Question:medium
In hemoglobin, the innate affinity of Heme for carbon monoxide is diminished by the presence of
In hemoglobin, the innate affinity of Heme for carbon monoxide is diminished by the presence of
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The distal histidine (His E7) sterically hinders the linear binding geometry that carbon monoxide prefers.
Updated On: Jul 8, 2026
- His F-8
- His E-7
- Gly B-6
- Thr C-4
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The Correct Option is B
Solution and Explanation
Step 1: Understanding the Concept:
Free heme by itself binds carbon monoxide far more strongly than oxygen because carbon monoxide likes to sit at a straight 180 degree angle to the iron. Inside the hemoglobin molecule this huge preference is cut down a lot, and the question asks which amino acid residue is responsible.
Step 2: Key Formula or Approach:
Two histidines flank the heme iron in each globin chain: the proximal His F8, which bonds directly to iron and anchors the heme, and the distal His E7, which sits in the binding pocket but does not bond to iron directly. The distal residue is the one that shapes how a gas molecule can approach the iron.
Step 3: Detailed Explanation:
Because His E7 occupies space directly across the pocket, it physically blocks carbon monoxide from lining up straight with the iron, the geometry carbon monoxide prefers. Carbon monoxide is forced into a bent, less stable angle instead, which weakens its binding. Oxygen, which naturally prefers a bent angle anyway, is not hindered nearly as much. This steric effect from His E7 brings hemoglobin's preference for carbon monoxide over oxygen down from about 25,000 times (as in free heme) to about 200 times. His F8 is important for holding the heme in place but sits on the far side from the gas binding site, so it does not create this steric block. Gly B6 and Thr C4 are part of the protein scaffold and are not positioned to interfere with gas geometry at the iron.
Step 4: Final Answer:
His E7, the distal histidine, is the residue that lowers heme's natural affinity for carbon monoxide, option (2).
Free heme by itself binds carbon monoxide far more strongly than oxygen because carbon monoxide likes to sit at a straight 180 degree angle to the iron. Inside the hemoglobin molecule this huge preference is cut down a lot, and the question asks which amino acid residue is responsible.
Step 2: Key Formula or Approach:
Two histidines flank the heme iron in each globin chain: the proximal His F8, which bonds directly to iron and anchors the heme, and the distal His E7, which sits in the binding pocket but does not bond to iron directly. The distal residue is the one that shapes how a gas molecule can approach the iron.
Step 3: Detailed Explanation:
Because His E7 occupies space directly across the pocket, it physically blocks carbon monoxide from lining up straight with the iron, the geometry carbon monoxide prefers. Carbon monoxide is forced into a bent, less stable angle instead, which weakens its binding. Oxygen, which naturally prefers a bent angle anyway, is not hindered nearly as much. This steric effect from His E7 brings hemoglobin's preference for carbon monoxide over oxygen down from about 25,000 times (as in free heme) to about 200 times. His F8 is important for holding the heme in place but sits on the far side from the gas binding site, so it does not create this steric block. Gly B6 and Thr C4 are part of the protein scaffold and are not positioned to interfere with gas geometry at the iron.
Step 4: Final Answer:
His E7, the distal histidine, is the residue that lowers heme's natural affinity for carbon monoxide, option (2).
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