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56.3 FORMATION OF PEPTIDES AND PROTEINS

Peptides are the products formed by the condensation of two or more amino acids through their amino and carboxylic groups involving elimination of water molecules. They may be classified as dipeptides, tripeptides, tetrapeptides, etc., depending upon whether the number of amino acid molecules taking part in condensation is two, three or four respectively. When the number of such amino acids is more than ten, the product is called polypeptide. A polypeptide having molecular mass more than 10000 u is called protein. Thus, polypeptides and proteins are condensation polymers of α-amino acids. The linkage (-CO-NH-) which unites various amino acid units in a peptide molecule is called peptide linkage or peptide bond.

A simple convention is used to write the structure and name of the peptide. The amino acid unit having free -NH2 group is called N-terminal end whereas the amino acid unit with free -COOH group is called C-terminal end. The structure is written with N-terminal end to the left and C-terminal end to the right. The base name of the peptide is taken from the C-terminal amino acid unit. Other amino acid units are taken to be substituents of this acid and the suffix ine of their name is replaced with yl. In case of polypeptides and proteins, the abbreviated names of amino acid units are used. Let us write the structure of a tripeptide formed from glycine, alanine and serine .

A section of polypeptide with different amino acid units is given below:

HYDROLYSIS OF PROTEINS

Polypeptides and proteins can be hydrolysed by dilute acids or enzymes. The ultimate product of hydrolysis of proteins is a mixture of α-amino acids.

When food is digested, the proteins present in it are broken into constituent amino acid molecules. During digestion the peptide linkage that joins the amino acids in proteins gets hydrolysed. Hydrolysis of proteins takes place in the stomach and small intestine and the amino acids produced in the process are absorbed from the intestine by the blood. These amino acids are then regrouped to form specific proteins in the cells of our body.

Hydrolysis of tripeptide glycylalanylglycine (gly.ala.gly) is shown below: