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Enzymes

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<p><strong>Enzymes</strong> are <a title="Protein" href="http://en.wikipedia.org/wiki/Protein">proteins</a> that <a title="Catalyst" href="http://en.wikipedia.org/wiki/Catalyst">catalyze</a> (<em>i.e.</em> <a title="Reaction rate" href="http://en.wikipedia.org/wiki/Reaction_rate">accelerate</a>) <a title="Chemical reaction" href="http://en.wikipedia.org/wiki/Chemical_reaction">chemical reactions</a>. In these reactions, the <a title="Molecule" href="http://en.wikipedia.org/wiki/Molecule">molecules</a> at the beginning of the process are called <a title="Substrate (biochemistry)" href="http://en.wikipedia.org/wiki/Substrate_%28biochemistry%29">substrates</a>, and the enzyme converts them into different molecules, the products. Almost all processes in the <a title="Cell (biology)" href="http://en.wikipedia.org/wiki/Cell_%28biology%29">cell</a> need enzymes in order to occur at significant rates. Since enzymes are extremely selective for their substrates and speed up only a few reactions from among many possibilities, the set of enzymes made in a cell determines which <a title="Metabolic pathway" href="http://en.wikipedia.org/wiki/Metabolic_pathway">metabolic pathways</a> occur in that cell.</p>
<p>Like all catalysts, enzymes work by lowering the <a title="Activation energy" href="http://en.wikipedia.org/wiki/Activation_energy" style="">activation energy</a> (ΔG<sup>&Dagger;</sup>) for a reaction, thus dramatically accelerating the rate of the reaction. Most enzyme reaction rates are millions of times faster than those of comparable uncatalyzed reactions. As with all catalysts, enzymes are not consumed by the reactions they catalyze, nor do they alter the <a title="Chemical equilibrium" href="http://en.wikipedia.org/wiki/Chemical_equilibrium">equilibrium</a> of these reactions. However, enzymes do differ from most other catalysts by being much more specific. Enzymes are known to catalyze about 4,000 biochemical reactions.<sup class="reference" id="_ref-0"><a title="" href="http://en.wikipedia.org/wiki/Enzyme#_note-0">[1]</a></sup> Not all biochemical catalysts are proteins, since some <a title="RNA" href="http://en.wikipedia.org/wiki/RNA">RNA</a> molecules called <a title="Ribozyme" href="http://en.wikipedia.org/wiki/Ribozyme">ribozymes</a> also catalyze reactions.</p>
<p>Enzyme activity can be affected by other molecules. <a title="Enzyme inhibitor" href="http://en.wikipedia.org/wiki/Enzyme_inhibitor">Inhibitors</a> are molecules that decrease enzyme activity; activators are molecules that increase activity. Many <a title="Drug" href="http://en.wikipedia.org/wiki/Drug">drugs</a> and <a title="Poison" href="http://en.wikipedia.org/wiki/Poison">poisons</a> are enzyme inhibitors. Activity is also affected by <a title="Temperature" href="http://en.wikipedia.org/wiki/Temperature">temperature</a>, <a title="PH" href="http://en.wikipedia.org/wiki/PH">pH</a>, and the concentration of substrate. Some enzymes are used commercially, for example, in the synthesis of <a title="Antibiotic" href="http://en.wikipedia.org/wiki/Antibiotic">antibiotics</a>. In addition, some household products use enzymes to speed up biochemical reactions (<em>e.g.</em>, enzymes in biological washing powders break down protein or <a title="Fat" href="http://en.wikipedia.org/wiki/Fat">fat</a> stains on clothes; enzymes in meat tenderizers break down proteins, making the meat easier to chew).</p>
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<h2><span class="mw-headline">Etymology and history</span></h2>
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<div style="width: 177px;" class="thumbinner"><a title="Eduard Buchner" class="internal" href="http://en.wikipedia.org/wiki/Image:Eduardbuchner.jpg"><img width="175" height="245" class="thumbimage" longdesc="/wiki/Image:Eduardbuchner.jpg" alt="Eduard Buchner" src="http://upload.wikimedia.org/wikipedia/commons/thumb/b/b2/Eduardbuchner.jpg/175px-Eduardbuchner.jpg" /></a>
