Enzymes are complex proteins that act as catalysts for the countless biochemical reactions that keep humans, animals, plants, and microorganisms alive. Constituents of every living cell, enzymes have relatively large molecules that contain one or more amino acid chains. The sequence of amino acids within the chains and the distinctive way each chain folds into its own characteristic three-dimensional shape help determine the enzyme's particular activity. In order for them to act, many enzymes also need to be attached to a nonprotein substance called a coenzyme. In most cases, these coenzymes are composed wholly or partially of vitamins, especially those in the water-soluble B family. The typical animal cell (roughly one-billionth the size of a drop of water) contains about three thousand different enzymes, almost all programmed to perform specific chemical reactions necessary for metabolism.
For example, in the digestive tract certain enzymes are involved in breaking down oversized fat, carbohydrate, and protein molecules into smaller and easier-to-absorb molecules; a different set of enzymes assists in moving these molecules into the bloodstream; then other enzymes utilize some of these molecules in the biosynthesis of new cellular structures.
Enzymes have important industrial and commercial uses as well. Since ancient times, people have observed enzymes at work fermenting their wine and beer, turning their sour milk into cheese, and causing their bread dough to rise. However, these reactions were generally considered as fermentations of some mysterious kind and only vaguely understood. Then, in the early 1800s, biochemists began taking a closer look at the "ferments" causing some of these reactions.
In 1833, French chemist Anselme Payen separated a substance from an extract of malt that, he realized, seemed capable of speeding up the conversion of starch to sugar. Payen called the substance diastase, the first enzyme to be isolated and prepared in concentrated form. Three years later, German physiologist Theodor Schwann prepared an extract containing tissues from an animal's stomach, mixed it with hydrochloric acid, and demonstrated that the animal tissue extract greatly increased the acid's meat-dissolving properties. Schwann then isolated the substance in the extract that appeared to provide the added potency, and named the substance pepsin from the Greek word meaning " to digest." Pepsin proved to be a vitally important enzyme and was the first such to be prepared from animal tissues.
In 1876, another German physiologist, Wilhelm Kühne (1837-1900), pointed out that certain ferments--such as pepsin and the trypsin he himself had recently isolated from pancreatic juice--should be given a separate name. He suggested the name enzyme, meaning "in yeast," because these substances merely resembled the more important ferments found in living cells, notably yeast, which were then believed to be governed by so-called "vital forces." In 1896, however, Eduard Buchner (who isolated the enzyme zymase from "dead" yeast cells) proved that the ferments in yeast were no different from those in digestive juice and from then on all were called enzymes, though scientists had yet to prove their protein structure because of their fragility.
In the 1920s, the highly respected German chemist Richard Willstätter declared that enzymes were not proteins, and his views dominated the scientific world for many years, despite the pioneering protein classification research of Ernst Hoppe-Selyer beginning in the 1870s. In fact, in 1926, when American biochemist James Sumner isolated an enzyme in pure form (the enzyme urease) and proved it was a protein, his work was hotly challenged. Another ten years passed before the corroborative research of John Northrop settled the issue.
When Payen named the first enzyme diastase, he started the tradition of ending the names of most enzymes with the suffix "ase." Today, the suffix is usually also added to the names of classes of enzymes, and these names typically indicate the reaction that the enzyme catalyzes. For example, the name transferase indicates that the enzymes in this general classification all act to help transfer chemical groups from one molecule to another. More specifically, the transaminase members of this classification transfer amino groups, while the transmethylase members transfer methyl groups.
Whatever their classification, most enzymes tend to act on only one kind of substance (called a substrate) and trigger only one kind of reaction. Because enzymes are relatively large molecules, only a section of the enzyme actually reacts with the substrate, a section called the active site. Most researchers believe that a particular substrate can only fit into the active site of a particular enzyme. This lock and key hypothesis was first suggested by Emil Fischer in 1894. (In related research begun in 1913, Leonor Michaelis, 1875-1949, was able to define the rates of reactions with the Michaelis-Menten equation, which supported the theory of such unions.) Because a perfect fit between active site and substrate is so important, any change in either could obviously impede the catalytic reaction.
More serious problems can result when a particular enzyme is actually missing. In a number of hereditary human diseases--such as phenylketonuria ( PKU) and galactosemia--geneticists have discovered that the affected individuals are born missing certain specific enzymes. Some of these enzyme-deficiency diseases can now be effectively treated and many researchers are concentrating on the search for more of these disorders, which may ultimately revolutionize the practice of medicine.
Another current research focus is in the development of "artificial enzymes." These are enzymes, synthesized in a lab, which mimic the action of naturally occurring enzymes. These artificial enzymes can catalyze reactions more specifically and efficiently than their natural counterparts. This is accomplished through combining known binding sites with different catalytic groups. The new enzyme is then studied to determine its ability to carry out selective reactions in an efficient manner. Artificial enzymes have been designed which catalyze such reactions as selective nucleic acid cleavage, chemical synthesis, and cell response control. Through the creation of such enzymes, researchers hope to both gain a better understanding of enzyme mechanics as well as discover practical applications for these enzymes, such as medicinal use.
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