Hb’s capacity to bind and carry oxygen in the presence of carbon monoxide is
[ Select ]
decreased
increased
remains unchanged
because CO binds to
[ Select ]
globin chains (allosteric binding sites)
heme (oxygen’s primary binding site)
.

Question

Hb’s capacity to bind and carry oxygen in the presence of carbon monoxide is 
[ Select ]
decreased
increased
remains unchanged
 because CO binds to  
[ Select ]
globin chains (allosteric binding sites)
heme (oxygen’s primary binding site)
 .

BlackTom AI · Accepted Answer

Carbon monoxide (CO) binds to the heme iron with much greater affinity than oxygen, so hemoglobin’s oxygen-carrying capacity is decreased when CO is present.

1) Relevant knowledge points and concepts
- Hemoglobin (Hb) contains heme prosthetic groups (each heme has an Fe2+ atom) that are the primary binding sites for O2.
- CO competes directly with O2 for the same heme iron and forms carboxyhemoglobin (HbCO).
- CO affinity for heme is roughly 200 times (commonly cited 200–250×) greater than O2. CO occupancy both removes binding sites and increases the affinity of remaining heme sites for O2 (left shift), impairing O2 unloading to tissues.

2) Step-by-step reasoning
- Because CO and O2 bind the same site (heme), they compete. High-affinity CO preferentially occupies heme even at low partial pressures.
- Occupied heme by CO cannot bind O2; therefore total O2 content carried by Hb falls.
- Additionally, CO-bound Hb causes a conformational change that increases the O2 affinity of the remaining unoccupied heme groups, making it harder to release O2 to tissues.

3) Simple illustrative calculation (intermediate steps)
- Model relative occupancy proportional to (affinity × partial pressure).
- Let O2 affinity = 1, pO2 = 100 mmHg → O2 term = 1 × 100 = 100.
- Let CO affinity = 200, pCO = 0.5 mmHg → CO term = 200 × 0.5 = 100.
- Relative contributions are equal (100 vs. 100) → roughly 50% of heme sites would be CO-bound despite much lower pCO. This demonstrates how small amounts of CO markedly reduce available O2-binding sites.

4) Why the correct answer is correct
- CO binds heme (oxygen’s primary binding site) with very high affinity, directly preventing O2 binding and impairing O2 release, so Hb’s capacity to bind and carry O2 is decreased.

5) Why other options are incorrect
- “Increased” is wrong because CO does not enhance O2 binding or transport; it reduces available sites and inhibits unloading.
- “Remains unchanged” is wrong because CO occupancy directly reduces functional Hb available for O2.
- “Globin chains (allosteric binding sites)” is incorrect because CO binds the heme iron, not distant allosteric globin sites.

6) Conclusion
These points confirm the provided answers: decreased; heme (oxygen’s primary binding site).

Hb’s capacity to bind and carry oxygen in the presence of carbon monoxide is [ Select ] decreased increased remains unchanged because CO binds to [ Select ] globin chains (allosteric binding sites) heme (oxygen’s primary binding site) .多重下拉选择题

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