Principles of Biochemistry

4.2.5 Charges of amino acids

Amino acids may have positive, negative, or zero net charge. Amino acids bear at least two ionizable weak acid groups, a -COOH and an -NH3⁺. In solution, two forms of these groups, one charged and one un-charged, exist in protonic equilibrium:

R-COOH ↔ R-COO^- + H⁺

R-NH₃⁺ ↔ R-NH₂+H⁺

R-COOH and R-NH₃⁺ are the protonated, or acidic partners.

The R-COO^- and R-NH₂ are the conjugate bases (proton acceptors). Although both R-COOH and R-NH₃⁺ are weak acids, R-COOH is a far stronger acid than is R-NH₃⁺. At the pH of blood plasma or the intracellular space(pH 7.4), carboxyl groups exist almost entirely as carboxylate ions, R-COO^-. Most amino groups are predominantly in the protonated form, R-NH₃⁺.

In acid medium:

H⁺+ NH₃⁺RCOO^– ↔ NH₃+RCOOH

In Alkaline medium: In alkaline medium the amino acid acts as acids, yielding an anion, and move towards anode in an electrical field.

NH₃⁺RCOO^– + H₂O ↔ NH₂RCOO^– + -OH

On the basis of these opposite reactions depending upon the acidity and alkalinity of the solution, the amino acids are called ampholyte.

Isoelectric pH (pI) of an amino acid is the pH at which it has no net charge and hence does not move in an electric feld. The ion at pI carries both + and – charges and is called ‘Zwitterion’. It is an ampholyte as it has both a proton donor and proton acceptor.

In an acid medium the amino acid acts as bases, yielding cation and move towards anode in an electrical field.