Question

is the pka of a histidine side chain always the same as that of the free amino acid? Why?

how do titration curves differ between non ionizable and ionizable r groups? why?

Biochemistry

Answer 1

hi,

A)

Charge of the amino acid side chains

Charged side chains

When an amino acid (AA) is incorporated into a polypeptide, the charges on the amino and carboxyl groups disappear. Among the 20 common amino acids, five have a side chain which can be charged. At pH=7, two are negative charged: aspartic acid (Asp, D) and glutamic acid (Glu, E) (acidic side chains), and three are positive charged: lysine (Lys, K), arginine (Arg, R) and histidine (His, H) (basic side chains).

The charge on the amino acid side chain depends on the pK of the AA (Table 1) and on the pH of the solution.

1. Negative charged (acidic side chains): aspartic acid and glutamic acid
   
   At a pH superior to their pK (Table 2), the carboxylic side chains lose an H⁺ ion (proton) and are negative charged. They are therefore acid.

   

   At a pH inferior to their pK, the aspartic acid and glutamic acid side chains are uncharged.

2. Positive charged (basic side chains): lysine, arginine and histidine
   
   At a pH superior to their pK (Table 2), the amine side chains are uncharged.
   At a pH inferior to their pK, the lysine, arginine and histidine side chains accept an H⁺ ion (proton) and are positive charged. They are therefore basic.

   

Table 1: pK of the amino acid side chain group. The pK of the side chain group is the pH at which exactly half of a carboxylic or amine group is charged.

Amino acid	pK of the side chain group
Aspartic acid	3.9
Glutamic acid	4.2
Lysine	10.5
Arginine	12.5
Histidine	6.0

b) Alpha amino acids are the building blocks of proteins. Almost all proteins consist of various combinations of the same 20 amino acids. Amino acids are compounds containing an amine group, -NH2, and a carboxylic acid group, -COOH. In addition there is an "R" group that differs for each amino acid. The symbol "R" is used here to represent a generalized abbreviation for an organic group. H2N C COOH H R

In physiological systems where the pH is near neutrality, the amino group of an amino acid will be protonated and the carboxylic acid group will be deprotonated. This is called the zwitterion form.

In strongly acidic solutions the carboxylic acid group will also be protonated, while in strongly basic solutions both the carboxylic acid group and the amino group will both be unprotonated.

The acid-base behavior of amino acids is best described by the Br