When the amino groups of an amino acid combine with carboxyl group of another amino acid, a peptide bond is formed. Peptides containing more than ten amino acids are referred as polypeptides.

The amino acids are held together in protein by a covalent, amide bond or peptide bond. These bonds are rather strong and serve as cementing material between the individual amino acids.

The existence of different ionic forms of amino acids can be more easily understood by the titration curves as shown in Figure 3.1. At low pH, leucine exists in a fully protonated form as cation. As titration proceeds with NaOH, leucine loses its proton, and at isoelectric pH (pI), it blames a zwitterion. Further titration results…

Isoelectric point (pI) is defined as the pH at which a molecule exists as zwitterion or dipolar ion and carries no net charge. Example: pI for leucine is 6.

The amino acids differ in their physical chemical properties, which ultimately determine the characteristics of proteins.

L-ornithine, α-citrulline, β-alanine, and γ-aminobutyrate, are non-protein amino acids. L-ornithine and α-citrulline Both play an important role in metabolic intermediate of urea cycle. β-alanine – isomer of alanine – vitamin pantothenic acid Creatinine – plays an important role in energy storage process. γ-aminobutyrate – free from brain.

In addition to the above twenty amino acids, there are other amino acids, which have limited distribution and high amounts in few proteins. Examples Hydroxy proline – 12% collagen Hydroxy lysine – 1% collagen N-methyl lysine – Myosin γ-carboxyglutamate – Blood-clotting protein α-aminoadipate – Corn protein α-έ-diaminopimelate – Bacterial protein

The remaining ten amino acids are non-essential amino acids. The body synthesises all body skeletons, and hence, their absence in food will not adversely affect growth.