protein, any of the group of highly complex organic compounds found in all living cells and comprising the most abundant class of all biological molecules. Protein comprises approximately 50% of cellular dry weight. Hundreds of protein molecules have been isolated in pure, homogeneous form; many have been crystallized. All contain carbon, hydrogen, and oxygen, and nearly all contain sulfur as well. Some proteins also incorporate phosphorous, iron, zinc, and copper. Proteins are large molecules with high molecular weights (from about 10,000 for small ones [of 50–100 amino acids] to more than 1,000,000 for certain forms); they are composed of varying amounts of the same 20 amino acids amino acid (əmē`nō)
..... Click the link for more information. , which in the intact protein are united through covalent chemical linkages called peptide peptide, organic compound composed of amino acids linked together chemically by peptide bonds. The peptide bond always involves a single covalent link between the α-carboxyl (oxygen-bearing carbon) of one amino acid and the amino nitrogen of a second amino acid.
..... Click the link for more information. bonds. The amino acids, linked together, form linear unbranched polymeric structures called polypeptide chains; such chains may contain hundreds of amino-acid residues; these are arranged in specific order for a given species of protein.

Types of Proteins

A protein molecule that consists of but a single polypeptide chain is said to be monomeric; proteins made up of more than one polypeptide chain, as many of the large ones are, are called oligomeric. Based upon chemical composition, proteins are divided into two major classes: simple proteins, which are composed of only amino acids, and conjugated proteins, which are composed of amino acids and additional organic and inorganic groupings, certain of which are called prosthetic groups prosthetic group, non-amino acid portions of certain protein molecules. The key part of the prosthetic group may be either organic (such as a vitamin ) or inorganic (such as a metal) and is usually required for biological activity, especially when the prosthetic
..... Click the link for more information. . Conjugated proteins include glycoproteins glycoprotein (glī'kōprō`tēn)
..... Click the link for more information. , which contain carbohydrates; lipoproteins lipoprotein (lĭp'əprō`tēn)
..... Click the link for more information. , which contain lipids; and nucleoproteins, which contain nucleic acids nucleic acid, any of a group of organic substances found in the chromosomes of living cells and viruses that play a central role in the storage and replication of hereditary information and in the expression of this information through protein synthesis.
..... Click the link for more information. .

Classified by biological function, proteins include the enzymes enzyme, biological catalyst . The term enzyme comes from zymosis, the Greek word for fermentation , a process accomplished by yeast cells and long known to the brewing industry, which occupied the attention of many 19th-century chemists.
..... Click the link for more information. , which are responsible for catalyzing the thousands of chemical reactions of the living cell; keratin keratin (kĕr`ətĭn), any one of a class of fibrous protein molecules that serve as structural units for various living tissues.
..... Click the link for more information. , elastin, and collagen collagen (kŏl`əjən)
..... Click the link for more information. , which are important types of structural, or support, proteins; hemoglobin hemoglobin (hē`məglō'bĭn)
..... Click the link for more information. and other gas transport proteins; ovalbumin, casein casein (kā`sēn), well-defined group of proteins found in milk, constituting about 80% of the proteins in cow's milk, but only 40% in
..... Click the link for more information. , and other nutrient molecules; antibodies antibody, protein produced by the immune system (see immunity ) in response to the presence in the body of antigens: foreign proteins or polysaccharides such as bacteria, bacterial toxins , viruses, or other cells or proteins.
..... Click the link for more information. , which are molecules of the immune system (see immunity immunity, ability of an organism to resist disease by identifying and destroying foreign substances or organisms. Although all animals have some immune capabilities, little is known about nonmammalian immunity.
..... Click the link for more information. ); protein hormones hormone, secretory substance carried from one gland or organ of the body via the bloodstream to more or less specific tissues, where it exerts some influence upon the metabolism of the target tissue.
..... Click the link for more information. , which regulate metabolism metabolism, sum of all biochemical processes involved in life. Two subcategories of metabolism are anabolism, the building up of complex organic molecules from simpler precursors, and catabolism , the breakdown of complex substances into simpler molecules, often
..... Click the link for more information. ; and proteins that perform mechanical work, such as actin actin, a protein abundantly present in many cells, especially muscle cells, that significantly contributes to the cell's structure and motility. Actin can very quickly assemble into long polymer rods called microfilaments.
..... Click the link for more information. and myosin myosin (mī`əsĭn), one of the two major protein constituents responsible for contraction of muscle.
..... Click the link for more information. , the contractile muscle proteins.

