Classification of proteins

Classification of proteins
The word protein is derived from Greek word, “proteios” which means primary. As the name shows, the proteins are of paramount importance for biological systems. Out of the total dry body weight, 3/4ths are made up of proteins. Proteins are used for body building; all the major structural and functional aspects of the body are carried out by protein molecules. Abnormality in protein structure will lead to molecular diseases with profound alterations in metabolic
functions.
Proteins contain Carbon, Hydrogen, Oxygen and Nitrogen as the major components while Sulfur and Phosphorus are minor constituents. Nitrogen is characteristic of proteins. On an average, the nitrogen content of ordinary proteins is 16% by weight. All proteins are polymers of amino acids.
Amino Acids are Linked by Peptide Bonds Alpha carboxyl group of one amino acid reacts with alpha amino group of another amino acid to form a peptide bond or CO-NH bridge (Fig. 4.1A). Proteins are made by polymerization of amino acids through peptide bonds. Two amino acids are combined to form a dipeptide; three amino acids form a tripeptide; four will make a tetrapeptide; a few amino acids together will make an oligopeptide; and combination of 10 to 50 amino acids is called as a polypeptide. By convention, long polypeptide chains containing more than 50 amino acids are called proteins. In a tripeptide, there are 3 amino acids, but these 3 can be any of the total 20 amino acids. Thus 203 = 8000 different permutations and combinations are possible