<prompt> Amino acid with dissociation constant closest to physiological pH is -SerineProlineHistidineThreonine  <response>: Each amino acid have two ionizable group: r c-carboxyl group -COOH, and ii c-amino group -NHr. Beside these two groups, some amino acids have special ionizable groups. e.g. -SH of cysteine, imidazole of histidine. Dissociation constant pka is the value of pH at which any ionizable group of amino acid is in 50 ionized form and 50 non-ionized form. Thus, each amino acid has two pka values :- i pk1 - pH at which 50 of amino acid molecules have a-carboryl group in ionized form COO- and 50 molecules have it in unionized form COOH. ii pk2 - pH at which 50 of amino acid molecules have a-amino group in ionized form NH3 and 50 molecules have it in unionized form NH2. iii The value of pk-l and pk-2 is different for different amino acids. But it ranges between 3.5-4.0 for pk-l i.e. pka of a-carboxyl group is 3.5-4.0 and 8.0-9.0 for pk-2 i.e. pka of a-amino group is 8.0-9.0. iv Beside a-carboxyl and a-amino group, some amino acids contain special ionizable group, which will provide one more pka value pk3. For example pka of imidazole group of histidine ranges between 6.5-7.4. That means somewhere between pH 6.5-7 .4 ,50 of histidine molecules in a solution will have imidazole group in ionized form and 50 molecules will have in unionized form. v Thus, imidazole group of histidine has pka value closest to physiological pH. 