This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: None, Subject name: Biochemistry
In hemoglobin, the innate affinity of heme for carbon monoxide is diminished by the presence of - 
Select and explain. 
A. His F8, B. His E7, C. Gly B6, D. Thr C4
B: His E7
Answer: B: His E7 
Explanation: Isolated heme binds carbon monoxide (CO) 25,000 times more strongly than oxygen. However, in myoglobin and hemoglobin, heme binds carbon monoxide to only about 200 times more strongly than oxygen.
This is because of distal histidine (His E7 of protein part of hemoglobin and myoglobin create a hindered environment which precludes the high-affinity orientation of CO.
The details of this mechanism are not required at this level.