Proteins are large, organic, nitrogen-containing molecules that are essential both to the structure and to the function of all living cells. Exceedingly complex themselves, the protein molecules have as their structural units much simpler compounds called amino acids. Typically, the amino acids in each molecule are strung together in chain-like fashion, with the longer chains folded into ribbons, spirals, and other three-dimensional forms.
The types of amino acids on the chain (over 22 varieties have so far been identified in animal protein), the number of them (a chain can contain a few amino acids or several thousand), and the particular configuration all help differentiate one protein from another. Because proteins have such a wide range of specific functions and characteristics, they can be classified in a number of ways. Probably the simplest way is by function. One of protein's main functions is to serve as "raw material" for the growth, maintenance and repair of all living tissues. Proteins make up at least 50 percent of most animal cells and they constitute much of the solid matter in muscles, organs, and endocrine glands. Specialized proteins (keratins, for instance) are used to form skin, hair, and nails; other proteins (collagen) help form connective tissue; and still others ( globulins and albumins) make up the soluble or semi-soluble molecules of all cells.
A number of proteins also have highly specialized functions in the regulation of body processes. Some of these regulators include the nucleoproteins (protein plus DNA and RNA) that contain the blueprints for the synthesis of all proteins; catalytic proteins (or enzymes) used to activate biological processes; hormonal proteins that start and stop cellular metabolism; contractile proteins (myosin and actin) that regulate muscular contraction; blood proteins (hemoglobin and lipoprotein) that transport proteins throughout the system; and the serum albumin and serum protein that regulate osmotic pressure during osmosis and maintain water balance.
When the amount of fats and carbohydrates in the diet is insufficient, protein can also serve as a source of energy. However, the protein " burned" to provide energy can no longer be used to synthesize new protein, resulting over time in a serious deficiency problem. While protein deficiency is rare in developed countries, it still occurs in young children in many poorer countries, usually in the form of kwashiorkor, in which the child's body is swollen and growth-retarded, or marasmus, in which the child suffers from emaciation.
Proteins were first recognized as natural organic molecules in the 1830s when Dutch chemist Gerardus Mulder began systematically analyzing a number of plant and animal products. In several apparently unrelated test materials--silk, egg white, blood, and gelatin--Mulder discovered the same nitrogen-containing organic substance, a substance that appeared much more complicated in constitution than either fats or carbohydrates. After further research, Mulder proclaimed it "unquestionably the most important of all known substances in the organic kingdom. Without it, no life appears possible on the planet." Mulder (perhaps at the suggestion of a contemporary, Jöns Berzelius) decided to name the substance protein, from the Greek word proteius, roughly meaning "to come first."
Even before protein had a name, a few researchers were aware that nitrogen-containing foods were somehow important to life. In 1816, French physiologist François Magendie demonstrated in dogs that nitrogen was essential to life and that the nitrogen could only be supplied in the diet through certain foods. In 1827, English biochemist William Prout classified foods into four main categories: water, saccharinous (carbohydrates), oleaginous (fats), and albuminous (proteins). In 1842, Justus von Liebig found that the protein acquired through nutritional sources was necessary for building body tissues, and protein, along with fats and carbohydrates, quickly became recognized as a dietary essential. Liebig also suggested that the urinary level of nitrogen could be used to measure the amount of protein eaten by an individual.
In the 1870s, Karl von Voit established the principles of nitrogen equilibrium in the body that are used as the criteria for human protein requirements. By this time, protein chemistry was being studied by a number of investigators, including German biochemist Ernst Hoppe-Seyler, who devised a classification system in 1875 that is still employed today (because of the large number of proteins, several classification systems are possible; chemical composition, physical properties, and solubility are among the key criteria used for grouping proteins).
By the turn of the century, more than half of protein's structural units—the amino acids--had been identified. The exact nature of these compounds was still unknown, however. The first clues were provided by Frederick Gowland Hopkins, who discovered tryptophan in 1900 and conducted countless nutritional experiments thereafter. Through his feeding tests, Hopkins suggested that certain amino acids were more nutritionally important than others and that, while some could be synthesized in the body, others had to be supplied in the diet. By 1907, Emil Fischer greatly furthered structural research by demonstrating precisely how amino acids were linked within proteins. In 1911, Thomas B. Osborne, after investigating with Lafayette B. Mendel (1872-1935) the protein in seed plants, provided definite proof that certain amino acids, such as lysine and tryptophan, were not only nutritionally essential but could not be synthesized using laboratory rats. The puzzle was finally finished by William Cummings Rose (once Mendel's student), who in 1935 isolated the last nutritionally important amino acid to be discovered, threonine. Rose was able to prove that while roughly half the amino acids in protein could be synthesized in the body, the other half--the so-called essential amino acids--had to be supplied through various "essential" foods.
Current research efforts have been focused on protein folding, the three-dimensional structure of a protein molecule. Scientists are trying to discover a way to predict a protein's folded structure from the amino acid sequence. Knowing the three-dimensional structure of a protein gives insight into its interaction with other molecules, reaction kinetics and dynamics, and evolutionary relationships with similar proteins in other organisms. Because many factors and intermediate structures influence the folding of a particular protein, this has become a laborious task which will require further study.
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