General Veterinary Biochemistry

PROPERTIES OF AMINO ACIDS

Physical properties

• Amino acids are white, crystalline substances soluble in water and insoluble in organic solvents.
• They generally have high melting points, above 200^οC.
• Amino acids may be tasteless (leucine), sweeter in taste (glycine and alanine) or bitter in taste (arginine).
• All amino acids except glycine possess asymmetric carbon atom and hence, optically active.
• Amino acids are amphoteric in nature, because, they contain both acidic (- COOH) and basic (-NH₂) groups. They can donate or accept a proton. Hence, they are also known as ampholytes.
• Aromatic amino acids have an absorption maxima at 280 nm

Chemical properties

• The chemical properties of amino acids are due to the presence of carboxyl and amino groups.

Peptide Bonds

• The combination of an α – amino group of one amino acid with the carboxyl group of a second amino acid, with the elimination of water, results in the formation of a peptide bond. The resulting compound is a dipeptide. A tri peptide contains three amino acid residues joined by two peptide bond, an oligopeptide contains around 20 amino acid (oligo=few) and a polypeptide contains more than 80 amino acid residues. The bonds are very stable. The distinction between peptide and protein is not clear. The term protein is used with more molecular weight and the polypeptide is for lower molecular weight.
• By convention, the amino acid sequence of a polypeptide is written from the amino terminus on the left and the carboxy terminus on the right. Each amino acid is abbreviated by a three letter symbol, usually the first three letters of its name or by a one letter symbol( this symbol is used for long sequence).
• Polypeptides are named as derivatives of amino acid with the free carboxyl group. The name of each amino acid in a polypeptide chain is changed to yl except the one at the C terminal. For example the tri-peptide glutathione, which is made up of glutamate, cysteine and glycine is represented by Gly--- Cys--------Gly ( glutamyl –cysteinyl—glycine).