Why is the oxygen dissociation curve S-shaped?

• A. Random binding of oxygen to subunits
• B. Cooperative binding of oxygen to the four subunits
• C. Oxygen binding is unaffected by subunit interactions
• D. Temperature effects influence the curve

Answer: (B)

The S-shaped curve arises from cooperative binding among hemoglobin’s four subunits. Each subunit can hold an oxygen molecule, and when the first oxygen binds, it triggers a conformational change in hemoglobin that increases the affinity of the remaining sites. This positive cooperativity makes it easier for subsequent oxygen molecules to bind, so the binding accelerates in the middle of the curve, producing the characteristic steep rise. As oxygen saturation nears its maximum, the curve levels off.

If binding were random or unaffected by subunit interactions, you’d expect a hyperbolic, not sigmoidal, curve. Temperature can affect affinity and the curve’s position, but the sigmoidal shape specifically reflects cooperative, allosteric interaction among the four subunits.