Principles of Biochemistry

5.3.1.1 Amphoteric Property

Many substances contain acidic and basic groups and react with both acids and bases to form salts. Such property is called an amphoteric property and the substances are called ampholytes. Notable among these are the amino acids of which the proteins are composed. Proteins like amino acids exist in solution as zwitterions and have isoelectric pH values at which they are least soluble and migrate least in an electric field.

Above the isoelectric pH they act as acids and form negative protein ions, whereas below the isoelectric pH they act as bases and form positive ions. Because protein molecules contain number of groups that interact reversibly with H ⁺ ions over a vide range of pH, their solutions have excellent buffer action, and much of the buffer capacity of body tissues is due to the protein buffers

              Protein+  ↔      H+ ^-Protein⁺     ↔           H⁺ +  Protein^-

                Cation                 Zwitterion                               anion