Histidine is the most impoant amino acid for buffering in normal conditions because

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.