Proteins are molecules essential to maintaining the structure and function of all living organisms. The term protein is derived from the Greek word proteios, meaning "primary." Proteins have many different properties and function in a variety of ways. For example, enzymes; hemoglobin; the collagen of bones, tendons, and skin; and certain hormones all are proteins.
Proteins were first recognized as unique molecules by Gerardus J. Mulder in the 1830s. The chemical structure and the peptide bond of amino acids--the primary units of proteins--were elucidated in 1902 by Emil Fischer. Another 25 years passed before James B. Sumner, an American biochemist, proved that the enzyme urease was a protein. In the 1950s, Frederick Sanger first described the linear sequence of amino acids of a simple, two-chain protein, the hormone insulin. Nearly a decade later, in 1961, the British biochemists John C. Kendrew and Max F. Perutz described the three-dimensional structure of myoglobin, which is muscle protein.
Proteins exist in diverse, complex structures that specify their particular function. Despite the variety of structures, however, all proteins comprise about 20 amino acids. Each amino acid is composed of an amino group and a carboxyl group as well as a carbon side chain, which specifies the characteristics of the particular amino acid. A primary protein is simply a long chain of amino acids linked together by a peptide bond between the amino group of one and the carboxyl group of another. In addition, the sequence of the amino acids in the chain varies with each type of protein. The amino acids constituting a protein are arranged in such a way as to give rise to periodic, secondary structures. The way in which a protein folds into its final conformation, or shape, is vital to its function.
Most proteins are polymers of amino acids, but some have other chemical groups attached to them. Lipoproteins contain lipid subunits in addition to the amino acid, glycoproteins contain carbohydrate subunits, phosphoproteins contain phosphoric acid, and nucleoproteins contain nucleic acids. Some proteins have important smaller molecules, known as prosthetic groups, attached to their surfaces; heme, a porphyrin ring containing an iron atom, is an example. It gives hemoglobin and myoglobin the ability to transport and store oxygen.
Protein is critically important part of the diet. Plants synthesize all the amino acids required for building all the necessary proteins. Animals, however, cannot synthesize eight essential amino acids and therefore depend on food to obtain them. Dietary protein is broken down into amino acids during digestion. These amino acids are absorbed into the bloodstream, where they travel to tissues throughout the body. Cells build up new proteins from these amino acids to serve a specific function.
Proteins play a role in virtually every cellular function. For instance, proteins regulate muscle contraction, antibody production, and blood vessel expansion and contraction to maintain normal blood pressure.
Generally, a lack of protein in the diet retards growth in children and causes a decrease in energy. A protein-deficiency disease common in developing countries of Asia, Africa, and South America is kwashiorkor, which afflicts children of ages 1 to 4 who are being weaned on starchy food. In general, however, the average daily protein intake by adults in the United States exceeds the 0.8 to 1.6 gram per kilogram of body weight recommended by the National Academy of Sciences.
Excess protein intake puts a strain on the liver and kidneys during excretion, and increased risks of certain cancers and coronary heart disease have been associated with high protein diets.