Classification of proteins
Proteins are large molecules formed by the combination of a number of aminoacids. About 20 amino acids have been found to occur in proteins and are important from the point of view of human nutrition. Amino acids can be classified as follows:
An essential amino acid may be defined as one which is necessary for the growth and health of all living organisms and which cannot be synthesised in the body and must therefore be supplied through dietary intake. There are 9 amino acids considered essential for the human infant, out of which Histidine is considered non-essential for the adult.
Sometimes a non-essential amino acid can become essential. During illness or conditions of trauma, or in other special circumstances the need for an amino acid that is normally non-essential may become greater than the body’s ability to produce it. In such circumstances, that amino acid becomes essential for the ill person. Amino acids that behave this way are referred to as ‘Conditionally essential’ amino acids for critically ill people.
Methionine can be converted to cystine, but cystine cannot be converted to methionine. Similarly, phenylalanine can be converted to tyrosine, but not vice-versa. yet these spare the requirements of the corresponding essential amino acid. Hence, cystine and tyrosine are sub-classed as semi- essential amino acids.
These amino acids can be synthesized in the body and not necessarily obtained through dietary intake.
The nutritional classification of amino acids is presented in table 10.1
a. Simple proteins: It is composed entirely of amino acids only.
b. Conjugated or Complex proteins: It is made up of amino acids and other organic or inorganic compounds.
The non-amino acid group is termed as Prosthetic group (e.g.)
Lipoproteins – Chylomicrons
c. Derived proteins: These are derivatives of proteins resulting from the action of heat, enzymes or chemical reagents. This group also includes the artificially-produced polypeptides(e.g.)Fibrin
Proteins are classified into two types based on nutrition view point as follows:
a. Complete proteins: These contain all the essential amino acids in sufficient quantity to supply the needs of the body. They support life even if supplied as the sole source of protein. These proteins are of animal origin (e.g) milk, meat, poultry, egg and fish. The quality of these proteins is much superior to those of incomplete proteins.
b. 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 (i.e) vegetables, fruits, cereals, pulses, nuts and oilseeds contain incomplete proteins to varying degrees.
c. Complementary proteins : If two sources of incomplete proteins are combined in the same meal, the resulting protein may be of better quality. These are called as Complementary proteins (e.g) Pongal prepared using moong dhal and rice is of better quality than rice or dhal cooked separately. Rice is deficient in aminoacid lysine, but rich in methionine. Pulses are rich in lysine, but deficient in methionine. So, rice and pulse combination will complement each other. Rice Kheer is another example, where animal and vegetable proteins –milk and rice are cooked together.