The word ‘Protein’ is derived from a Greek word ‘protos’ meaning ‘primary or holding first place’ which is appropriate name for an essential life forming and life sustaining substance of all organisms. Proteins contain nitrogen, but the nutritive value of protein- rich foods does not depend upon the total nitrogen content, but on the constituent of aminoacids. The nitrogen content of proteins varies from about 14 to 20 % and in most of the proteins, the value is about 16%.This average figure of 16% is used commonly for converting nitrogen content of foodstuffs or tissues into proteins.
Origin and composition of proteins:
Amino acids are small units that combine to form a protein molecule. Plants synthesise amino acids with the help of bacteria and fungi from:(i) soil, which supplies the necessary nitrogen and sulphur; (ii) water, which provides oxygen and hydrogen; and (iii) atmospheric carbon dioxide, which supplies carbon and oxygen. Animals cannot synthesise amino acids from basic elements, but derive them from ingested plants. Thus, the primary source of all proteins is the vegetable kingdom.
Proteins are chemical compounds that contain the same atoms as carbohydrate and lipid – carbon(C), hydrogen (H) and oxygen (O) – but proteins are different in that they also contain nitrogen (N) atoms. These nitrogen atoms give the name ‘amino’ (nitrogen containing) to the amino acids that are the links in the chains referred to as proteins.