What has 210x the binding affinity for hemoglobin compared to oxygen?

• A. Nitrogen
• B. Carbon Monoxide
• C. Methane
• D. Oxygen

Answer: (B)

Binding affinity describes how tightly a molecule binds to a site, such as the heme iron in hemoglobin. Carbon monoxide binds to that iron about 210 times more strongly than oxygen does. Because of this, CO effectively occupies the same binding sites that oxygen would, forming carboxyhemoglobin and blocking oxygen binding and release where it’s needed. This results in a double hit: less oxygen is carried overall, and the remaining oxygen bound to the other hemoglobin sites is less readily released to tissues because CO shifts the oxygen dissociation curve to the left. The other options don’t fit because nitrogen and methane don’t bind hemoglobin in this context, and oxygen is the natural ligand that normally binds, not one that binds far more tightly.