Proteins are organic substances that on digestion yield their constituent, unit-building blocks– the aminoacids. In other words amino acids unite to form the complex molecule of a protein. Proteins are important for tissue building and maintenance. 1 gram of protein yields 4.0 calories in the body. Chemical composition–
- Proteins are complex organic compounds containing carbon (C), hydrogen(H) and oxygen(O). Amino acids are building blocks which unite to form a protein molecule.
- Amino acids are linked together to form a protein molecule through NH2 group of one amino acid condensing with the COOH group of another amino acid with the elimination of one molecule of water, and a compound thus formed is called a Peptide and the linkage is called as a Peptide linkage.
- When two amino acids join they form Dipeptide, three join they form Tripeptide.
- A chain containing more than three amino acids is called a polypeptide, which ultimately forms a protein.
A protein may contain hundreds or thousands of amino acids.
Classification–
- Complete proteins- These proteins contain all the essential amino acids in sufficient quantity and ratio to supply the body’s needs. They support life even if supplied as a sole source of protein. These proteins are of animal origin.e.g. milk,meat,poultry,products and fish. A variety of complete proteins in the diet are one way of assuring that the body’s amino acid needs are met.
The best quality protein is one which provides essential amino acids pattern very close to the pattern of tissue protein.Egg protein and human milk protein are classified as high quality proteins and serve as reference proteins for defining the quality of other proteins.
The proteins of animal foods like milk,meat ,fish etc. are good quality proteins and are also easily digestible.
- Incomplete proteins- These proteins are deficient in one or more of the essential amino acids and therefore, they do not support life on their own. All plant sources of proteins such as vegetable, fruits, pulses, cereals, nuts and oilseeds contain proteins incomplete to varying degrees.
If two sources of protein are combined in the same meal, the resulting protein may be of better quality, for example khichdi prepared using tur or moong dal and rice is of better protein quality than rice or dal cooked separately.
Rice kheer is another example where animal and vegetable proteins milk and rice are cooked together.
Gelatin is the only animal protein which is an incomplete protein.