Haemoglobin A

Haemoglobin A (HbA) is the most common type of haemoglobin found in adults. It is a protein in red blood cells that carries oxygen from the lungs to the body's tissues and returns carbon dioxide from the tissues back to the lungs. Haemoglobin A is composed of two alpha (α) and two beta (β) globin chains, forming a heterotetramer (α2β2). This composition distinguishes it from other forms of haemoglobin, such as haemoglobin F (HbF), which is predominant in the fetus and newborns, and haemoglobin A2 (HbA2), which is present in small amounts in adults.

Structure and Function

The structure of haemoglobin A is crucial for its function in oxygen transport. Each of the four globin chains in HbA contains a heme group, which is a porphyrin ring with an iron (Fe) atom at its center. This iron atom binds one molecule of oxygen, allowing each haemoglobin molecule to carry up to four oxygen molecules. The binding of oxygen causes a conformational change in the haemoglobin molecule, enhancing its ability to pick up more oxygen at high oxygen concentrations (in the lungs) and release oxygen at lower concentrations (in the tissues).

Genetics

The genes encoding the alpha and beta chains of haemoglobin A are located on different chromosomes. The alpha globin genes are found on chromosome 16, while the beta globin genes are located on chromosome 11. Mutations in these genes can lead to various haemoglobinopathies, such as sickle cell disease and beta-thalassemia, which affect the structure and function of haemoglobin.

Clinical Significance

Haemoglobin A levels are measured as part of a complete blood count (CBC) and are important in diagnosing and managing various medical conditions. Abnormal levels can indicate diseases such as anemia, polycythemia, and haemoglobinopathies. Additionally, the glycated haemoglobin (HbA1c) test, which measures the percentage of haemoglobin A that is bound to glucose, is used to monitor long-term glucose control in diabetes mellitus.

Variants

There are several genetic variants of haemoglobin A, resulting from mutations in the alpha or beta globin genes. These variants can affect the oxygen-carrying capacity and stability of the haemoglobin molecule. Some common variants include haemoglobin S (HbS), which causes sickle cell disease, and haemoglobin C (HbC), which can cause a mild form of haemolytic anemia.

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