Haemoglobin (AQA A-Level Biology): Revision Notes

Haemoglobin

Introduction

Haemoglobin is a group of chemically similar protein molecules found in many organisms, specialised for transporting oxygen. These molecules have evolved a highly efficient quaternary structure that allows them to load oxygen under certain conditions and unload it under different conditions.

Structure of haemoglobin molecules

Haemoglobin molecules demonstrate all four levels of protein structure, each contributing to their oxygen-carrying function:

• Primary structure refers to the sequence of amino acids within the four polypeptide chains that make up each haemoglobin molecule.
• Secondary structure occurs when each polypeptide chain coils into a helix formation, providing the basic three-dimensional shape for each chain.
• Tertiary structure involves each polypeptide chain folding into a precise, complex shape. This specific folding pattern is essential for the molecule's ability to bind with oxygen effectively.
• Quaternary structure represents the final level, where all four polypeptide chains link together to form an almost spherical molecule. Each polypeptide chain associates with a haem group containing a $Fe^{2+}$ ion. Since each $Fe^{2+}$ ion can combine with one oxygen molecule ($O\_2$), a single haemoglobin molecule can transport four $O\_2$ molecules in humans.

Loading and unloading oxygen

The transport of oxygen by haemoglobin involves two complementary processes:

1. Loading (or associating) describes when haemoglobin binds with oxygen. In humans, this process occurs primarily in the lungs where oxygen concentration is high.
2. Unloading (or dissociating) describes when haemoglobin releases its oxygen. This happens in body tissues where oxygen is needed for cellular respiration.

The efficiency of these processes depends on haemoglobin's affinity for oxygen. Molecules with high affinity readily take up oxygen but release it less easily, while those with low affinity take up oxygen less readily but release it more easily.

Environmental effects on oxygen transport

Haemoglobin exhibits a remarkable property where its oxygen affinity changes based on environmental conditions. The presence of carbon dioxide causes shape changes in the haemoglobin molecule, which affects how tightly it binds to oxygen.

Location	Oxygen concentration	Carbon dioxide concentration	Haemoglobin affinity	Result
Gas exchange surface	High	Low	High	Oxygen associates
Respiring tissues	Low	High	Low	Oxygen dissociates

At gas exchange surfaces (such as lungs), high oxygen and low carbon dioxide concentrations result in high affinity, promoting oxygen loading. In respiring tissues, low oxygen and high carbon dioxide concentrations reduce affinity, encouraging oxygen unloading exactly where it's needed.

Role of haemoglobin in oxygen transport

For haemoglobin to function effectively as an oxygen transporter, it must fulfil two key requirements:

• Readily associate with oxygen at surfaces where gas exchange occurs
• Readily dissociate from oxygen in tissues that require it for respiration

Although these requirements might seem contradictory, haemoglobin achieves both through its ability to change affinity based on local conditions, particularly the presence of carbon dioxide.

Variation in haemoglobin types

Different organisms possess haemoglobin molecules with varying oxygen affinities. This variation exists because each species has evolved a slightly different amino acid sequence in their haemoglobin. These sequence differences result in distinct tertiary and quaternary structures, leading to different oxygen-binding properties.

The range extends from haemoglobins with very high oxygen affinity to those with relatively low affinity. This diversity allows different species to optimise their oxygen transport according to their specific environmental conditions and metabolic requirements.

Key Points to Remember