Q.24 Which compound in red cells is ctucial in determining the oxygen affinity of haemoglobin ?
1. Inositol triphosphate
2. Glyceraldehyde-3-phosphate
3.2, 3- Bisphosphoglycerate
4. Phosphatidyl inositol
2,3-Bisphosphoglycerate (2,3-BPG) in red blood cells is the key allosteric regulator that decreases hemoglobin’s oxygen affinity, promoting O2 unloading in tissues.
Correct Answer
3. 2, 3- Bisphosphoglycerate
Option Analysis
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1. Inositol triphosphate: Incorrect. Inositol triphosphate (IP3) is a second messenger in signaling pathways, releasing Ca2+ from ER stores to trigger muscle contraction or secretion, unrelated to hemoglobin O2 binding.
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2. Glyceraldehyde-3-phosphate: Incorrect. This glycolysis intermediate precedes 1,3-BPG in the pathway; it does not bind hemoglobin or modulate O2 affinity, serving only as a metabolic precursor to 2,3-BPG via the Rapoport-Luebering shunt.
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3. 2, 3- Bisphosphoglycerate: Correct. 2,3-BPG binds deoxyhemoglobin’s central cavity (T-state), stabilizing low-affinity conformation via salt bridges with β-chain residues (His143, Lys82), shifting the O2 dissociation curve right (P50 ↑ from 26 to 33 mmHg).
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4. Phosphatidyl inositol: Incorrect. This membrane phospholipid generates IP3/DAG upon hydrolysis; it anchors proteins in RBC membranes but does not interact with hemoglobin or affect O2 affinity.
2,3-Bisphosphoglycerate hemoglobin oxygen affinity regulation in red cells ensures efficient O2 delivery. Produced via Rapoport-Luebering shunt, 2,3-BPG binds deoxy-Hb, lowering affinity and aiding unloading in tissues.
Mechanism of Action
2,3-BPG fits the deoxy-Hb β-chain cleft, forming ionic bonds that favor T-state (low affinity). This right-shifts the curve: at venous PO2 (40 mmHg), saturation drops from 75% to 25% without BPG. Fetal Hb (γ-chains) binds less, maintaining high affinity.
Physiological Regulation
Levels rise at altitude/hypoxia (↑ via pH drop, ↑ glycolysis), anemia, or stored blood; fall in alkalosis. Unlike inositol triphosphate (signaling) or glyceraldehyde-3-phosphate (glycolytic), only 2,3-BPG directly modulates Hb.
Clinical Relevance
↓2,3-BPG in banked blood impairs O2 delivery; ↑ aids high-altitude adaptation. Phosphatidyl inositol aids membrane integrity, not affinity.
