The Bohr effect describes the influence of pH and CO2 on haemoglobin oxygen affinity. At an exercising muscle where pH is low and PCO2 is high, the oxygen dissociation curve shifts RIGHT. The molecular basis is:

• A Low pH directly oxidises the haem iron from Fe2+ to Fe3+, reducing O2 binding
• B CO2 competes with O2 at the haem iron binding site
• C Protons and CO2 (as carbamino-CO2) bind to specific residues on deoxyhaemoglobin, stabilising the T (tense/deoxy) conformation and reducing O2 affinity ✓
• D Acid conditions denature the alpha-beta subunit interface, dissociating the tetramer into high-affinity monomers

Explanation

In the Bohr effect, protons protonate specific histidine residues (His-146 of beta-chain is key) and CO2 forms carbamino compounds at N-terminal amino groups, both stabilising the low-affinity T-state of deoxyhaemoglobin. This shifts the O2 dissociation curve rightward (lower O2 affinity), promoting O2 unloading at metabolically active tissues. The reverse occurs in the lungs (high pH, low PCO2), where the R-state is favoured, enhancing O2 loading — the physiological basis of the Bohr effect.

Reference: Harper's Illustrated Biochemistry, 32nd ed.

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