What differentiates fetal hemoglobin from adult hemoglobin?

Hemoglobin, usually written with the acronym Hb, is the protein responsible for transporting oxygen and carbon dioxide in erythrocytes or red blood cells. It is a protein with a quaternary structure formed by four globin subunits and four heme groups and a prosthetic group consisting of iron and porphyrin.

There are several types of globins that can be part of hemoglobin: alpha, beta, delta, gamma, epsilon and zeta globin, each encoded by a different gene. Except for the first few weeks of embryonic development, of the four globin subunits in each hemoglobin molecule, there are always two alpha and two non-alpha, resulting in the different types of hemoglobin that appear during fetal development. hemoglobin A).

structural differences

Hemoglobin A is adult hemoglobin. There are two types, hemoglobin A1 It is composed of two subunits of alpha globin and two subunits of beta globin.Hbα2β2). Hemoglobin A2 is Hbα2δ2 (2 alpha globins and 2 delta globins). Hemoglobin F: is the hemoglobin of the fetus and is composed of two subunits of alpha globin and two of gamma globin, it is Hbα2γ2. Hemoglobin E: is embryonic hemoglobin. There are four types, the most abundant being Gower’s hemoglobin 1 (HbE Gower 1) and consisting of two zeta globin chains and two epsilon globin chains (Hbζ2ε2). the Gower 2 is Hbα2ε2 (2 alpha, 2 epsilon). Hemoglobin also appears in the embryo. Portland 1 (Hbζ2γ2 or HbE Portland-1; has 2 zeta and 2 gamma globins) and Portland 2 (Hbζ2β2 or HbE Portland-2; has 2 zeta and 2 beta globins). ζ-chain (zeta) α-chain (alpha) ε-chain (epsilon) HbE Gower 1 HbE Gower 2 γ-chain (gamma) HbE Portland 1 HbF (fetal) β-chain (beta) HbE Portland 2 HbA1 (adult) δ-chain (delta) ) N/A HbA2 (minority adult)
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During the embryonic phase, approximately during the first two months of pregnancy, the most abundant hemoglobin is Gower’s hemoglobin 1, a type of hemoglobin exclusive to this phase. Low amounts of HbE Gower 2 and HbE Portland 1 and 2 also appear.

In the next 7 months, throughout fetal life, the most abundant hemoglobin is HbF. Low amounts of embryonic Gower 1 and Portland 1 and 2 hemoglobins also appear. At parturition, fetal hemoglobin represents 50% to 95% of the total hemoglobin.

Beginning approximately six months after birth, fetal hemoglobin levels drop rapidly and levels of hemoglobin A or adult hemoglobin increase until they become predominant. Around the year of life, hemoglobin A1 already represents about 95% of the total hemoglobin A2, about 3% and the rest low amounts of HbF.

functional differences

Beta and gamma subunits are very similar, although their small differences are enough to generate different conformations (spatial arrangement of globins) when comparing fetal hemoglobin and adult hemoglobin.

These conformational differences also cause functional differences. Hemoglobin’s main function is to transport oxygen, and for that, both HbA and HbF must have the proper affinity for oxygen that allows them to capture and release it as needed.

The developing fetus does not breathe air on its own; It totally depends on the oxygen present in the mother’s blood. HbF has a greater affinity for oxygen than HbA, which allows for the transfer of oxygen from the mother to the fetus. After birth, when the lungs begin to function, fetal hemoglobin levels drop rapidly and HbA levels rise, which has better characteristics for delivering oxygen to the tissues.

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Hereditary persistence of fetal hemoglobin

Hereditary persistence of fetal hemoglobin is a benign condition characterized by the presence of significant concentrations of HbF during adulthood. The cause is mutations in the gene cluster for alpha globins or beta globins.

The amount of HbF present can represent from 10% to almost 100% of the total hemoglobin, and it is generally higher in homozygous individuals than in heterozygotes, since the expression of the mutation is lower in these.

It is usually an asymptomatic condition and is only detected in tests for other pathologies related to hemoglobin.

HbF also improves symptoms of sickle cell anemia, and therefore sickle cell disease patients with inherited persistence of HbF have a less severe form of the disease.

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