Amino acid with dissociation constant closest to physiological pH is
Correct Answer: Histidine
Description: To understand dissociation constant (pKa) of amino acids one must know the ionizable groups of amino acids.
Each amino acid has two ionizable groups (i) α-carboxyl group (-COOH), and (ii) α-amino group (-NH2).
Besides these two groups, some amino acids have special ionizable groups. e.g. -SH of cysteine, imidazole of histidine.
The dissociation constant (pKa) is the value of pH at which any ionizable group of the amino acid is in 50% ionized form and 50% non-ionized form.
Thus, each amino acid has two pKa values:-
pk1:- pH at which 50% of amino acid molecules have a-carboxyl group in the ionized form (COO-) and 50% molecules have it in unionized form (COOH).
pk2:- pH at which 50% of amino acid molecules have an a-amino group in the ionized form (NH3+) and 50% molecules have it in unionized form (NH2).
The value of pk-1 and pk-2 is different for different amino acids. But it ranges between 3.5-4.0 for pk-1 (i.e. pKa of α -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).
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
Category:
Biochemistry
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