Molecule of the Month: Fetal Hemoglobin

Growing fetuses obtain oxygen and other essential nutrients from their mothers. This poses a special challenge. There needs to be a special mechanism to share oxygen from the mother’s blood to the fetus, or else the fetus will not receive enough for proper development. Fortunately, the fetus has something on its side–fetal hemoglobin! Fetal hemoglobin is the main oxygen-carrying protein in the red blood cells of human fetuses, and subtle differences between fetal hemoglobin and maternal hemoglobin enable the efficient exchange of oxygen from the mother to the fetus.

Fetal hemoglobin solves an essential dilemma: how is a hemoglobin molecule able to pass its oxygen to another hemoglobin molecule? Fetal hemoglobin achieves this feat by possessing a higher oxygen affinity than adult hemoglobin under normal, healthy conditions. During pregnancy, the mother's hemoglobin gives up oxygen, which crosses the placenta and is taken up by fetal hemoglobin. Fetal hemoglobin predominates during the last two trimesters of gestation, but by the end of the first year of life, our bodies make almost entirely adult hemoglobin and fetal hemoglobin is nearly absent.

Both fetal and adult hemoglobin are composed of four subunits: both have two identical alpha subunits, but in fetal hemoglobin, the two beta subunits found in adult hemoglobin are replaced with two gamma subunits (shown here from PDB entries 4hhb and 1fdh). These gamma subunits give fetal hemoglobin its increased affinity for oxygen. Red blood cells and placental cells make a small organic phosphate molecule called 2,3-BPG (2,3-bisphosphoglycerate). 2,3-BPG binds to adult hemoglobin and reduces its oxygen affinity, but it doesn’t bind strongly to fetal hemoglobin. So, 2,3-BPG causes adult hemoglobin to release its oxygen, allowing fetal hemoglobin to capture it. These differences in oxygen affinity work together to pass oxygen from the mother to the fetus.

Fetal hemoglobin is also being tested as a treatment for sickle cell disease. Sickle cell disease is caused by a mutation in the beta subunit that causes hemoglobin to form fibers that distort red blood cells into an abnormal sickle shape. Since fetal hemoglobin lacks the beta subunit, it lacks the ability to acquire this disease mutation. So, infants that carry the sickle cell gene don’t suffer from the symptoms of sickle cell disease, since they are making mostly fetal hemoglobin, but they start having problems when their bodies switch to adult hemoglobin later in childhood.

A possible treatment was discovered by looking at individuals with a rare genetic condition: Hereditary Persistence of Fetal Hemoglobin. They naturally have higher levels of fetal hemoglobin even when they are adults, and individuals that also have the sickle cell mutation have less severe forms of the disease. Studies have shown that a level of about 20% fetal hemoglobin in the blood is enough to reduce the symptoms of sickle cell disease, so researchers are using gene therapy to increase the levels of fetal hemoglobin in people that carry the sickle cell mutation. The treatment suppresses a silencer of the fetal hemoglobin gene, BCL11A (shown here from PDB ID 6ki6). This allows fetal hemoglobin to be made and has been recently shown to improve the prognosis of individuals with sickle cell disease.