The term protein signifies first or foremost and proteins are the most abundant macromolecules in cells and constitute over 50% of dry weight of most of the organisms. Proteins are the instruments by which genetic information is expressed. There are thousands of different proteins in each species of organisms. There are perhaps 10 million of different species. There are twenty amino acids in enough sequences constitute a protein.
Amino acid sequences of different protein enable them to serve their different function. Considering structure shape physiochemical properties of proteins these are classified under different classes and groups. But there are following major classes of proteins according to their biological function.
I. Enzymes (Enzymoproteins):
Those proteins which are highly specialized in their function with catalytic activity are called enzymes. These proteins regulate almost all biological reactions going on inside all living cells. There are about 2000 different enzymes has been recognized; each capable of catalyzing a different kind of biochemical reaction. Without enzymes no life forms are possible.
[b]II. Transport proteins:
Transport proteins are those proteins which help in transportation of life sustaining chemicals vital gases and nutrients. Hemoglobin is a globular protein present in RBC of blood can binds with oxygen when blood passes though longs and distributes oxygen through out the body cells to affect cellular respiration. Blood plasma contains lipoprotein which carries lipids from the liver to other organs.
Other kinds of transport proteins present in cell membrane called carrier proteins which are specially adopted to bind and transport glucose, amino acids and other nutrients across the membrane into the cells.
III. Nutrient and storage proteins:
Storage proteins are those stored inside the cells or tissue as reserved food and can be mobilized at the time of nutrient requirement to provide energy.
The seeds of many store nutrient protein required for the growth of embryonic plant. These include proteins in wheat corn and rice stored in endosperm, ovalbumin (egg white) stored in egg. Casein in milk, Ferritin in animal tissue are the nutrient proteins.
IV. Contractile or motile proteins:
Some proteins endow cells, and organisms with the ability to contract to change the shape or to move about. These proteins includes. Acting and myosin; which are present in form of filamentous protein in muscle cells for functioning in the contractile systems.
Tubulin is another contractile protein present in each cell type inform of micro tubes. Micro tubes are main constituent of cilice and flagella which help in movement of cells.
V. Structural proteins:
Many proteins contribute to serve a supporting filaments, cables or sheets to give biological structures strength and protection. This type of protein form major component of tendons, cartilages and bones. These are fibrous proteins named collagen. Ligaments are contains special structural protein capable of stretching in two dimensions called as elastin. Hairs finger nails, feathers of birds consists of tough insoluble protein named keratin. Major component of silk fibers, threads of spider web contain structural protein named fibroin.
VI. Defense proteins:
Many proteins in body of organisms posses defending action against the invasion and attack of foreign entities or protect the body from injury. Among these proteins special globular protein named immunoglobulin's or antibodies in vertebrate’s body is the most indispensible protein. It synthesized by lymphocytes and they can neutralize the foreign protein produced by bacteria, virus and other harmful microbes called antigens through precipitation or glutination.
Fibrinogen and thrombin are blood proteins belong to this class of proteins which help in blood clotting and protect blood loss from injury and help in repair.
VII. Regulatory proteins:
Some proteins help to regulate cellular or physiological activity. Among them are many hormones, such as insulin; which is a regulatory protein formed in pancreatic tissue help to regulate the blood sugar level.
Growth hormones of pituitary and parathyroid hormones regulate Ca++ and phosphate transport in body. Other proteins called repressors regulate biosynthesis of enzymes.
VIII. Other functional proteins:
There are number of proteins whose functions are not yet specified and are rather exotic. These includes –
Monelin: - A protein of an African plant has an intensely sweet taste and used as non toxic food sweetener for human use.
Antifreeeze: A protein present in blood plasma of Antarctic fisher which protect their blood freezing in ice cold water.
Resillin: A type of protein present in wing hinges of some insects with elastic properties.
Above all proteins can also be divided according to shape as globular protein, fibrous protein, etc on basis of their shape and chromo protein, metal protein glycoprotein, nucleoprotein etc, according to the compounds with which they are found in conjugated form.[/b][/b][/b]