Biological Importance of Proteins
Of all the molecules encountered in living organisms, proteins have the most diverse functions, as the following list suggests.
1. Catalysis. Catalytic proteins called the enzymes accelerate thousandsof biochemical reactions in such processes as digestion, energy capture, and biosynthesis. These molecules have remarkable properties. For example, enzymes can increase reaction rates by factors of between 106 and 1012. They can perform this feat under mild conditions of pH and temperature because they can induce or stabilize strained reaction intermediates. For example, ribulose bisphosphate carboxylase is an important enzyme in photosynthesis, and the protein complex nitrogenase is responsible for nitrogen fixation.
2. Structure. Structural proteins often have very specialized properties.For example, collagen (the major components of connective tissues) and fibroin (silkworm protein) have significant mechanical strength. Elastin, the rubberlike protein found in elastic fibers, is found in blood vessels and skin that must be elastic to function properly.
3. Movement. Proteins are involved in all cell movements. Actin,tubulin, and other proteins comprise the cytoskeleton. Cytoskeletal proteins are active in cell division, endocytosis, exocytosis, and the ameboid movement of white blood cells.
4. Defense. A wide variety of proteins are protective. In vertebrates,keratin, a protein found in skin cells, aids in protecting the organism against mechanical and chemical injury. The blood-clotting proteins fibrinogen and thrombin prevent blood loss when blood vessels are damaged. The immunoglobulins (or antibodies) are produced by lymphocytes when foreign organisms such as bacteria invade an organism. Binding antibodies to an invading organism is the first step in its destruction.
5. Regulation. Binding a hormone molecule or a growth factor tocognate receptors on its target cell changes cellular function. For example, insulin and glucagon are peptide hormones that regulate
blood glucose levels. Growth hormone stimulates cell growth and division. Growth factors are polypeptides that control animal cell division and differentiation. Examples include platelet-derived growth factor (PDGF) and epidermal growth factor (EGF).
6. Transport. Many proteins function as carriers of molecules or ionsacross membranes or between cells. Examples of membrane transport proteins include the enzyme Na_-K_ ATPase and the glucose transporter. Other transport proteins include hemoglobin, which carries O2 to the tissues from the lungs, and the lipoproteins LDL and HDL, which transport waterinsoluble lipids in the blood from the liver. Transferrin and ceruloplasmin are serum proteins that transport iron and copper, respectively.
7. Storage. Certain proteins serve as a reservoir of essential nutrients.For example, ovalbumin in bird eggs and casein in mammalian milk are rich sources of organic nitrogen during development. Plant proteins such as zein perform a similar role in germinating seeds.
8. Stress response. The capacity of living organisms to survive a varietyof abiotic stresses is mediated by certain proteins. Examples include cytochrome P450, a diverse group of enzymes found in animals and plants that usually convert a variety of toxic organic contaminants into less toxic derivatives, and metallothionein, a cysteine-rich intracellular protein found in virtually all mammalian cells that binds to and sequesters toxic metals such as cadmium, mercury, and silver. Excessively high temperatures and other stresses result in the synthesis of a class of proteins called the heatshock proteins (hsps) that promote the correct refolding of damaged proteins. If such proteins are severely damaged, hsps promote their degradation. (Certain hsps function in the normal process of protein folding). Cells are protected from radiation by DNA repair enzymes.