Hemoglobin fetal, oxygen affinity

A similar adaptive mutation is found in human fetal hemoglobin. This has a lower oxygen affinity than the adult in the absence of phosphate, but it also has a lower affinity for DPG, due mainly to the replacement His H21(143)/3 — Ser. As a result, the oxygen affinity of fetal blood exceeds that of the adult, which helps the transfer of oxygen from the maternal to the fetal circulation across the placenta (5). 

Figure 10.24. Oxygen Affinity of Fetal Red Blood Cells. Fetal red blood cells have a higher oxygen affinity than that of maternal red blood cells because fetal hemoglobin does not bind 2,3-BPG as well as maternal hemoglobin does.

Fetal hemoglobin is composed of different subunits than adult hemoglobin and binds BPG less strongly. As a result, the affinity of fetal hemoglobin for oxygen is higher, and the fetus can extract the O2 that is transported in maternal blood. 

The quantity P50, a measure of O2 concentration, is the partial pressure of Oj that half-saturates a given hemoglobin. Depending on the organism, P50 can vary widely, but in all instances it will exceed the PO2 of the peripheral tissues. For example, values of P50 for HbA and fetal HbF are 26 and 20 mm Hg, respectively. In the placenta, this difference enables HbF to extract oxygen from the HbA in the mother s blood. However, HbF is suboptimal postpartum since its high affinity for O2 dictates that it can deliver less Oj to the tissues. 

HbA now has a greater affinity for oxygen than does HbF. When BPG is reintroduced, the 02-saturation curves return to normal, as shown in the graph. What is the effect of BPG on the 02 affinity of hemoglobin How can the above information be used to explain the different 02 affinities of fetal and maternal hemoglobin ... 

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