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<a title="Eduard Buchner" href="http://en.wikipedia.org/wiki/Eduard_Buchner">Eduard Buchner</a></div>
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<p>As early as the late <a title="18th century" href="http://en.wikipedia.org/wiki/18th_century">1700s</a> and early <a title="19th century" href="http://en.wikipedia.org/wiki/19th_century">1800s</a>, the digestion of <a title="Meat" href="http://en.wikipedia.org/wiki/Meat">meat</a> by stomach secretions<sup class="reference" id="_ref-Reaumur1752_0"><a title="" href="http://en.wikipedia.org/wiki/Enzyme#_note-Reaumur1752">[2]</a></sup> and the conversion of <a title="Starch" href="http://en.wikipedia.org/wiki/Starch">starch</a> to <a title="Sugar" href="http://en.wikipedia.org/wiki/Sugar">sugars</a> by plant extracts and <a title="Saliva" href="http://en.wikipedia.org/wiki/Saliva">saliva</a> were known. However, the mechanism by which this occurred had not been identified.<sup class="reference" id="_ref-1"><a title="" href="http://en.wikipedia.org/wiki/Enzyme#_note-1">[3]</a></sup></p>
<p>In the 19th century, when studying the <a title="Fermentation (food)" href="http://en.wikipedia.org/wiki/Fermentation_%28food%29">fermentation</a> of sugar to <a title="Alcohol" href="http://en.wikipedia.org/wiki/Alcohol">alcohol</a> by <a title="Yeast" href="http://en.wikipedia.org/wiki/Yeast">yeast</a>, <a title="Louis Pasteur" href="http://en.wikipedia.org/wiki/Louis_Pasteur">Louis Pasteur</a> came to the conclusion that this fermentation was catalyzed by a vital force contained within the yeast cells called "<a title="Vitalism" href="http://en.wikipedia.org/wiki/Vitalism">ferments</a>", which were thought to function only within living organisms. He wrote that "alcoholic fermentation is an act correlated with the life and organisation of the yeast cells, not with the death or putrefaction of the cells."<sup class="reference" id="_ref-2"><a title="" href="http://en.wikipedia.org/wiki/Enzyme#_note-2">[4]</a></sup></p>
<p>In 1878 German physiologist <a title="Wilhelm Kühne" href="http://en.wikipedia.org/wiki/Wilhelm_K%C3%BChne">Wilhelm Kühne</a> (1837–1900) coined the term <em><a title="wiktionary:enzyme" class="extiw" href="http://en.wiktionary.org/wiki/enzyme">enzyme</a></em>, which comes from <a title="Greek language" href="http://en.wikipedia.org/wiki/Greek_language">Greek</a> <em>ενζυμον</em> "in leaven", to describe this process. The word <em>enzyme</em> was used later to refer to nonliving substances such as <a title="Pepsin" href="http://en.wikipedia.org/wiki/Pepsin">pepsin</a>, and the word <em>ferment</em> used to refer to chemical activity produced by living organisms.</p>
<p>In <a title="1897" href="http://en.wikipedia.org/wiki/1897">1897</a> <a title="Eduard Buchner" href="http://en.wikipedia.org/wiki/Eduard_Buchner">Eduard Buchner</a> began to study the ability of yeast extracts to ferment sugar despite the absence of living yeast cells. In a series of experiments at the <a title="Humboldt University of Berlin" href="http://en.wikipedia.org/wiki/Humboldt_University_of_Berlin">University of Berlin</a>, he found that the sugar was fermented even when there were no living yeast cells in the mixture.<sup class="reference" id="_ref-3"><a title="" href="http://en.wikipedia.org/wiki/Enzyme#_note-3">[5]</a></sup> He named the enzyme that brought about the fermentation of sucrose "<a title="Zymase" href="http://en.wikipedia.org/wiki/Zymase">zymase</a>".