Q.36

Which of the following condition(s) lead(s) to reduced affinity of O2 for human

hemoglobin?

(A) Reduction of pH of blood plasma from pH 7.4 to 7.2

(B) Decrease of partial pressure of CO2 in the lungs from 6 to 2 kPa

(C) Enhancement of intracellular 2, 3-bis phosphoglycerate (BPG) level from 5 to 8 mM

(D) Increase in ambient CO level from 1 to 600 ppm

Reduced hemoglobin oxygen affinity occurs under conditions like low pH or high BPG levels, shifting the oxygen dissociation curve rightward via the Bohr effect. This article explains the correct answers for the MCQ on conditions lowering O2 affinity for hemoglobin, targeting biology students and researchers.

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Correct Answer

Options (A) and (C) lead to reduced affinity of O2 for human hemoglobin. Lower pH from 7.4 to 7.2 triggers the Bohr effect, stabilizing deoxyhemoglobin and promoting O2 release in tissues. Increased BPG from 5 to 8 mM binds deoxyhemoglobin, further decreasing O2 affinity to enhance unloading at active tissues.

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Option A: pH Reduction

Reduction of blood plasma pH from 7.4 to 7.2 decreases hemoglobin’s O2 affinity through protonation of key residues, favoring the T-state conformation. This Bohr effect is crucial in metabolically active tissues producing lactic acid or CO2, ensuring efficient oxygen delivery.

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Option B: CO2 Partial Pressure Drop

Decreasing lung CO2 partial pressure from 6 to 2 kPa increases O2 affinity, as lower PCO2 raises pH and shifts the dissociation curve leftward. This opposes oxygen unloading, making (B) incorrect for reduced affinity.

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Option C: Elevated BPG Levels

Enhancing intracellular 2,3-bisphosphoglycerate (BPG) from 5 to 8 mM reduces O2 affinity by stabilizing the deoxy form via electrostatic interactions in the central cavity. BPG levels rise in hypoxia, adapting hemoglobin for better O2 release at high altitudes.

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Option D: Ambient CO Increase

Increasing CO from 1 to 600 ppm forms carboxyhemoglobin, which has higher O2 affinity than oxyhemoglobin, impairing release rather than reducing binding. CO competes with O2 at heme sites but stabilizes the R-state, so (D) does not reduce affinity.

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