In hemoglobin, the innate affinity of heme for carbon monoxide is diminished by the presence of :
**Core Concept**
Hemoglobin's ability to bind carbon monoxide (CO) is a critical aspect of its function, as CO can displace oxygen from the heme iron. The innate affinity of heme for CO is influenced by the presence of other substances that can bind to the heme group.
**Why the Correct Answer is Right**
The presence of oxygen (O2) is known to displace CO from the heme iron, thereby reducing the affinity of hemoglobin for CO. This phenomenon is known as the "oxygen-carbon monoxide competition" for the heme iron. The binding of oxygen to the heme iron causes a conformational change that makes it more difficult for CO to bind, thereby reducing its affinity. This is an important mechanism that helps protect the body from the toxic effects of CO.
**Why Each Wrong Option is Incorrect**
**Option A:** There is no known substance that increases the affinity of heme for CO in the presence of other molecules.
**Option B:** While histidine is an amino acid that is part of the hemoglobin molecule, it does not directly influence the affinity of heme for CO.
**Option C:** Deoxyhemoglobin (hemoglobin without oxygen bound) actually has a higher affinity for CO than oxyhemoglobin (hemoglobin with oxygen bound), not the other way around.
**Clinical Pearl / High-Yield Fact**
It's worth noting that carbon monoxide poisoning can occur when CO displaces oxygen from hemoglobin, leading to tissue hypoxia. This is often seen in cases of accidental inhalation of CO from vehicle exhaust or other sources.
**Correct Answer:** D. O2