<sup class="reference" id="_ref-4"><a title="" href="http://en.wikipedia.org/wiki/Enzyme#_note-4">[6]</a></sup> In 1907 he received the <a title="Nobel Prize in Chemistry" href="http://en.wikipedia.org/wiki/Nobel_Prize_in_Chemistry">Nobel Prize in Chemistry</a> "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 <em>-ase</em> is added to the name of the <a title="Substrate (biochemistry)" href="http://en.wikipedia.org/wiki/Substrate_%28biochemistry%29">substrate</a> (<em>e.g.</em>, <a title="Lactase" href="http://en.wikipedia.org/wiki/Lactase">lactase</a> is the enzyme that cleaves <a title="Lactose" href="http://en.wikipedia.org/wiki/Lactose">lactose</a>) or the type of reaction (<em>e.g.</em>, <a title="DNA polymerase" href="http://en.wikipedia.org/wiki/DNA_polymerase">DNA polymerase</a> forms DNA polymers).</p>
<p>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 <a title="Richard Willstätter" href="http://en.wikipedia.org/wiki/Richard_Willst%C3%A4tter">Richard Willstätter</a>) argued that proteins were merely carriers for the true enzymes and that proteins <em>per se</em> were incapable of catalysis. However, in 1926, <a title="James B. Sumner" href="http://en.wikipedia.org/wiki/James_B._Sumner">James B. Sumner</a> showed that the enzyme <a title="Urease" href="http://en.wikipedia.org/wiki/Urease">urease</a> was a pure protein and crystallized it; Sumner did likewise for the enzyme <a title="Catalase" href="http://en.wikipedia.org/wiki/Catalase">catalase</a> in 1937. The conclusion that pure proteins can be enzymes was definitively proved by <a title="John Howard Northrop" href="http://en.wikipedia.org/wiki/John_Howard_Northrop">Northrop</a> and <a title="Wendell Meredith Stanley" href="http://en.wikipedia.org/wiki/Wendell_Meredith_Stanley">Stanley</a>, who worked on the digestive enzymes pepsin (1930), trypsin and chymotrypsin. These three scientists were awarded the 1946 Nobel Prize in Chemistry.<sup class="reference" id="_ref-5"><a title="" href="http://en.wikipedia.org/wiki/Enzyme#_note-5">[7]</a></sup></p>
<p>This discovery that enzymes could be crystalised eventually allowed their structures to be solved by <a title="X-ray crystallography" href="http://en.wikipedia.org/wiki/X-ray_crystallography">x-ray crystallography</a>. This was first done for <a title="Lysozyme" href="http://en.wikipedia.org/wiki/Lysozyme">lysozyme</a>, an enzyme found in tears, saliva and <a title="Egg white" href="http://en.wikipedia.org/wiki/Egg_white">egg whites</a> that digests the coating of some bacteria; the structure was solved by a group led by <a title="David Chilton Phillips" href="http://en.wikipedia.org/wiki/David_Chilton_Phillips">David Chilton Phillips</a> and published in 1965.<sup class="reference" id="_ref-6"><a title="" href="http://en.wikipedia.org/wiki/Enzyme#_note-6">[8]</a></sup> This high-resolution structure of lysozyme marked the beginning of the field of <a title="Structural biology" href="http://en.wikipedia.org/wiki/Structural_biology">structural biology</a> and the effort to understand how enzymes work at an atomic level of detail.</p>
<p><a id="Structures_and_mechanisms" name="Structures_and_mechanisms"></a></p>
<h2><span class="editsection">[<a title="Edit section: Structures and mechanisms" href="http://en.wikipedia.org/w/index.php?title=Enzyme&action=edit&section=2">edit</a>]</span> <span class="mw-headline">Structures and mechanisms</span></h2>
<dl><dd><span class="boilerplate seealso"><em>See also: <a title="Enzyme catalysis" href="http://en.wikipedia.org/wiki/Enzyme_catalysis">Enzyme catalysis</a></em></span></dd></dl>
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<div style="width: 302px;" class="thumbinner"><a title="Ribbon-diagram showing carbonic anhydrase II. The grey sphere is the zinc cofactor in the active site. Diagram drawn from PDB 1MOO." class="internal" href="http://en.wikipedia.org/wiki/Image:Carbonic_anhydrase.png"><img width="300" height="274" class="thumbimage" longdesc="/wiki/Image:Carbonic_anhydrase.png" alt="Ribbon-diagram showing carbonic anhydrase II. The grey sphere is the zinc cofactor in the active site. Diagram drawn from PDB 1MOO." src="http://upload.wikimedia.org/wikipedia/en/thumb/4/40/Carbonic_anhydrase.png/300px-Carbonic_anhydrase.png" /></a>
