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
Which of the following is having Maximum buffering capacity 
Select and explain. 
A. Histidine, B. Cysteine, C. Tyrosine, D. Arginine
A: Histidine
Answer: A: Histidine 
Explanation: Answer-A- HistidineMaximum buffering capacity occurs at a pH equal to the pKa, So amino acid which has pKa range near physiologic pH can act as an effective buffer * lmidazole group of histidine - 6.5- 7.4 pKa range * SH group of cysteine- 8.5-9.0 pKa range * OH group of tyrosine - 9.5-10.5 pKa range* Arginine - > 12