Why is haemoglobin so good at carrying oxygen, and why is carbon monoxide so deadly?
Haemoglobin holds an Fe²⁺ ion in a porphyrin ring; oxygen binds reversibly to this iron centre as a ligand, so blood can pick up O₂ in the lungs and release it in the tissues.
Haemoglobin holds an Fe²⁺ ion in a porphyrin ring; oxygen binds reversibly to this iron centre as a ligand, so blood can pick up O₂ in the lungs and release it in the tissues. Carbon monoxide binds to the same iron site but around 200–250 times more strongly and won't let go, so it blocks oxygen transport — that's why CO is a silent, lethal poison.
Key point
O₂ binds reversibly to the Fe centre of haemoglobin; CO binds the same site far more tightly and blocks it.
Common mistake
thinking CO poisons by being toxic itself rather than by out-competing O₂.
Memory tip
CO hogs haemoglobin's iron site → no oxygen delivery.
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