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What is so specific about fetal hb that is so different from adult hb?

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Fetal haemoglobin (HbF) has two alpha and two gamma subunits (nonionized serine). Adult haemoglobin (HbA) has two alpha and two beta subunits (positively charged histidine). HbA's histidine binds to negatively charged 2,3 DPG while the neutral serine in HbF has minimal interaction with 2,3 DPG. This gives fetal haemoglobin a greater affinity to oxygen than adult haemoglobin. Oxygen binds to fetal haemoglobin more easily and is more reluctant to let go. This is important, as fetal haemoglobin needs to “steal” oxygen away from the mother’s haemoglobin when nearby in the placenta. If the fetal and maternal haemoglobin had the same affinity for oxygen, there would be no incentive for the oxygen to switch from the maternal blood to fetal blood.

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