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: The heme has high affinity for carbon monoxide (about 25000 more than for 02). But when heme molecule is associated with Histidine E 7 (as occurs in myoglobin), its affinity for CO is drastically reduced because His E 7 creates a hindered environment for CO by disturbing the orientation of atoms in heme molecules.