Fetal haemoglobin (HbF, α2γ2) has a higher oxygen affinity than adult HbA. This is because:

• A HbF has a higher concentration of iron in the haem group
• B HbF has more alpha chains than HbA
• C Fetal blood has a lower pH that shifts the curve left
• D HbF binds 2,3-DPG less avidly than HbA, resulting in a left-shifted oxygen-haemoglobin dissociation curve ✓

Explanation

HbF (α2γ2) binds 2,3-DPG much less avidly than HbA (α2β2) because the γ-globin chain has a serine instead of histidine at position 143, reducing 2,3-DPG binding. Since 2,3-DPG stabilizes the T-state (deoxy) conformation of haemoglobin and reduces O2 affinity, HbF — with less 2,3-DPG effect — remains in the R-state (oxy) configuration more readily, left-shifting its dissociation curve. This enables HbF to extract oxygen from maternal HbA across the placenta (Bohr/Haldane effects facilitate this double Bohr effect).

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

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Written and medically reviewed by the StethoPrep medical team.