The oxygen dissociation curve of hemoglobin shows a sigmoidal shape reflecting cooperative binding. The Hill coefficient (n) of hemoglobin is approximately 2.8 (theoretical maximum for 4 subunits is 4). A Hill coefficient close to 1 would indicate what change in hemoglobin behavior?

• A Increased cooperativity — hemoglobin would saturate more rapidly at lower PO₂ but unload O₂ less efficiently at tissue PO₂
• B The Bohr effect would be eliminated because cooperativity and Bohr effect share the same allosteric mechanism
• C Loss of cooperativity — hemoglobin would behave like myoglobin with a hyperbolic O₂ dissociation curve, greatly impairing O₂ delivery to tissues ✓
• D Hemoglobin would become irreversibly locked in the R-state (oxy conformation) unable to release O₂

Explanation

The sigmoidal (cooperative) shape of the Hb-O₂ dissociation curve — driven by the T→R allosteric transition — means Hb loads efficiently at high pulmonary PO₂ (sigmoid plateau) and unloads efficiently at low tissue PO₂ (steep portion). A Hill coefficient approaching 1 would indicate loss of cooperativity — the curve becomes hyperbolic like myoglobin. Myoglobin would remain >90% saturated at tissue PO₂ of 20–40 mmHg, barely releasing O₂. A non-cooperative hemoglobin would be an extremely poor O₂ carrier, extracting very little O₂ from the lung-to-tissue gradient. The 2,3-BPG effect on the T-state and the Bohr effect are separate from cooperativity but share the allosteric framework.

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

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