Explain about the Proteins?

Proteins, as you may already know, are made up of carbon, hydrogen, nitrogen, oxygen and usually sulphur. Proteins are built up of a large number of amino acid molecules inter linked by peptide bonds as illustrated in figure. All the amino acids have a trivial or common name, many a times is related to the source from which they have been first isolated. For example:

Amino acids are twenty in number, and you have read in the Nutritional Biochemistry Course that they are classified according to the nature of the side chain (R group).