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Ribbon-diagram showing <a title="Carbonic anhydrase" href="http://en.wikipedia.org/wiki/Carbonic_anhydrase">carbonic anhydrase II</a>. The grey sphere is the <a title="Zinc" href="http://en.wikipedia.org/wiki/Zinc">zinc</a> cofactor in the active site. Diagram drawn from <a rel="nofollow" title="http://www.rcsb.org/pdb/explore.do?structureId=1MOO" class="external text" href="http://www.rcsb.org/pdb/explore.do?structureId=1MOO">PDB 1MOO</a>.</div>
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<p>The activities of enzymes are determined by their <a title="Quaternary structure" href="http://en.wikipedia.org/wiki/Quaternary_structure">three-dimensional structure</a>.<sup class="reference" id="_ref-7"><a title="" href="http://en.wikipedia.org/wiki/Enzyme#_note-7">[9]</a></sup></p>
<p>Most enzymes are much larger than the substrates they act on, and only a very small portion of the enzyme (around 3–4 <a title="Amino acid" href="http://en.wikipedia.org/wiki/Amino_acid">amino acids</a>) is directly involved in catalysis.<sup class="reference" id="_ref-8"><a title="" href="http://en.wikipedia.org/wiki/Enzyme#_note-8">[10]</a></sup> The region that contains these catalytic residues, binds the substrate, and then carries out the reaction is known as the <a title="Active site" href="http://en.wikipedia.org/wiki/Active_site">active site</a>. Enzymes can also contain sites that bind <a title="Cofactor (biochemistry)" href="http://en.wikipedia.org/wiki/Cofactor_%28biochemistry%29">cofactors</a>, which are needed for catalysis. Some enzymes also have binding sites for small molecules, which are often direct or <a title="" href="http://en.wikipedia.org/wiki/Enzyme#Metabolic_pathways">indirect</a> products or substrates of the reaction catalyzed. This binding can serve to increase or decrease the enzyme's activity, providing a means for <a title="Feedback" href="http://en.wikipedia.org/wiki/Feedback">feedback</a> regulation.</p>
<p>Like all proteins, enzymes are made as long, linear chains of amino acids that <a title="Protein folding" href="http://en.wikipedia.org/wiki/Protein_folding">fold</a> to produce a <a title="Tertiary structure" href="http://en.wikipedia.org/wiki/Tertiary_structure">three-dimensional product</a>. Each unique amino acid sequence produces a unique structure, which has unique properties. Individual protein chains may sometimes group together to form a <a title="Protein complex" href="http://en.wikipedia.org/wiki/Protein_complex">protein complex</a>. Most enzymes can be <a title="Denaturation (biochemistry)" href="http://en.wikipedia.org/wiki/Denaturation_%28biochemistry%29">denatured</a>—that is, unfolded and inactivated—by heating, which destroys the <a title="Tertiary structure" href="http://en.wikipedia.org/wiki/Tertiary_structure">three-dimensional structure</a> of the protein. Depending on the enzyme, denaturation may be reversible or irreversible.</p>
<p><a id="Specificity" name="Specificity"></a></p>
<h3><span class="editsection">[<a title="Edit section: Specificity" href="http://en.wikipedia.org/w/index.php?title=Enzyme&action=edit&section=3">edit</a>]</span> <span class="mw-headline">Specificity</span></h3>
