Classification of proteins based on their functions
Proteins are responsible for many different functions in the living cell. Classification of proteins on the basis of functions is as follows :
Enzymes : Proteins that catalyze chemical and biochemical reactions within living cell and outside. This group of proteins probably is the biggest and most important group of the proteins. Enzymes are responsible for all metabolic reactions in the living cells. Well known and very interesting examples are : DNA and RNA-polymerases, dehydrogenases etc.
Hormones: Proteins that are responsible for the regulation of many processes in organisms. Hormones are usually quite small and can be classifies as peptides. Most known protein hormones are : insulin, growth factor, lipotropin, prolactin etc. Many protein hormones are predecessor of peptide hormones, such as endorfme, enkephalin etc. It is possible to increase this group of proteins by adding of all protein venoms.
Transport proteins : These proteins are transporting or store some other chemical compounds and ions. Some of them are well known : cytochrome C-electron transport; haemoglobin and myoglobin – oxygen transport; albumin – fatty acid transport in the blood stream etc. It is possible to classify trans membrane protein channels as transport proteins as well.
Immunoglobulin or Antibodies : Proteins that are involved in the immune response of the organism to neutralize large foreign molecules, which can be a part of an infection. Sometimes antibodies can act as enzymes. Sometimes this group of proteins is considered as a bigger group of protective proteins with adding such proteins as lymphocyte antigen¬recognizing receptors, antiviral agents such as interferon, tumor necrosis factor (TNF). Probably the clotting of blood proteins, such as fibrin and thrombin should be classified as protective proteins as well.
Structural proteins : These proteins are maintaining structures of other biological components, like cells and tissues. Collagen, elastin, a-keratin, sklerotin, fibroin – these proteins are involved into formation of the whole organism body. Bacterial proteoglycans and virus coating proteins also belongs to this group of proteins. Currently we do not know about other functions of these proteins.
Motor proteins : These proteins can convert chemical energy into mechanical energy. Actin and myosin are responsible for muscular motion. Sometimes it is difficult to make a strict separation between structural and motion proteins.
Receptors : These proteins are responsible for signal detection and translation into other type of signals. Sometimes these proteins are active only in complex with low molecular weight compounds. Very well known member of this protein family idopsin/rhodopsin – light detecting protein. Many receptors are transmembrane proteins.
Signalling proteins: This group of proteins is involved into signalling translation process. Usually they significantly change conformation in presence of some signalling molecules. These proteins can act as enzymes. Other proteins, usually small, can interact with receptors. Classical example of this group of proteins is GTPases.
Storage proteins: These proteins contain energy, which can be released during metabolism processes in the organism. Egg ovalbumin and milk casein are such proteins. Almost all proteins can be digested and used as a source of energy and building material by other organisms.