Structure
and Functions of Haemoglobin
Structure:
Haemoglobin is a
complex protein found in red blood cells (erythrocytes) and is primarily
responsible for transporting oxygen from the lungs to tissues throughout the
body. The structure of haemoglobin consists of four protein subunits, each containing
a heme group. Each heme group contains an iron atom at its center, which binds
to oxygen molecules. In humans, adult haemoglobin, known as haemoglobin A
(HbA), consists of two alpha globin chains and two beta globin chains, forming
a tetrameric structure.
Functions:
1. Oxygen Transport: Haemoglobin’s primary function is to transport
oxygen from the lungs to tissues and organs throughout the body. In the lungs,
oxygen binds to the iron atoms within the heme groups of haemoglobin molecules,
forming oxyhaemoglobin. This oxygenated haemoglobin then travels through the
bloodstream to tissues where it releases oxygen molecules, allowing cells to
undergo cellular respiration and generate energy (ATP).
2. Carbon Dioxide Transport: In addition to transporting oxygen, haemoglobin
also aids in the removal of carbon dioxide from tissues. Carbon dioxide
produced during cellular metabolism diffuses into the bloodstream and is
transported back to the lungs. In the blood, carbon dioxide binds to haemoglobin
and is carried back to the lungs where it is released and exhaled.
3. Buffering Capacity: Haemoglobin acts as a buffer against changes in
blood pH by binding to hydrogen ions (H+) released during cellular metabolism.
This helps maintain the blood's pH within a narrow range, ensuring optimal
conditions for cellular function and metabolic processes.
4. Regulation of Nitric Oxide: Haemoglobin also plays a role in
regulating the levels of nitric oxide (NO) in the bloodstream. Nitric oxide is
a vasodilator that helps relax blood vessels, improving blood flow and oxygen
delivery to tissues. Haemoglobin can bind to and scavenge nitric oxide,
modulating its availability and regulating vascular tone and blood pressure.
5. Heme Recycling: Haemoglobin turnover in the body involves the
breakdown of aged or damaged red blood cells by macrophages in the spleen and
liver. The heme groups released during this process are broken down further
into biliverdin, bilirubin, and iron. Bilirubin is then excreted by the liver
into bile, while iron is recycled and used for the synthesis of new haemoglobin
molecules.
Understanding the
structure and functions of haemoglobin is essential for comprehending oxygen
transport, carbon dioxide exchange, and the regulation of blood pH and vascular
tone. Disruptions in haemoglobin structure or function can lead to various
disorders, including anemia, hemoglobinopathies (such as sickle cell disease),
and conditions affecting oxygen delivery to tissues.