<p>Enzymes are usually very specific as to which reactions they catalyze and the <a title="Substrate (biochemistry)" href="http://en.wikipedia.org/wiki/Substrate_%28biochemistry%29">substrates</a> that are involved in these reactions. Complementary shape, charge and <a title="Hydrophilic" href="http://en.wikipedia.org/wiki/Hydrophilic">hydrophilic</a>/<a title="Hydrophobic" href="http://en.wikipedia.org/wiki/Hydrophobic">hydrophobic</a> characteristics of enzymes and substrates are responsible for this specificity. Enzymes can also show impressive levels of <a title="Stereospecificity" href="http://en.wikipedia.org/wiki/Stereospecificity">stereospecificity</a>, <a title="Regioselectivity" href="http://en.wikipedia.org/wiki/Regioselectivity">regioselectivity</a> and <a title="Chemoselectivity" href="http://en.wikipedia.org/wiki/Chemoselectivity">chemoselectivity</a>.<sup class="reference" id="_ref-9"><a title="" href="http://en.wikipedia.org/wiki/Enzyme#_note-9">[11]</a></sup></p>
<p>Some of the enzymes showing the highest specificity and accuracy are involved in the copying and expression of the <a title="Genome" href="http://en.wikipedia.org/wiki/Genome">genome</a>. These enzymes have "proof-reading" mechanisms. Here, an enzyme such as <a title="DNA polymerase" href="http://en.wikipedia.org/wiki/DNA_polymerase">DNA polymerase</a> catalyses a reaction in a first step and then checks that the product is correct in a second step.<sup class="reference" id="_ref-10"><a title="" href="http://en.wikipedia.org/wiki/Enzyme#_note-10">[12]</a></sup> This two-step process results in average error rates of less than 1 error in 100 million reactions in high-fidelity mammalian polymerases.<sup class="reference" id="_ref-11"><a title="" href="http://en.wikipedia.org/wiki/Enzyme#_note-11">[13]</a></sup> Similar proofreading mechanisms are also found in <a title="RNA polymerase" href="http://en.wikipedia.org/wiki/RNA_polymerase">RNA polymerase</a><sup class="reference" id="_ref-12"><a title="" href="http://en.wikipedia.org/wiki/Enzyme#_note-12">[14]</a></sup>, <a title="Aminoacyl tRNA synthetase" href="http://en.wikipedia.org/wiki/Aminoacyl_tRNA_synthetase">aminoacyl tRNA synthetases</a><sup class="reference" id="_ref-13"><a title="" href="http://en.wikipedia.org/wiki/Enzyme#_note-13">[15]</a></sup> and <a title="Ribosome" href="http://en.wikipedia.org/wiki/Ribosome">ribosomes</a>.<sup class="reference" id="_ref-14"><a title="" href="http://en.wikipedia.org/wiki/Enzyme#_note-14">[16]</a></sup></p>
<p>Some enzymes that produce <a title="Secondary metabolite" href="http://en.wikipedia.org/wiki/Secondary_metabolite">secondary metabolites</a> are described as promiscuous, as they can act on a relatively broad range of different substrates. It has been suggested that this broad substrate specificity is important for the evolution of new biosynthetic pathways.<sup class="reference" id="_ref-15"><a title="" href="http://en.wikipedia.org/wiki/Enzyme#_note-15">[17]</a></sup></p>
<p><a name=".22Lock_and_key.22_model"></a></p>
<h4><span class="editsection">[<a title="Edit section: "Lock and key" model" href="http://en.wikipedia.org/w/index.php?title=Enzyme&action=edit&section=4">edit</a>]</span> <span class="mw-headline">"Lock and key" model</span></h4>
<p>Enzymes are very specific, and it was suggested by <a title="Emil Fischer" href="http://en.wikipedia.org/wiki/Emil_Fischer">Emil Fischer</a> in 1894 that this was because both the enzyme and the substrate possess specific complementary geometric shapes that fit exactly into one another.<sup class="reference" id="_ref-16"><a title="" href="http://en.wikipedia.org/wiki/Enzyme#_note-16">[18]</a></sup> This is often referred to as "the lock and key" model. However, while this model explains enzyme specificity, it fails to explain the stabilization of the transition state that enzymes achieve.</p>
<p><a id="Induced_fit_model" name="Induced_fit_model"></a></p>
<h4><span class="editsection">[<a title="Edit section: Induced fit model" href="http://en.wikipedia.org/w/index.php?title=Enzyme&action=edit&section=5">edit</a>]</span> <span class="mw-headline">Induced fit model</span></h4>
