Haemoglobin

Composition:

• Haemoglobin is a protein made up of four polypeptide chains, each containing a haem group.

• The haem group includes an iron ion, allowing each haemoglobin molecule to carry up to four oxygen molecules.

Oxygen Transport:

• The concentration of oxygen in air or solution is expressed as the partial pressure of oxygen (pO2).

• Oxygen saturation of haemoglobin depends on pO2:

  • High pO2 (e.g., in blood near the lungs): Haemoglobin binds oxygen.

  • Low pO2 (e.g., in blood near respiring tissues): Haemoglobin releases oxygen.

Cooperative Binding and the Dissociation Curve

Cooperative Binding:

• When one oxygen molecule binds to haemoglobin, the protein undergoes a shape change due to alterations in its tertiary structure.

• This makes it progressively easier for the second and third oxygen molecules to bind.

• However, binding the fourth oxygen molecule becomes more difficult since there is now a low probability an oxygen molecule will colide successfully with the remaining binding site.

Dissociation Curve:

• The binding behavior produces a characteristic S-shaped (sigmoidal) dissociation curve.

• This curve represents the average oxygen dissociation across multiple haemoglobin molecules.

Haemoglobin oxygen dissociation curve.

The Bohr Effect: Impact of Carbon Dioxide

Role of pCO2 (Partial Pressure of Carbon Dioxide):

• Low pCO2: Haemoglobin has a higher affinity for oxygen, shifting the dissociation curve left.

• High pCO2: Haemoglobin’s affinity for oxygen decreases, shifting the curve right.

Adaptation to Respiration:

• In the lungs (low pCO2): Haemoglobin binds oxygen more effectively.

• In respiring tissues (high pCO2): Oxygen is released more readily.

• This phenomenon is known as the Bohr effect.

The Bohr effect on oxygen dissociation.

Adaptations in Haemoglobin Affinity

Environmental Adaptations:

• Organisms in low pO2 environments (e.g., high altitudes, aquatic habitats):

  • Haemoglobin has a higher affinity for oxygen (curve shifts left), aiding oxygen capture.

• Organisms with high metabolic rates (e.g., small mammals, active birds):

  • Haemoglobin has a lower affinity for oxygen (curve shifts right), facilitating oxygen delivery to tissues.

Structural Variations:

• Haemoglobin’s affinity for oxygen is influenced by its quaternary and tertiary structures, allowing for species-specific adaptations.

Oxygen dissociation in different species.

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