The word protein is derived from the Greek word, Proteios meaning ‘primary’ in English. Proteins are of utmost importance for all biological systems in the body. Out of the total dry body weight, three-fourth is made up of proteins. They are used for all body building processes including all major structural and functional aspects of the body that are carried out by protein molecular diseases. They contain carbon, hydrogen, oxygen and nitrogen as major components while sulphur and phosphorus are minor constituents.
Proteins form a significant part of human nutrition. They are complex organic nitrogenous compounds and defer from carbohydrates and fats as they contain nitrogen amounting to about 16%.
Essential amino acids: Proteins are made up of smaller unites called amino acids. Some 24 amino acids are needed by the human body of which 9 are ‘essential’ because the body cannot synthesize them in amounts corresponding to its needs, and therefore, they must be obtained through diet. They include lucine, isolucine, lysine, methionine, phenylalanine, threonine, valine, tryptophan and hystidine. Essential amino acids include arginine, asparaginic acid, serine, glutamic acid, praline and glycine. Both essential and nonessential amino acids are needed for the synthesis of tissue proteins. The former must be supplied through diet whereas the latter can be synthesized by the body provided other building blocks are present. New tissues cannot be formed unless all the essential amino acids are present in the diet. A protein is biologically complete only if it contains all the essential amino acids in amounts required by the human body. From the nutritional standpoint, animal proteins are rated superior to vegetable proteins because they are biologically complete.
Functions of proteins:
Proteins are needed by the body for carrying out various functions.
Body building: They are required for building muscular mass and are important for the maintenance of body tissues. They aid in the process of repair of body tissues and help increase muscle mass.
Proteins play a significant role in the maintenance of osmotic pressure and in the synthesis of certain substances like antibodies, hemoglobin, enzymes, hormones and coagulations factors,
Proteins are connected with the immune mechanism of the body. The cell mediated immune response and the bactericidal activity of leucocytes has been found to be lowered in several forms of protein energy malnutrition.
Proteins are also capable of supplying energy when the calorie intake is inadequate.
Dietary Sources of Proteins: There are two main dietary sources of obtaining proteins.
Animal sources: Proteins of animal origin include milk, meat, eggs, cheese and fowl. These proteins contain all the essential amino acids in adequate amounts. Egg proteins are considered to be the best food proteins because of their high biological value and digestibility.
Vegetable sources: They include pulses, cereals, beans, nuts, oilseeds etc. They are poor in essential amino acids but are easily available.
Classifications of proteins:
Proteins are classified on various basis:
Functional basis:
1. Catalytic Proteins- Enzymes
2. Structural Proteins – Collagen, Elastin, Keratin
3. Transport proteins -Hemoglobin, Myoglobin, albumin, transferrin
4. Regulatory Proteins - growth hormone
5. Genetic Proteins - histones
6. Protective Proteins-immunoglobulins, interferons
Composition and solubility: Proteins are divided into three major groups- simple proteins, conjugated proteins and derived proteins.
Simple Proteins:
Albumins: They are soluble in water and coagulated by heat e.g. egg albumin.
Globulins: They are insoluble in pure water but soluble in dilute salt solutions e.g. egg globulin, serum globulin and legumin of peas.
Protamines: They are soluble in water, dilute acids and alkalis. They are not coagulated by heating and contain large number of arginine and lysine residues. They can combine with other acidic proteins.
Prolamines: They are soluble in 70-80% alcohol but insoluble in pure water.
Lecithins: They have high affinity to sugar groups. They are mostly derived from plant sources and also occur in the animal kingdom.
Seleroproteins: They are insoluble in water, salt solutions, and organic solvents and soluble only in hot strong acids. They form supporting tissues such as collagen of bone, cartilage and tendon, keratin of hair and nail.
Conjugated Proteins:
Glycoproteins: They are combined with carbohydrates and are also known as mucoproteins or proteoglycans. They include blood group antigens and many serum proteins.
Lipoproteins: These proteins are loosely combined with lipid components. They occur in blood and on cell membranes.
Dietary proteins consist of both essential and non-essential amino acids. They are required for the process of protein synthesis, whereas certain amino acids can also be used for energy and gluconeogenesis.
RDA of proteins:
For adults, the recommended dietary allowance for proteins is about 0.6 g/kg per day of desirable body weight. Protein needs increase during growth, pregnancy, lactation and rehabilitation and during the treatment of malnutrition. The tolerance to dietary protein is decreased in renal insufficiency and liver failure.
Digg 
Delicious
reditt
Stumble Upon