Protein Structure

Every protein molecule has a characteristic three-dimensional shape, or conformation. Fibrous proteins, such as collagen and keratin, consist of polypeptide chains arranged in roughly parallel fashion along a single linear axis, thus forming tough, usually water-insoluble, fibers or sheets. Globular proteins, e.g., many of the known enzymes, show a tightly folded structural geometry approximating the shape of an ellipsoid or sphere.

Because the physiological activity of most proteins is closely linked to their three-dimensional architecture, specific terms are used to refer to different aspects of protein structure. The term primary structure denotes the precise linear sequence of amino acids that constitutes the polypeptide chain of the protein molecule. Automated techniques for amino-acid sequencing have made possible the determination of the primary structure of hundreds of proteins.

The physical interaction of sequential amino-acid subunits results in a so-called secondary structure, which often can either be a twisting of the polypeptide chain approximating a linear helix (α-configuration), or a zigzag pattern (β-configuration). Most globular proteins also undergo extensive folding of the chain into a complex three-dimensional geometry designated as tertiary structure. Many globular protein molecules are easily crystallized and have been examined by X-ray diffraction, a technique that allows the visualization of the precise three-dimensional positioning of atoms in relation to each other in a crystal.

The tertiary structure of several protein molecules has been determined from X-ray diffraction analysis. Two or more polypeptide chains that behave in many ways as a single structural and functional entity are said to exhibit quaternary structure. The separate chains are not linked through covalent chemical bonds but by weak forces of association.

The precise three-dimensional structure of a protein molecule is referred to as its native state and appears, in almost all cases, to be required for proper biological function (especially for the enzymes). If the tertiary or quaternary structure of a protein is altered, e.g., by such physical factors as extremes of temperature, changes in pH, or variations in salt concentration, the protein is said to be denatured; it usually exhibits reduction or loss of biological activity.

Protein Synthesis

The cell's ability to synthesize protein is, in essence, the expression of its genetic makeup. Protein synthesis is a sequence of chemical reactions that occur in four distinct stages, i.e., activation of the amino acids that ultimately will be joined together by peptide bonds; initiation of the polypeptide chain at a cell organelle known as the ribosome; elongation of the polypeptide by stepwise addition of single amino acids to the chain; and termination of amino-acid additions and release of the completed protein from the ribosome. The information for the synthesis of specific amino-acid sequences is carried by a nucleic acid molecule called messenger RNA (see nucleic acid). Proteins are needed in the diet mainly for their amino acids, which the body uses to build new proteins (see nutrition nutrition, study of the materials that nourish an organism and of the manner in which the separate components are used for maintenance, repair, growth, and reproduction. Nutrition is achieved in various ways by different forms of life.
..... Click the link for more information. ).

The mechanism of action of many widely used antibiotics, such as streptomycin streptomycin (strĕp'tōmī`sĭn), antibiotic produced by soil bacteria of the genus Streptomyces
..... Click the link for more information. , chloramphenicol chloramphenicol (klōr'ămfĕn`əkŏl')
..... Click the link for more information. , and tetracycline tetracycline (tĕ'trəsī`klēn), any of a group of antibiotics produced by bacteria of the genus Streptomyces.
..... Click the link for more information. , can be understood in terms of their ability to interfere with some stage of protein synthesis in bacteria.

protein

Any of numerous organic compounds, complex polymers of amino acids that are involved in nearly every aspect of the physiology and biochemistry of living organisms. Twenty different amino acids are common to proteins, linked in chains of hundreds to thousands of units. An active protein molecule has three important levels of structure: primary (the amino acid sequence), determined by the genes; secondary (the geometric shape, often a helix), determined by the angles of the covalent bonds between and within amino acids; and tertiary (the looped and folded overall shape), determined largely by attraction between oppositely charged groups (and repulsion between like charged groups) on amino-acid side chains and especially by hydrogen bonding. The tertiary structure, which can be globular or sheetlike with ridges, crevices, or pockets, often holds the key to a protein's biological activity. Proteins can serve, e.g., as structural material (as in connective tissue and hair; see collagen; keratin), as enzymes and hormones, as transporters of essential substances such as oxygen (see hemoglobin), as antibodies, or as regulators of gene expression. Some proteins are simple (amino acids only), some conjugated (see conjugation) to other groups, often vitamins or metal atoms needed in tiny amounts in the diet (see coenzyme; cofactor). Rhodopsin and hemoglobin are conjugated proteins. Proteins may be covalently linked to other atoms or molecules, as to sugars (glycoproteins), phosphate groups (phosphoproteins), or sulfur (sulfoproteins). Proteins are an essential human nutrient, obtained from both plant and animal foods. Their greatest commercial use is in food products; they are also employed in adhesives, plastics, and fibres.