2 alpha and 2 beta chains (α2β2). Variants exist (e.g., fetal hemoglobin α2γ2), but stoichiometry remains four subunits.

Step-by-Step Solution:

Verification / Alternative check:
Classical structural studies and oxygen-binding behavior (sigmoidal curve) depend on a four-subunit assembly enabling cooperativity.

Why Other Options Are Wrong:

Common Pitfalls:
Confusing subunit count with heme count; each of the four chains carries one heme, totaling four heme groups per tetramer.

Final Answer:
α2β2 (four chains: two alpha and two beta).