Sum the masses of hemoglobin's four polypeptide chains

Steps:

• Hemoglobin consists of two alpha chains and two beta chains.
• From the table, add the masses: 2 × alpha mass + 2 × beta mass.
• Convert any Da values to kDa by dividing by 1000.
• Total mass = sum of chain masses in kDa (heme contribution is typically included or negligible for whole molecule approximation).

Why D is correct:

• 64.5 kDa matches the standard molecular weight of tetrameric hemoglobin (2α + 2β chains ≈ 64.5 kDa total).

Why the others are wrong:

• A: 31.6 kDa is roughly one chain doubled, ignoring full tetramer.
• B: 33.5 kDa underestimates by halving the tetramer or misadding chains.
• C: 62.6 kDa is close but misses precise chain masses or conversion.

Final answer: D