Hemoglobin's Quaternary Structure

Steps:

• Recall hemoglobin is a protein in red blood cells that transports oxygen.
• Identify its composition: consists of four polypeptide chains (two alpha and two beta subunits).
• Note each chain binds one heme group, totaling four heme groups.
• Confirm the chains differ in sequence, forming a tetramer.

Why D is correct:

• Hemoglobin has two alpha and two beta polypeptide chains (not identical) and four heme groups, as defined by its quaternary structure enabling cooperative oxygen binding.

Why the others are wrong:

• A: Incorrectly states only one heme group; hemoglobin has four.
• B: Wrong on identical polypeptides and single heme group; chains vary and there are four hemes.
• C: Falsely claims all four polypeptides are the same; they include distinct alpha and beta types.

Final answer: D