The Acid and Base Characteristics of Amino Acids

When an amino acid is titrated (i.e., exposed to an acid or base), the resulting reactions can be described by a titration curve. The shape of this curve is influenced by the types and number of the functional groups capable of reacting with or exchanging a hydrogen ion. In an amino acid such as alanine, there are two titratable groups (Fig. 2-4). At a low pH (i.e., a high hydrogen ion concentration), both the amino group and the carboxylic acid group of alanine are protonated. As a result, alanine is positively charged and migrates toward the negative pole in an electrical field. If a base is added (to decrease the concentration of hydrogen ions or, alternatively to increase the concentration of hydroxyl ions), the carboxylic acid group and then the amino group lose their protons. At the midpoint of this process when the carboxylate group is unprotonated and negatively charged and the amino group is still protonated and positively charged, alanine is neutral. The addition of

Examples of Posttranslationally Modified Amino Acid Residues in Proteins

Tyrosine Sulfate With Different


Tyrosine Sulfate


Tyrosine Sulfate



7-Carboxyglutamate e-A/-Trimethyllysine

Metabolites of Free Amino Acids Not Commonly Found in Proteins

h2c ch2

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