One fifth of an adults total body weight is protein. Protein is found in every cell of our body.
All the tissues in our body such as muscle, blood, bone, skin and hair are made up of proteins.
Many hormones and enzymes are either protein or protein derivatives. The nucleic acids in the cell nucleus occur in combination with proteins as nucleoproteins.
Protein is thus essential to maintain cellular integrity and function and for health and reproduction.
Proteins contain carbon, hydrogen, oxygen and nitrogen. They are distinguished from carbohydrates and fats by the presence of nitrogen.
Protein is synthesized from basic units called amino acids. Protein molecules, which contain up to hundred amino acids are much larger than carbohydrates or lipid molecule.
Chemically amino acids are composed of a carbon atom to which is attached a carboxyl (COOH) group, a hydrogen atom (H), an amino group (NH2) and an amino acid radical (R) as shown below.
Structure of an Amino acid.
H C R
The carboxyl group, the amino group and the hydrogen atom are the same for all amino acids.
The R group however distinguishes one amino acid from another.
R varies from a single hydrogen atom as found in glycine, to longer chain of up to 7 carbon atoms.
A protein molecule is made upon of chains of amino acids joined to each other by a peptide linkage. The amino group of one amino acid is linked to the carboxyl group of another amino acid by removal of water.
Thus two amino acids form di-peptide and three form a tri-peptide. Proteins consist of hundreds of such linkages hence called Polypeptides.
ESSENTIAL AND NON-ESSENTIAL AMINO ACIDS
Amino acids are classified into two groups - essential (indispensable) and non-essential (dispensable).An essential amino acid is one that cannot be synthesized by the body to meet the physiological needs and hence should be supplied by the diet. The essential amino acids are histidine, isoleucine, leucine, lysine, methionine, phenylalanine threonine, tryptophan and valine. Non-essential amino acids are those that the body can synthesize. They are alanine, arginine, aspargine, aspartic acid, cysteine, glutamic acid, glutamine, glycine, proline, serine and tyrosine.