Hemoglobin A

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Hemoglobin A (HbA) is the main adult hemoglobin. It makes up about 97% of the hemoglobin in red blood cells and is built from four parts: two alpha chains and two beta chains (α2β2). Each chain carries a heme group with iron that grabs and releases oxygen as blood moves through the body. All four subunits are needed for normal oxygen transport.

There are a couple of other important adult hemoglobins:
- HbA2, made of two alpha and two delta chains (α2δ2), which accounts for about 1–3%.
- HbF is the fetal form (α2γ2) and is mostly replaced by HbA after birth.

How hemoglobin is made
- The heme part is created in mitochondria and the cell cytosol through several steps, ending with iron being inserted into a porphyrin ring to form heme.
- The globin protein parts are made on ribosomes in the cytosol. Two alpha chains and two other chains join with heme to form HbA.

Genes and location
- Alpha-globin genes are on chromosome 16.
- Beta-like globin genes are on chromosome 11.

When things go wrong
- Problems in making heme or globin can cause hemoglobin disorders (hemoglobinopathies).
- Alpha-thalassemia occurs when there isn’t enough alpha-globin. Severe cases can produce Hb Bart’s (a gamma tetramer) before birth, which can be fatal because it doesn’t carry oxygen well.
- Beta-thalassemia is caused by mutations in the beta-globin gene and can range from mild to life-threatening; many affected babies need regular blood transfusions.

Sickle cell disease
- The most common sickle variant, HbS, comes from a beta-globin mutation (glutamic acid to valine). Types include:
- HbSS: the most severe form
- HbSC: another form
- HbS with beta-thalassemia
- HbAS (sickle cell trait): usually healthy, but can cause problems under severe low-oxygen conditions
- People with sickle cell disease can have painful crises, anemia, and organ issues because misshapen red blood cells clog small vessels.

Treatment options
- Treatments vary and can include blood transfusions, bone marrow transplants, and therapies to manage symptoms and complications.

Bottom line: HbA is the most common adult form of hemoglobin and is essential for carrying oxygen from the lungs to tissues throughout the body.