Proteins are the chief substance which constitutes the cells of both plant and animal body. Next to water, protein is the most abundant component of the body. It accounts for about 1/6th of the live body weight. Out of which a third of it is found in the muscles, a fifth in the bones and cartilage, a tenth in the skin and the remainder is in other tissues and body fluid.
Proteins are extremely complex nitrogenous organic compounds, built up by smaller units of structure called amino acids. They contain the elements- carbon, hydrogen, oxygen and nitrogen. Most of them contain sulphur and some also contain phosphorous. In addition, some specialized protein too contains other trace elements like iron, iodine and copper.
The presence of nitrogen about 16% distinguishes protein from carbohydrate and fat. Protein is more complex than fats and carbohydrates, as the size of the molecules is large and there is a great variation in the units from which it is formed.
Proteins are made up of 23 of simpler compounds called amino acids. The amino acids contain an amino (basic) and an acid (Carboxyl) group in their molecules. The structure of the amino acids may be represented thus:
By verifying the grouping (R) that is attached to the carbon containing the amino group, many different amino acids are possible. The molecules weight or protein varies from 13,000 or less too many millions.
Proteins may be classified on the basis of their physical and chemical properties, their amino acid structure or according to their nutritional qualities.
I. Physical and Chemical Properties
This classification is sub-divided into further three groups according to the solubility.
(i) Simple proteins:
Simple proteins yield only amino acids upon hydrolysis by acids, alkalis or enzymes. Examples are albumins and globulins found within all body cells. Keratin, collagen and elastin in supportive tissues of the body and in hair and nails.
(ii) Conjugated proteins:
They are composed of simple proteins combined with a non-protein substance. Examples are phosphor-proteins as casein in milk, nucleoprotein as protein of the cell nucleus and chromoprobins- as hemoglobin.
(iii) Derived proteins:
They are substances resulting from the decomposition of simple and conjugated proteins as in peptones, peptides and proteosis.
II. According to the Amino Acid Structure
There are about twenty two amino acids commonly found in dietary proteins. The body can synthesis only some of these twenty-two amino acids in sufficient quantity. These are called as the ‘non-essential amino-acids’. However, ten amino acids can not be synthesised by the body and these have to be supplied through food. These are called as the ‘essential amino acids’. The extent to which the amino acids are present determines the quality of protein in any food. The list of essential and non-essential amino-acids is given in Table-1.
III. According to nutritional qualities
Proteins may be broadly divided into three groups in regard to their nutrition value.
(i) Complete protein:
A complete protein contains essential amino acids to maintain body tissues and to promote a normal rate of growth and is referred as having a high biological value. Examples are egg, milk and meat (including poultry and fish) proteins wheat germ and dried yeast have a biological value approaching that of animal source.
(ii) Partially complete proteins:
They will maintain life, but lack sufficient amounts of some of the essential amino acids necessary for growth. Adults under no physiological stress can maintain satisfactory nutrition for indefinite period when consuming sufficient amount of protein from certain cereals or legumes- gladden of wheat.
(iii) Totally incomplete proteins:
These type of proteins are incapable of replacing or building new tissues and hence cannot support life and promote growth. Zein in corn and gelatin are examples of this type.