Proteins have many different and varied biological functions and in addition to their size, shape and orientation, can be classified according to their biological roles within the cell.

Almost every chemical reaction between organic bio molecules in living cells are catalysed by enzymes. Enzymes are the most varied and specialised proteins, and many thousands of different types, each capable of catalysing a different type of chemical reaction, have been discovered in different organisms.

Nutrient and Storage Proteins
Many plants store nutrient proteins in their seeds. Such proteins are vital for the growth and survival of the germinating seedlings. Particularly well-studied examples are the seed proteins found in corn, wheat and rice. Other examples of nutrient proteins are ovalbumin, the major component protein of egg white and casein, found in milk.
Ferritin, found in some bacteria, and also in plant and animal tissues, stores iron.

Contractile Proteins
Some proteins provide cells and organisms with the ability to contract, to change conformation, or to move about, and are known as contractile or motile proteins.
Actin and myosin function in the contractile system of skeletal muscle, and are also found in many non-muscle cells. Microtubules are formed from the protein tubulin, and act in conjunction with the protein dyenin in the flagella and cilia of bacteria, which propel the organisms, allowing motility.

Transport Proteins
These proteins allow substances to be carried to their destination. In blood plasma, transport proteins bind and transport specific molecules or ions from one organ to another. Haemoglobin in erythrocytes binds oxygen as blood passes through the lungs, transporting it to the peripheral tissues, and releases it to contribute in the energy-yielding oxidation of nutrients. Blood plasma contains lipoproteins, which carry lipids from the liver to other organs. Other types of transport proteins are present in the plasma membranes and intracellular membranes of all organisms; these are adapted to bund glucose, amino acids and other substances, and transport them across the membrane to the point at which they are utilised.

Structural Proteins
In order to give biological structures strength or protection, many proteins serve as supporting filaments, cables or sheets. The chief constituent of tendons and cartilage is collagen, which is a fibrous protein, having a very high tensile strength. One example of this is leather, which comprises of almost pure collagen. Ligaments contain elastin, which is a structural protein which can be stretched in two dimensions. Hair, fingernails, feathers and horn all contain great amounts of keratin, which is a tough, insoluble protein. The main component of silk fibres and spider webs is fibroin and the wing hinges of some insects contain resilin. Resilin has ideal elastic properties.

Regulatory Proteins
Cellular or physiological activity is also regulated by some proteins. Amongst those proteins are many hormones. Insulin, which regulates sugar metabolism and the growth hormone secreted by the pituitary gland are two examples. The cellular response to many hormonal signals is often mediated by a class of GTP-binding proteins, known as G proteins. GTP is closely associated to ATP, where guanine replaces the adenine section of the molecule. Other regulatory proteins attach to DNA and control the biosynthesis of enzymes and RNA molecules concerned in cell division, in both prokaryotes and eukaryotes.

Defence Proteins
Many proteins defend organisms against invasion by another species or protect them from injury. The immunoglobins or antibodies, which are specialised proteins produced by the lymphocytes of vertebrates, can recognise, precipitate or neutralise invading microorganisms, and also foreign proteins from another species. Fibrinogen and thrombin are blood clotting proteins which stop the loss of blood when damage occurs to the vascular system. Some snake venom, bacterial toxins and toxic plant proteins, such as ricin, also appear to have defensive functions. Some of these, including fibrinogen, thrombin and some types of venom, are also enzymes.

Other Proteins
There are numerous other proteins whose functions are exotic in nature and therefore, are not easily classified. Monellin, a protein from an African plant, has an intensely sweet taste and has been studied for human use as a food sweetener. Some Antarctic fish possess antifreeze proteins within their blood plasma, which prevents their blood from freezing.

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