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
[{'content': 'Histidine is the most impoant amino acid for buffering in normal conditions because \nSelect and explain. \nA. It regulates kidney acid-base balance, B. PKa value is higher than pH, C. PKa value is closer to pH, D. PKa value is very low', 'role': 'user'}, {'content': "Answer: C \nExplanation: Ans. is 'c' i.e., PKa value is closer to pH Maximal buffering capacity occurs at pH equal to pka of buffer.Therefore, to work as a best buffer at physiological pH amino acid should have pka value close to physiological pH (7.4).Amino acids can have buffering action due to three ionizable groups :a-carboxyl group :- Different amino acids have pka value of a-carboxyl group between 3.5-4. So, carboxyl group of amino acids has maximum buffering capacity between pH 3.5-4.a-amino group :- Different amino acids have pka value of a-amino group between 8.0-9.0. Thus, a-amino group has maximum buffering capacity between pH 8.0-9.0.Special ionizable group (in some amino acids):- Among special ionizable group of amino acids, imidazole group of histidine has pka value 6.5-7.4, which is closest to physiological pH. Hence, histidine (due to imidazole group) has maximum buffering capacity at physiologial pH.", 'role': 'assistant'}]