This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: Haematology, Subject name: Anatomy
Hb is a good buffer because of 
Select and explain. 
A. Histidine residues, B. Protein nature, C. Acidic nature, D. Iron molecules
A: Histidine residues
Answer: A: Histidine residues 
Explanation: Ref Robbins 9/e p650 Haemoglobin also functions in CO2 and proton transpo from tissues to lungs . Release of O2 from Oxy Hb at the tissue is accompanied by uptake of proton due to lowering of the pKa of histamine residues