The cooperative nature of oxygen binding to show that the change in shape…

The idea

Cooperative binding explains the sigmoid dissociation curve. In deoxygenated haemoglobin, binding the first oxygen molecule is difficult, making the curve shallow at low partial pressures. Binding this first oxygen alters the tertiary and quaternary structure of the protein. This conformational change increases the affinity of the remaining haem groups, so subsequent oxygen molecules bind much more easily, creating the steep middle section. As haemoglobin approaches full saturation, the curve levels off into a plateau because most binding sites are already occupied. This cooperative effect works in reverse: releasing the first oxygen molecule makes it easier for the remaining oxygens to dissociate during unloading in respiring tissues.

What earns the marks

• Stating that the first oxygen molecule binding causes a change in the tertiary or quaternary structure of haemoglobin.
• Explaining that this conformational change increases the affinity of the remaining haem groups, making further oxygen bind more easily.
• Linking this increased affinity to the steep middle section of the dissociation curve.
• Explaining the plateau at high partial pressures as the point where most binding sites are occupied and haemoglobin approaches full saturation.
• Applying the cooperative principle in reverse to explain rapid unloading of oxygen in respiring tissues.

Common mistakes and how to put them right

• Stating that affinity rises simply because there is more oxygen present, failing to mention the conformational shape change of the protein.

• Claiming all four oxygen molecules bind simultaneously rather than sequentially.

• Describing the haem group itself changing shape rather than the tertiary and quaternary structure of the whole protein.

• Explaining the plateau by stating the lungs run out of oxygen or that the final binding site is 'hard to find', rather than the sites simply becoming saturated.

• Using cooperative binding to explain the Bohr shift, which is a different mechanism.

Exam tips

• The key phrase examiners look for is that the change in shape of haemoglobin makes the binding of further oxygen easier.
• Match each part of your explanation to a specific region of the curve: shallow start, steep middle, and the saturation plateau.
• While you can name 'positive cooperativity', you must explicitly describe the change in tertiary or quaternary structure to secure the marks.