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<div style="width: 452px;" class="thumbinner"><a title="Diagrams to show the induced fit hypothesis of enzyme action." class="internal" href="http://en.wikipedia.org/wiki/Image:Induced_fit_diagram.svg"><img width="450" height="176" class="thumbimage" longdesc="/wiki/Image:Induced_fit_diagram.svg" alt="Diagrams to show the induced fit hypothesis of enzyme action." src="http://upload.wikimedia.org/wikipedia/commons/thumb/2/24/Induced_fit_diagram.svg/450px-Induced_fit_diagram.svg.png" /></a>
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<div style="float: right;" class="magnify"><a title="Enlarge" class="internal" href="http://en.wikipedia.org/wiki/Image:Induced_fit_diagram.svg"><img width="15" height="11" alt="" src="http://en.wikipedia.org/skins-1.5/common/images/magnify-clip.png" /></a></div>
Diagrams to show the induced fit hypothesis of enzyme action.</div>
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<p>In 1958 <a title="Daniel Koshland" class="new" href="http://en.wikipedia.org/w/index.php?title=Daniel_Koshland&action=edit">Daniel Koshland</a> suggested a modification to the lock and key model: since enzymes are rather flexible structures, the active site can be reshaped by interactions with the substrate as the substrate interacts with the enzyme.<sup class="reference" id="_ref-17"><a title="" href="http://en.wikipedia.org/wiki/Enzyme#_note-17">[19]</a></sup> As a result, the amino acid <a title="Side chain" href="http://en.wikipedia.org/wiki/Side_chain">side chains</a> which make up the active site are molded into the precise positions that enable the enzyme to perform its catalytic function. In some cases, such as glycosidases, the substrate molecule also changes shape slightly as it enters the active site.<sup class="reference" id="_ref-18"><a title="" href="http://en.wikipedia.org/wiki/Enzyme#_note-18">[20]</a></sup></p>
<p><a id="Mechanisms" name="Mechanisms"></a></p>
<h3><span class="editsection">[<a title="Edit section: Mechanisms" href="http://en.wikipedia.org/w/index.php?title=Enzyme&action=edit&section=6">edit</a>]</span> <span class="mw-headline">Mechanisms</span></h3>
<p>Enzymes can act in several ways, all of which lower ΔG<sup>&Dagger;</sup>:<sup class="reference" id="_ref-19"><a title="" href="http://en.wikipedia.org/wiki/Enzyme#_note-19">[21]</a></sup></p>
<ul>
<li>Lowering the <a title="Activation energy" href="http://en.wikipedia.org/wiki/Activation_energy">activation energy</a> by creating an environment in which the transition state is stabilised (e.g. straining the shape of a substrate - by binding the transition-state conformation of the substrate/product molecules, the enzyme distorts the bound substrate(s) into their transition state form, thereby reducing the amount of energy required to complete the transition).</li>
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<li>Providing an alternative pathway (e.g. temporarily reacting with the substrate to form an intermediate which would be impossible in the absence of the enzyme).</li>
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<li>Reducing the reaction entropy change by bringing substrates together in the correct orientation to react. Considering ΔH<sup>&Dagger;</sup> alone overlooks this effect.</li>
</ul>
<p><a id="Dynamics_and_function" name="Dynamics_and_function"></a></p>
<h4><span class="editsection">[<a title="Edit section: Dynamics and function" href="http://en.wikipedia.org/w/index.php?title=Enzyme&action=edit&section=7">edit</a>]</span> <span class="mw-headline">Dynamics and function</span></h4>
