A 40-year-old man is brought to the emergency department with headache and confusion after a house fire. He has cherry-red skin and a carboxyhemoglobin level of 30%. Which statement about carbon monoxide binding to hemoglobin is correct?
- A CO binds to the globin chain at the same site as 2,3-BPG
- B CO oxidizes heme iron to the ferric (Fe3+) state, forming methemoglobin
- C CO binds heme iron with approximately 200-fold greater affinity than oxygen and causes a leftward shift of the ODC ✓
- D CO binds only to the alpha chains, leaving beta chains free for oxygen transport
Correct answer: C. CO binds heme iron with approximately 200-fold greater affinity than oxygen and causes a leftward shift of the ODC
Explanation
CO binds to heme iron (Fe2+) with about 200-250 times greater affinity than oxygen. It also increases the affinity of the remaining heme sites for oxygen, causing a leftward shift of the ODC, which impairs tissue oxygen delivery. CO does not oxidize iron (that would form metHb) and binds to both alpha and beta chain heme groups.
Reference: Harrison's Principles of Internal Medicine, 21st ed.
High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP
Written and medically reviewed by the StethoPrep medical team.