Haemoglobin and oxygen transportAQA A-Level Biology: Revision notes

Haemoglobins are a group of chemically similar proteins found in many different organisms. Each differs slightly in its amino acid sequence, so different haemoglobins have different affinities for oxygen. Human haemoglobin is a protein with quaternary structure: four polypeptide chains, each with a haem group containing an iron ion (Fe²⁺) that binds one oxygen molecule. One haemoglobin molecule can therefore carry up to four oxygen molecules.

Haemoglobin is packed inside red blood cells, which carry oxygen from the lungs to the respiring tissues. Oxygen combines with haemoglobin to form oxyhaemoglobin.

The partial pressure of oxygen (pO₂, in kPa) is a measure of oxygen concentration. The oxyhaemoglobin dissociation curve plots the percentage saturation of haemoglobin against pO₂.

• In the lungs (alveolar capillaries), pO₂ is high, so haemoglobin is almost fully saturated: oxygen is loaded.
• Oxyhaemoglobin is transported in the blood.
• In respiring tissues, pO₂ is low, so haemoglobin releases (unloads) oxygen, which diffuses into the cells for aerobic respiration.

The curve is S-shaped (sigmoid). The steep middle section means a small fall in pO₂ releases a large amount of oxygen.

The S-shape is explained by cooperative binding. At low pO₂ the first oxygen molecule binds with difficulty, giving a shallow start. Its binding changes the shape of the haemoglobin molecule, which makes it easier for the second and third oxygen molecules to bind, so the curve rises steeply. The fourth binds less easily because few sites remain, so the curve flattens at the top.

Respiring tissues release carbon dioxide, which dissolves to lower the pH of the blood. A higher carbon dioxide concentration reduces the affinity of haemoglobin for oxygen, because it changes the shape of the molecule. The curve shifts to the right, so haemoglobin unloads more oxygen at a given pO₂. This is the Bohr effect.

It is useful because the most active tissues (which make most carbon dioxide) receive most oxygen. In the lungs, the low carbon dioxide concentration raises affinity, which helps loading.

Different animals have haemoglobins with different oxygen transport properties.

• Higher affinity (curve to the left): animals at high altitude (such as the llama), or those living where oxygen is scarce, load more oxygen at low pO₂. The fetus has haemoglobin with higher affinity than its mother's so it takes oxygen from her blood in the placenta.
• Lower affinity (curve to the right): small animals with a high metabolic rate (such as a shrew) unload oxygen readily to supply fast aerobic respiration.

To link structure and function, remember that a different amino acid sequence gives a differently shaped haemoglobin with a different affinity.