Functions Of Proteins
Proteins are the agents of biological function. Virtually every cellular activity is dependent on one or more particular proteins. Thus, a convenient way to classify the enormous number of proteins is by the biological roles they fill. The various functions of proteins are as follows.
By far the largest class of proteins is enzymes. More than 3000 different enzymes are listed in Enzyme Nomenclature, the standard reference volume on enzyme classification. Enzymes are catalysts that accelerate the rates of biological reactions. Each enzyme is very specific in its function and acts only in a particular metabolic reaction. Virtually every step in metabolism is catalyzed by an enzyme. Enzymes are systematically classified according to the nature of the reaction that they catalyze, such as the transfer of a phosphate group (phosphotransferase) or an oxidation–reduction (oxidoreductase). The formal names of enzymes come from the particular reaction within the class that they catalyze, as in ATP: D-fructose-6-phosphate 1-phosphotransferase. Often, enzymes have common names in addition to their formal names. ATP:
D-fructose-6-phosphate 1-phosphotransferase is more commonly known as phosphofructokinase (kinase is a common name given to ATP-dependent phosphotransferases).
A number of proteins do not perform any obvious chemical transformation but nevertheless can regulate the ability of other proteins to carry out their physiological functions. Such proteins are referred to as regulatory proteins. A well-known example is insulin, the hormone regulating glucose metabolism in animals. Insulin is a relatively small protein and consists of two polypeptide chains held together by disulfide cross-bridges. Other hormones that are also proteins include pituitary somatotropin and thyrotropin, which stimulates the thyroid gland.
A third class of proteins is the transport proteins. These proteins function to transport specific substances from one place to another. One type of transport is exemplified by the transport of oxygen from the lungs to the tissues by haemoglobin or by the transport of fatty acids from adipose tissue to various organs by the blood protein serum albumin.Membrane transport proteins take up metabolite molecules on one side of a membrane, transport them across the membrane, and release them on the other side. Examples include the transport proteins responsible for the uptake of essential nutrients into the cell, such as glucose or amino acids.
Proteins whose biological function is to provide a reservoir of an essential nutrient are called storage proteins. Because proteins are amino acid polymers and because nitrogen is commonly a limiting nutrient for growth, organisms have exploited proteins as a means to provide sufficient nitrogen in times of need. For example, ovalbumin, the protein of egg white, provides the developing bird embryo with a source of nitrogen during its isolation within the egg. Casein is the most abundant protein of milk and thus the major nitrogen source for mammalian infants. The seeds of higher plants often contain as much as 60% storage protein to make the germinating seed nitrogen-sufficient during this
crucial period of plant development. In corn (Zea mays or maize), a family of low molecular weight proteins in the kernel called zeins serve this purpose. Ferritin is a protein found in animal tissues that binds iron, retaining this
essential metal so that it is available for the synthesis of important ironcontaining proteins such as hemoglobin.
Contractile and Motile Proteins
Certain proteins endow cells with unique capabilities for movement. Cell division, muscle contraction, and cell motility represent some of the ways in which cells execute motion. Examples include actin and myosin, the filamentous
proteins forming the contractile systems of cells, and tubulin, the major component of microtubules.
An apparently passive but very important role of proteins is their function in creating and maintaining biological structures. Structural proteins provide strength and protection to cells and tissues. Monomeric units of structural proteins typically polymerize to generate long fibers (as in hair). a-Keratins are insoluble fibrous proteins making up hair, horns, and fingernails. Collagen, another insoluble fibrous protein, is found in bone, connective tissue, tendons, and cartilage, where it forms inelastic fibrils of great strength. One-third of the total protein in a vertebrate animal is collagen. A structural protein having elastic properties is, appropriately, elastin, an important component of ligaments.
Certain insects make a structurally useful protein, fibroin (a a-keratin), the major constituent of cocoons (silk) and spider webs.
Scaffold Proteins (Adapter Proteins)
Some proteins play a recently discovered role in the complex pathways of cellular response to hormones and growth factors. These proteins, the scaffold or adapter proteins, have a modular organization in which specific parts (modules) of the protein’s structure recognize and bind certain structural elements in other proteins through protein–protein interactions.
Protective and Exploitive Proteins
In contrast to the passive protective nature of some structural proteins, another group can be more aptly classified as protective or exploitive proteins because of their biologically active role in cell defense, protection, or exploitation.
Prominent among the protective proteins are the immunoglobulins or antibodies produced by the lymphocytes of vertebrates. Antibodies have the remarkable ability to specifically recognize and neutralize “foreign” molecules resulting from the invasion of the organism by bacteria, viruses, or other infectious agents. Another group of protective proteins is the blood-clotting proteins, thrombin and fibrinogen, which prevent the loss of blood when the circulatory system is damaged. Arctic and Antarctic fishes have antifreeze proteins to protect their blood against freezing in the below-zero temperatures of high-latitude seas. Another class of exploitive proteins includes the toxins produced by bacteria, such as diphtheria toxin and cholera toxin. It is worth repeating that the great diversity of function in proteins, as reflected is attained using just 20 amino acids.