<p>Recent investigations have provided new insights into the connection between internal dynamics of enzymes and their mechanism of catalysis.<sup class="reference" id="_ref-20"><a title="" href="http://en.wikipedia.org/wiki/Enzyme#_note-20">[22]</a></sup><sup class="reference" id="_ref-21"><a title="" href="http://en.wikipedia.org/wiki/Enzyme#_note-21">[23]</a></sup><sup class="reference" id="_ref-22"><a title="" href="http://en.wikipedia.org/wiki/Enzyme#_note-22">[24]</a></sup> An enzyme's internal dynamics are described as the movement of internal parts (<em>e.g.</em> amino acids, a group of amino acids, a loop region, an alpha helix, neighboring beta-sheets or even entire domain) of these biomolecules, which can occur at various time-scales ranging from <a title="Femtoseconds" href="http://en.wikipedia.org/wiki/Femtoseconds">femtoseconds</a> to seconds. Networks of protein residues throughout an enzyme's structure can contribute to catalysis through dynamic motions.<sup class="reference" id="_ref-23"><a title="" href="http://en.wikipedia.org/wiki/Enzyme#_note-23">[25]</a></sup><sup class="reference" id="_ref-24"><a title="" href="http://en.wikipedia.org/wiki/Enzyme#_note-24">[26]</a></sup><sup class="reference" id="_ref-25"><a title="" href="http://en.wikipedia.org/wiki/Enzyme#_note-25">[27]</a></sup><sup class="reference" id="_ref-26"><a title="" href="http://en.wikipedia.org/wiki/Enzyme#_note-26">[28]</a></sup> Protein motions are vital to many enzymes, but whether small and fast vibrations or larger and slower conformational movements are more important depends on the type of reaction involved. These new insights also have implications in understanding allosteric effects, producing designer enzymes and developing new drugs.</p>
<p><a id="Allosteric_modulation" name="Allosteric_modulation"></a></p>
<h3><span class="editsection">[<a title="Edit section: Allosteric modulation" href="http://en.wikipedia.org/w/index.php?title=Enzyme&action=edit&section=8">edit</a>]</span> <span class="mw-headline">Allosteric modulation</span></h3>
<p><a title="Allosteric" href="http://en.wikipedia.org/wiki/Allosteric">Allosteric</a> enzymes change their structure in response to binding of <a title="Effector (biology)" href="http://en.wikipedia.org/wiki/Effector_%28biology%29">effectors</a>. Modulation can be direct, where the effector binds directly to <a title="Binding site" href="http://en.wikipedia.org/wiki/Binding_site">binding sites</a> in the enzyme, or indirect, where the effector binds to other proteins or <a title="Protein subunit" href="http://en.wikipedia.org/wiki/Protein_subunit">protein subunits</a> that interact with the allosteric enzyme and thus influence catalytic activity.</p>
<p><a id="Cofactors_and_coenzymes" name="Cofactors_and_coenzymes"></a></p>
<h2><span class="editsection">[<a title="Edit section: Cofactors and coenzymes" href="http://en.wikipedia.org/w/index.php?title=Enzyme&action=edit&section=9">edit</a>]</span> <span class="mw-headline">Cofactors and coenzymes</span></h2>
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<div class="noprint"><em>Main articles: <a title="Cofactor (biochemistry)" href="http://en.wikipedia.org/wiki/Cofactor_%28biochemistry%29">Cofactor (biochemistry)</a> and <a title="Coenzyme" href="http://en.wikipedia.org/wiki/Coenzyme">Coenzyme</a></em></div>
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<p><a id="Cofactors" name="Cofactors"></a></p>
<h3><span class="editsection">[<a title="Edit section: Cofactors" href="http://en.wikipedia.org/w/index.php?title=Enzyme&action=edit&section=10">edit</a>]</span> <span class="mw-headline">Cofactors</span></h3>
