Proteins are made up of building blocks called amino acids (i.e. on hydrolysis, proteins yield these units). The idea that proteins are built up of amino acids was first proposed by Emil Fischer. Amino acids are carboxylic acids which have at least one COOH group and one NH2 group. Other functional groups may also be present. There are at least 22 different types of amino acids encountered in plants.

The amino acids are bound to one another in a protein chain by means of linkages or bonds called peptide linkages. Peptide linkages are formed between the COOH group of one amino acid and the NH2 group of the other, with the elimination of a molecule of water. Several amino acids combine together with the peptide bond and form a peptide chain. Several of these peptides constitute a polypeptide chain.

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The polypeptide chain folds and refolds in a characteristic fashion to form a protein molecule.

Peptides:

These are made up of amino acids linked with CONH (peptide) bonds. Peptides are intermediate compounds produced during the synthesis or degradation of proteins. Amino acids— peptides—– proteins

Proteins:

These are the final products in the N2 metabolism, and are made up of folded peptide chains which have many additional linkages (depends upon the point of folding) in addition to the CONH bond. Proteins have a three dimensional configuration and have very high molecular weight reaching up to 10? -10?. Although proteins contain no more than 20 amino acids, they are of immense variety and complexity due to variations in combination of amino acids.

In general the three following aspects pertaining to the amino acids Decide the type of protein. These are – the quality, quantity and sequence of amino acids in the peptide chains. In other words, the type of amino acid, its quantity and the sequence in which these acids are arranged forms the basis for the categorization of proteins.

Besides, proteins may differ in the number of peptide chains per molecule, type of secondary linkages (SH) etc.