3.4.3 The Role of Haemoglobin

Structure of Haemoglobin:

1. Composition:

• A water-soluble globular protein made of:
• Two alpha polypeptide chains.
• Two beta polypeptide chains.
• Each chain contains a haem group with an iron ion $(Fe²⁺)$.

2. Oxygen Binding:

• Each haemoglobin molecule can bind up to four oxygen molecules.

Oxygen Affinity and Partial Pressure ($pO₂$):

1. Definition of Partial Pressure:

• $pO₂$ is the concentration of oxygen in the surrounding environment.
• High $pO₂$: Found in the lungs.
• Low $pO₂$: Found in respiring tissues.

2. Loading and Unloading:

• Loading (Association):
• In the lungs, where $pO₂$ is high, haemoglobin binds oxygen tightly.
• Unloading (Dissociation):
• In respiring tissues, where $pO₂$ is low, haemoglobin releases oxygen to meet metabolic demands.

Oxygen Dissociation Curve:

1. Shape of the Curve:

• The curve is sigmoidal (S-shaped), showing the relationship between $pO₂$ and haemoglobin saturation.
• Initial Shallow Slope:
• The first oxygen molecule is difficult to bind.
• Steep Increase:
• Binding the first oxygen changes haemoglobin's shape, increasing its affinity for subsequent oxygen molecules (positive cooperativity).

• Flattened Slope:
• Saturation levels off as it becomes harder for the fourth oxygen molecule to bind.

2. Fetal Haemoglobin:

• Higher affinity for oxygen than adult haemoglobin.
• This ensures oxygen is efficiently absorbed from the mother's blood, even at low $pO₂$ in the placenta.

Bohr Effect:

1. Definition:

• The Bohr effect describes how carbon dioxide ($pCO₂$) affects oxygen binding.

2. Mechanism:

• High $pCO₂$ from respiring tissues lowers haemoglobin's affinity for oxygen.
• Carbon dioxide forms carbonic acid, causing a slightly acidic environment that changes haemoglobin's shape, facilitating oxygen release.

3. Impact:

• Ensures oxygen is delivered to actively respiring tissues that produce more carbon dioxide.

Key Adaptations of Haemoglobin:

1. High-Altitude Adaptation:

• Organisms in low-oxygen environments have haemoglobin with a higher affinity for oxygen to ensure effective loading at lower $pO₂$.

2. Active Organisms:

• Species with high metabolic rates often have haemoglobin with a lower affinity for oxygen, allowing easier unloading to supply respiring tissues.

3. Foetal Haemoglobin:

• Higher oxygen affinity compared to adult haemoglobin, ensuring oxygen transfer across the placenta.

Summary of Processes:

Process	Location	Description
Loading (Association)	Lungs	Oxygen binds to haemoglobin at high $pO₂.$
Transport	Bloodstream	Oxyhaemoglobin carries oxygen.
Unloading (Dissociation)	Respiring tissues	Oxygen is released at low $pO₂$or high $pCO₂.$

Key Terms:

• Haem Group: Contains $Fe²⁺,$ responsible for oxygen binding.
• Oxyhaemoglobin: Haemoglobin bound to oxygen.
• Positive Cooperativity: Easier oxygen binding after the first molecule.
• Bohr Effect: Reduced oxygen affinity in the presence of carbon dioxide.

Tip for Exams:

• Be able to describe and explain the oxygen dissociation curve, including adaptations like foetal haemoglobin and the Bohr effect.
• Link haemoglobin's structure to its function in oxygen transport and release.