<prompt> Histidine is the most impoant amino acid for buffering in normal conditions becauseIt regulates kidney acid-base balancePKa value is higher than pHPKa value is closer to pHPKa value is very low  <response>: 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. 