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Enzyme activity can be affected by other molecules. [[enzyme inhibitor|Inhibitors]] are molecules that decrease enzyme activity; [[enzyme activator|activator]]s are molecules that increase activity. Many [[drug]]s and [[poison]]s are enzyme inhibitors. Activity is also affected by [[temperature]], chemical environment (e.g. [[pH]]), and the [[concentration]] of substrate. Some enzymes are used commercially, for example, in the synthesis of [[antibiotic]]s. In addition, some household products use enzymes to speed up biochemical reactions (''e.g.'', enzymes in biological [[washing powder]]s break down protein or [[fat]] stains on clothes; enzymes in [[meat tenderizer]]s break down proteins, making the meat easier to chew).
 
== Etymology and history ==
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[[Image:Eduardbuchner.jpg|thumb|180px|right|[[Eduard Buchner]]]]
As early as the late 1700s and early 1800s, the digestion of [[meat]] by stomach secretions<ref name="Reaumur1752">{{cite journal | last = de Réaumur | first = RAF | authorlink = René Antoine Ferchault de Réaumur | year = 1752 | title = Observations sur la digestion des oiseaux | journal = Histoire de l'academie royale des sciences | volume = 1752|pages = 266, 461}}</ref> and the conversion of [[starch]] to [[sugar]]s by plant extracts and [[saliva]] were known. However, the mechanism by which this occurred had not been identified.<ref>Williams, H. S. (1904) [http://etext.lib.virginia.edu/toc/modeng/public/Wil4Sci.html A History of Science: in Five Volumes. Volume IV: Modern Development of the Chemical and Biological Sciences] Harper and Brothers (New York) Accessed 04 April 2007</ref>
 
In the 19th century, when studying the [[fermentation (food)|fermentation]] of sugar to [[alcohol]] by [[yeast]], [[Louis Pasteur]] came to the conclusion that this fermentation was catalyzed by a vital force contained within the yeast cells called "[[Vitalism|ferments]]", which were thought to function only within living organisms. He wrote that "alcoholic fermentation is an act correlated with the life and organization of the yeast cells, not with the death or putrefaction of the cells."<ref>{{cite journal |author=Dubos J.|year= 1951|title= Louis Pasteur: Free Lance of Science, Gollancz. Quoted in Manchester K. L. (1995) Louis Pasteur (1822–1895)—chance and the prepared mind.|journal= Trends Biotechnol|volume=13|issue=12|pages=511–515|id= PMID 8595136}}</ref>
 
In 1878 German physiologist [[Wilhelm Kühne]] (1837–1900) first used the term ''[[wiktionary:enzyme|enzyme]]'', which comes from [[Greek language|Greek]] ''ενζυμον'' "in leaven", to describe this process. The word ''enzyme'' was used later to refer to nonliving substances such as [[pepsin]], and the word ''ferment'' used to refer to chemical activity produced by living organisms.
 
In 1897 [[Eduard Buchner]] began to study the ability of yeast extracts that lacked any living yeast cells to ferment sugar. In a series of experiments at the [[Humboldt University of Berlin|University of Berlin]], he found that the sugar was fermented even when there were no living yeast cells in the mixture.<ref>[http://nobelprize.org/nobel_prizes/chemistry/laureates/1907/buchner-bio.html Nobel Laureate Biography of Eduard Buchner at http://nobelprize.org] Accessed 04 April 2007</ref> He named the enzyme that brought about the fermentation of sucrose "[[zymase]]".<ref>[http://nobelprize.org/nobel_prizes/chemistry/laureates/1907/buchner-lecture.html Text of Eduard Buchner's 1907 Nobel lecture at http://nobelprize.org] Accessed 04 April 2007</ref> In 1907 he received the [[Nobel Prize in Chemistry]] "for his biochemical research and his discovery of cell-free fermentation". Following Buchner's example; enzymes are usually named according to the reaction they carry out. Typically the suffix ''-ase'' is added to the name of the [[substrate (biochemistry)|substrate]] (''e.g.'', [[lactase]] is the enzyme that cleaves [[lactose]]) or the type of reaction (''e.g.'', [[DNA polymerase]] forms DNA polymers).
 
Having shown that enzymes could function outside a living cell, the next step was to determine their biochemical nature. Many early workers noted that enzymatic activity was associated with proteins, but several scientists (such as Nobel laureate [[Richard Willstätter]]) argued that proteins were merely carriers for the true enzymes and that proteins ''per se'' were incapable of catalysis. However, in 1926, [[James B. Sumner]] showed that the enzyme [[urease]] was a pure protein and crystallized it; Sumner did likewise for the enzyme [[catalase]] in 1937. The conclusion that pure proteins can be enzymes was definitively proved by [[John Howard Northrop|Northrop]] and [[Wendell Meredith Stanley|Stanley]], who worked on the digestive enzymes pepsin (1930), trypsin and chymotrypsin. These three scientists were awarded the 1946 Nobel Prize in Chemistry.<ref>[http://nobelprize.org/nobel_prizes/chemistry/laureates/1946/ 1946 Nobel prize for Chemistry laureates at http://nobelprize.org] Accessed 04 April 2007</ref>
 
This discovery that enzymes could be crystallized eventually allowed their structures to be solved by [[x-ray crystallography]]. This was first done for [[lysozyme]], an enzyme found in tears, saliva and [[egg white]]s that digests the coating of some bacteria; the structure was solved by a group led by [[David Chilton Phillips]] and published in 1965.<ref>{{cite journal |author=Blake CC, Koenig DF, Mair GA, North AC, Phillips DC, Sarma VR.|year= 1965|title= Structure of hen egg-white lysozyme. A three-dimensional Fourier synthesis at 2 Angstrom resolution. |journal= Nature |volume=22|issue=206|pages=757–761|id= PMID 5891407}}</ref> This high-resolution structure of lysozyme marked the beginning of the field of [[structural biology]] and the effort to understand how enzymes work at an atomic level of detail.
 
==Structures and mechanisms==