Fetal Hemoglobin - Overview

Overview

Oxygenated blood is delivered to the fetus via the umbilical vein from the placenta, which is anchored to the wall of the mother's uterus. The chorion acts as a barrier between the maternal and fetal circulation so that there is no admixture of maternal and fetal blood. Blood in the maternal circulation is delivered via open ended arterioles to the intervillous space of the chorionic plate, where it bathes the chorionic villi that carry umbilical capillary beds, thereby allowing gas exchange to occur between the maternal and fetal circulation. Deoxygenated maternal blood drains into open ended intervillous venules to return to maternal circulation. Due to the admixture of oxygenated and deoxygenated blood, maternal blood in the intervillous space is lower in oxygen than arterial blood. As such, fetal hemoglobin must be able to bind oxygen with greater affinity than adult hemoglobin in order to compensate for the relatively lower oxygen tension of the maternal blood supplying the chorion.

Fetal hemoglobin's affinity for oxygen is substantially greater than that of adult hemoglobin. Notably, the P50 value for fetal hemoglobin (i.e., the partial pressure of oxygen at which the protein is 50% saturated; lower values indicate greater affinity) is roughly 19 mmHg, whereas adult hemoglobin has a value of approximately 26.8 mmHg. As a result, the so-called "oxygen saturation curve", which plots percent saturation vs. pO₂, is left-shifted for fetal hemoglobin in comparison to the same curve in adult hemoglobin.

This greater affinity for oxygen is explained by the lack of fetal hemoglobin's interaction with 2,3-bisphosphoglycerate (2,3-BPG or 2,3-DPG). In adult red blood cells, this substance decreases the affinity of hemoglobin for oxygen. 2,3 BPG is also present in fetal red blood cells, but interacts less efficiently with fetal hemoglobin than adult hemoglobin, due to a change in a single amino acid found in the 2,3 BPG 'binding pocket': from Histidine (positivity charged), interacts well with the negative charges found on the surface of 2,3 BPG) to serine (which has a neutrally charged side chain at physiological pH, and interacts less well) . This change results in 2,3 BPG binding less well to fetal Hb, and as a result, oxygen will bind to it with higher affinity than adult hemoglobin.

For mothers to deliver oxygen to a fetus, it is necessary for the fetal hemoglobin to extract oxygen from the maternal oxygenated hemoglobin across the placenta. This requires the fetal hemoglobin to have a higher oxygen affinity than that of the maternal carrier. This is achieved by a fetal hemoglobin subunit γ (gamma), instead of the b (beta) subunit. The γ subunit has fewer positive charges than the adult hemoglobin b subunit. This means that 2,3-BPG is less electrostatically bound to fetal hemoglobin as compared to adult hemoglobin. This means that 2,3-BPG is less effective in lowering the oxygen affinity of the fetal hemoglobin. This lowered affinity allows for adult hemoglobin (the maternal hemoglobin) to readily transfer its oxygen to the fetal hemoglobin.