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, iron is bound to 
Select and explain. 
A. Histidine, B. Leucine, C. Isoleucine, D. Valine
A: Histidine
Answer: A: Histidine 
Explanation: The iron atom of hemoglobin has six coordinated bonds : (i) Four bonds are formed between the iron and nitrogen atoms of porphyrin ring system; (ii) Fifth bond is formed between the nitrogen of histidine (proximal histidine: HisF8) of globin; and (iii) Sixth bond is formed with oxygen.