The fetal haemoglobin (HbF) oxygen dissociation curve is shifted to the LEFT compared to adult HbF. The primary reason is:
- A HbF has a higher affinity for 2,3-DPG, reducing O2 release at tissues
- B HbF gamma-chains bind 2,3-DPG less avidly than adult beta-chains, keeping Hb in the R (oxy) state ✓
- C HbF has a higher Bohr effect, enhancing O2 uptake in the placenta
- D Fetal erythrocytes contain less carbonic anhydrase, reducing CO2-mediated O2 release
Correct answer: B. HbF gamma-chains bind 2,3-DPG less avidly than adult beta-chains, keeping Hb in the R (oxy) state
Explanation
HbF consists of two alpha and two gamma chains. The gamma-subunit has a serine (instead of histidine-143 in beta-chains) at the 2,3-DPG binding site, making gamma-chains far less able to bind 2,3-DPG. Since 2,3-DPG stabilises the deoxy (T) state of haemoglobin, reduced 2,3-DPG binding keeps HbF in the oxy (R) state with higher O2 affinity (P50 ~19 mmHg vs 27 mmHg for HbA). This left shift allows HbF to extract O2 from maternal HbA across the placenta (double Bohr effect).
Reference: Guyton & Hall, Textbook of Medical Physiology, 14th ed.
High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP
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