Which statement correctly describes carbon monoxide's affinity for hemoglobin?

Which statement correctly describes carbon monoxide's affinity for hemoglobin?

• A. It binds less tightly than oxygen.
• B. It binds with about equal affinity to oxygen.
• C. It binds about 200 times more readily than oxygen.
• D. It does not bind to hemoglobin.

Answer: (C)

Carbon monoxide binds to hemoglobin far more tightly than oxygen. It forms carboxyhemoglobin, occupying binding sites so oxygen cannot attach where it’s needed. The affinity is about 200 times greater than that of oxygen, so CO quickly outcompetes O2 for Hb binding. This not only reduces the amount of oxygen carried in the blood, but also makes the remaining oxygen bind more tightly to the other heme sites, shifting the oxygen dissociation curve to the left and hindering oxygen release to tissues. That combination leads to tissue hypoxia even when oxygen levels are normal. The other statements don’t fit because CO does bind to hemoglobin, and it binds much more tightly, not less or equally, to Hb.

Carbon monoxide binds to hemoglobin far more tightly than oxygen. It forms carboxyhemoglobin, occupying binding sites so oxygen cannot attach where it’s needed. The affinity is about 200 times greater than that of oxygen, so CO quickly outcompetes O2 for Hb binding. This not only reduces the amount of oxygen carried in the blood, but also makes the remaining oxygen bind more tightly to the other heme sites, shifting the oxygen dissociation curve to the left and hindering oxygen release to tissues. That combination leads to tissue hypoxia even when oxygen levels are normal. The other statements don’t fit because CO does bind to hemoglobin, and it binds much more tightly, not less or equally, to Hb.