The sigmoidal oxygen binding curve of hemoglobin in the figure arises due to cooperativity, so the correct option is (C).

Introduction

Hemoglobin displays a characteristic sigmoidal oxygen binding curve when fractional saturation is plotted against oxygen pressure, unlike the hyperbolic curve of myoglobin. This S‑shaped oxygen binding profile of hemoglobin reflects positive cooperativity, a special mode of ligand binding in multimeric proteins that dramatically enhances oxygen loading in lungs and unloading in tissues.

Understanding the graph in the question

The given graph plots:

• X‑axis: O₂ pressure (in torrs)

• Y‑axis: Fraction saturation of hemoglobin with oxygen

The curve is:

• Shallow at low O₂ pressure (low affinity)

• Very steep in the middle region

• Plateauing near full saturation, producing an S‑shape

This sigmoidal oxygen binding curve is the hallmark of positive cooperativity, where binding of one O₂ molecule to hemoglobin increases the affinity of remaining sites for O₂.

Explanation of each option

Option (A) Quaternary structure

• Hemoglobin is a tetramer made of four polypeptide chains (usually two α and two β), giving it a quaternary structure.

• This quaternary arrangement allows subunits to communicate allosterically and is the structural basis that permits cooperativity, but the question asks what the profile “arises due to,” which is the functional phenomenon of cooperativity itself.

So, quaternary structure is necessary for the effect but not the most accurate direct cause of the sigmoidal binding profile; therefore option (A) is not the best answer.

Option (B) Subunit dissociation

• Under physiological conditions, hemoglobin functions as a stable tetramer and does not normally dissociate into independent subunits during routine oxygen binding and release.

• The oxygen binding curve’s shape is explained by changes in affinity of intact subunits within the tetramer, not by subunit dissociation, so this option is incorrect.

Option (C) Cooperativity (correct)

• Each hemoglobin molecule can bind up to four oxygen molecules; binding of the first O₂ triggers conformational changes that increase the affinity of remaining unoccupied heme sites, demonstrating positive cooperativity.

• This cooperative binding generates the sigmoidal oxygen binding curve: low affinity (T state) at low O₂ pressure, rapidly increasing affinity as more sites are occupied, and then a plateau as saturation is approached.

Therefore, the characteristic oxygen binding profile (S‑shaped curve) of hemoglobin arises due to cooperativity, making option (C) the correct choice.

Option (D) Conformational change

• Hemoglobin switches between a low‑affinity T (tense) state and a high‑affinity R (relaxed) state; oxygen binding induces conformational changes that underlie this T→R transition.

• These conformational changes are an essential molecular mechanism for cooperative binding but are not, by themselves, the full explanation of the profile; the curve is specifically described as cooperative binding behavior.

Thus, conformational change contributes mechanistically but the phenomenon named in the question is cooperativity; option (D) is less precise than option (C).

Why cooperativity gives a sigmoidal curve

• In deoxygenated blood, most hemoglobin is in the low‑affinity T state, so initial O₂ binding occurs slowly, giving a shallow slope at low pO₂.

• Binding of the first O₂ molecule promotes T→R state conversion and increases the affinity of the remaining subunits, producing a steep middle section of the curve as additional O₂ binds more easily.

• As most binding sites become occupied, the curve flattens toward a plateau because hemoglobin approaches full saturation, completing the S‑shape that is characteristic of cooperative ligand binding.

In summary, the oxygen binding profile of hemoglobin shown in the question arises due to cooperativity in oxygen binding among its subunits (option C).