The oxygen dissociation curve of hemoglobin is sigmoidal, while that of myoglobin is hyperbolic. The sigmoidal shape of the hemoglobin curve reflects:

• A The presence of heme groups in hemoglobin but not myoglobin
• B The higher molecular weight of hemoglobin compared to myoglobin
• C Cooperative binding of oxygen due to the quaternary structure of hemoglobin ✓
• D Allosteric inhibition of hemoglobin by methemoglobin formation

Explanation

Hemoglobin is a tetramer (alpha2beta2) whose four subunits undergo conformational transitions between the low-affinity T (tense) state and high-affinity R (relaxed) state. Binding of one oxygen molecule shifts the equilibrium toward the R state, increasing the oxygen affinity of remaining subunits — a positive cooperative effect described by the Hill equation with a Hill coefficient near 2.8. This cooperativity produces the sigmoidal curve and allows hemoglobin to function efficiently as both an oxygen loader in the lungs and an oxygen deliverer in tissues.

Reference: Harper's Illustrated Biochemistry, 32nd ed.

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