The following points highlight the twenty-one important functions of protein. Some of the functions are: 1. Protective Structures 2. Defence Proteins 3. Toxins 4. Structural Proteins 5. Compatibility Proteins 6. Contractile Proteins 7. Microtubules 8. P-Protein 9. Enzymes 10. Transport Proteins 11. Storage Proteins 12. Receptor Proteins 13. Protein Buffers 14. Hormones 15. Repressors 16. Sweet Proteins and Others.
Function # 1. Protective Structures:
Fibrous protein keratin forms external protective structures of animals like nails, hoofs, scales, hair, feathers, horny layer of skin, etc. Silkworms protect themselves in the cocoon stage by silk fibres. The latter are built up of protein fibroin. Spider webs are also built of this protein.
Function # 2. Defence Proteins:
Immunoglobulin’s or antibodies are proteins produced by lymphocytes. They are meant for recognising and neutralizing foreign proteins, toxins, viruses and other pathogens.
Function # 3. Toxins:
They are both defensive and offensive proteins produced by certain animals, bacteria and some plants, e.g., snake venom, ricin of castor, bacterial toxins, etc.
Function # 4. Structural Proteins:
Proteins constitute more than 50% of the dry weight of protoplast. They take part in formation of colloidal complex of protoplast, cell organelles, cell membranes and cell products. Many proteins form supporting structures, e.g., elastin of ligaments, collagen of tendons, cartilage, bone and connective tissue. Along with proteoglycans, the two above mentioned fibrous proteins form a major component of connective tissue.
Function # 5. Compatibility Proteins:
Pollen grains possess specific proteins in their walls for compatibility-incompatibility reaction with the stigma during pollination.
Function # 6. Contractile Proteins:
Actin and myosin are fibrous proteins which form the contractile system of muscles. Contractile system is basically made up of protein actin but association with myosin is essential for contraction.
Function # 7. Microtubules:
They are un-branched, hollow, sub-microscopic tubules which form the structural material of cilia, flagella, basal bodies, centrioles and spindle apparatus. Microtubules are built up of proto-filaments formed of two related proteins, α and β-tubulins.
Function # 8. P-Protein:
It is a special protein present in sieve tube elements. The protein is vibratile and is believed to actively participate in the transport of nutrients.
Function # 9. Enzymes:
There are over 2000 enzymes. Except a few, all others are built up of proteins alone or in conjugation with some non-protein materials called cofactors. Enzymes catalyse all the chemical reactions that occur in the living world, e.g., pepsin, trypsin, ribonuclease, flavoprotein, etc.
Function # 10. Transport Proteins (Carrier Proteins):
Haemoglobin of RBCs transports oxygen from lungs to different parts of the body. Myoglobin of muscles stores oxygen, α-globulin of blood carries thyroxine and bilirubin. β-globulin transports vitamins A, D and K, cholesterol and ions. Albumin carries calcium and fatty acids. A number of carrier proteins occur in the cell membranes for transporting specific materials to the inside, e.g., glucose, amino acids.
Function # 11. Storage Proteins:
They occur in milk, eggs and seeds to nourish the young ones. Important protein of milk is casein. Major protein of egg white is ovalbumin while it is glutelin in cereals. Iron storing protein commonly found in animal tissue is ferritin. Proteins having all the essential amino acids are called first class proteins.
Function # 12. Receptor Proteins:
They occur on the external surface of cell membranes. The proteins bind to specific information molecules like hormones and mediate in the cellular effects, generally through the formation of either с AMP or specific activators. Receptors are specific for different hormones, e.g., adrenaline receptor, β-adrenaline receptor (epinephrine receptor).
Function # 13. Protein Buffers:
The polar side chains of the proteins can combine with excess acid and base. This helps in keeping the pH constant.
Function # 14. Hormones:
Some hormones are proteinaceous, e.g., insulin (regulates sugar metabolism), growth hormone of pituitary, parathyroid hormone parathormone (regulates Ca and phosphate transport).
Function # 15. Repressors:
Most of the repressors of the genes are proteinaceous in nature. They regulate gene action.
Function # 16. Sweet Proteins:
Monellin (from Dioscoreothyllum cumminsii), thaumitin (Katempe fruit Thaumatococcus daniellii) and brazzein (from Pentadiplandra brazzeana) are sweetest chemicals which are natural proteins. They are some 2000 times as sweet as sugar. Their use can be boon to diabetic patients since they are non-fattening, non-caloric and nontoxic sweeteners.
Function # 17. Blood Clotting Proteins:
Fibrinogen and thrombin prevent blood loss from injured vessels by causing clotting of blood.
Function # 18. Antibodies:
They are formed of proteins.
Function # 19. Visual Pigments:
Rhodopsin and iodopsin are protein pigments respectively present in rods and cones of retina. They take part in perception of image.
Function # 20. Mucoproteins:
They are produced by salivary glands and mucous glands of alimentary canal. Mucoproteins form mucus. Mucus protects the lining layer of alimentary canal from friction and digestive juices, e.g., gastric mucoprotein is resistant to pepsin digestion.
Function # 21. Other Proteins:
Nucleoproteins are proteins associated with nucleic acid. Human memory is believed to be stored in specific proteins called memory proteins. Proteins being multivalent macromolecules can carry on a number of chemical reactions.