Fetal hemoglobin (HbF) has a higher oxygen affinity than adult HbA. The physiological basis for this difference and its functional significance in the fetus is:

• A HbF has a higher Bohr effect, releasing O2 more readily at lower pH in fetal tissues
• B HbF gamma chains bind 2,3-DPG less avidly than beta chains; reduced DPG binding shifts the ODC left, increasing O2 affinity and facilitating O2 transfer from maternal HbA to fetal HbF across the placenta ✓
• C HbF has more heme groups per molecule, increasing oxygen carrying capacity
• D HbF operates in an alkaline fetal environment that shifts the ODC leftward regardless of DPG

Explanation

HbF has gamma-globin chains replacing the beta-chains of adult HbA. The gamma chains interact less strongly with 2,3-bisphosphoglycerate (2,3-DPG) than beta chains, because 2,3-DPG binds in the central cavity between beta chains, and gamma chains have serine instead of histidine at position 143 (the key 2,3-DPG binding site). Reduced 2,3-DPG binding means less allosteric stabilization of the deoxy state, so HbF has higher O2 affinity (leftward shifted ODC, lower P50 ~19 mmHg vs ~27 mmHg for HbA). At the placenta, fetal blood with high O2 affinity extracts O2 from maternal blood of lower affinity—a double Bohr effect further facilitates O2 transfer. HbF has the same 4 heme groups as HbA.

Reference: Guyton & Hall, Textbook of Medical Physiology, 14th ed.

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