<p>Some enzymes do not need any additional components to show full activity. However, others require non-protein molecules to be bound for activity. Cofactors can be either <a title="Inorganic" href="http://en.wikipedia.org/wiki/Inorganic">inorganic</a> (<em>e.g.</em>, metal ions and <a title="Iron-sulfur cluster" href="http://en.wikipedia.org/wiki/Iron-sulfur_cluster">iron-sulfur clusters</a>) or <a title="Organic molecules" href="http://en.wikipedia.org/wiki/Organic_molecules">organic compounds</a>, (e.g., <a title="Flavin" href="http://en.wikipedia.org/wiki/Flavin">flavin</a> and <a title="Heme" href="http://en.wikipedia.org/wiki/Heme">heme</a>). Organic cofactors (coenzymes) are usually <a title="Prosthetic groups" href="http://en.wikipedia.org/wiki/Prosthetic_groups">prosthetic groups</a>, which are tightly bound to the enzymes that they assist. These tightly-bound cofactors are distinguished from other coenzymes, such as <a title="Nicotinamide adenine dinucleotide" href="http://en.wikipedia.org/wiki/Nicotinamide_adenine_dinucleotide">NADH</a>, since they are not released from the active site during the reaction.</p>
<p>An example of an enzyme that contains a cofactor is <a title="Carbonic anhydrase" href="http://en.wikipedia.org/wiki/Carbonic_anhydrase">carbonic anhydrase</a>, and is shown in the ribbon diagram above with a zinc cofactor bound in its active site.<sup class="reference" id="_ref-27"><a title="" href="http://en.wikipedia.org/wiki/Enzyme#_note-27">[29]</a></sup> These tightly-bound molecules are usually found in the active site and are involved in catalysis. For example, flavin and heme cofactors are often involved in <a title="Redox" href="http://en.wikipedia.org/wiki/Redox">redox</a> reactions.</p>
<p>Enzymes that require a cofactor but do not have one bound are called apoenzymes. An apoenzyme together with its cofactor(s) is called a holoenzyme (<em>i.e.</em>, the active form). Most cofactors are not covalently attached to an enzyme, but are very tightly bound. However, organic prosthetic groups can be covalently bound (<em>e.g.</em>, <a title="Thiamine pyrophosphate" href="http://en.wikipedia.org/wiki/Thiamine_pyrophosphate">thiamine pyrophosphate</a> in the enzyme <a title="Pyruvate dehydrogenase" href="http://en.wikipedia.org/wiki/Pyruvate_dehydrogenase">pyruvate dehydrogenase</a>).</p>
<p><a id="Coenzymes" name="Coenzymes"></a></p>
<h3><span class="editsection">[<a title="Edit section: Coenzymes" href="http://en.wikipedia.org/w/index.php?title=Enzyme&action=edit&section=11">edit</a>]</span> <span class="mw-headline">Coenzymes</span></h3>
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<div style="width: 152px;" class="thumbinner"><a title="Space-filling model of the coenzyme NADH" class="internal" href="http://en.wikipedia.org/wiki/Image:NADH-3D-vdW.png"><img width="150" height="167" class="thumbimage" longdesc="/wiki/Image:NADH-3D-vdW.png" alt="Space-filling model of the coenzyme NADH" src="http://upload.wikimedia.org/wikipedia/commons/thumb/e/ed/NADH-3D-vdW.png/150px-NADH-3D-vdW.png" /></a>
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<div style="float: right;" class="magnify"><a title="Enlarge" class="internal" href="http://en.wikipedia.org/wiki/Image:NADH-3D-vdW.png"><img width="15" height="11" alt="" src="http://en.wikipedia.org/skins-1.5/common/images/magnify-clip.png" /></a></div>
<a title="Molecular graphics" href="http://en.wikipedia.org/wiki/Molecular_graphics#Space-filling_models">Space-filling model</a> of the coenzyme NADH</div>
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<p>Coenzymes are small molecules that transport chemical groups from one enzyme to another.<sup class="reference" id="_ref-28"><a title="" href="http://en.wikipedia.org/wiki/Enzyme#_note-28">[30]</a></sup> Some of these chemicals such as <a title="Riboflavin" href="http://en.wikipedia.org/wiki/Riboflavin">riboflavin</a>, <a title="Thiamine" href="http://en.wikipedia.org/wiki/Thiamine">thiamine</a> and <a title="Folic acid" href="http://en.wikipedia.org/wiki/Folic_acid">folic acid</a> are <a title="Vitamins" href="http://en.wikipedia.org/wiki/Vitamins">vitamins</a>, this is when these compounds cannot be made in the body and must be acquired from the diet. The chemical groups carried include the hydride ion (H+ + 2e-) carried by <a title="Nicotinamide adenine dinucleotide" href="http://en.wikipedia.org/wiki/Nicotinamide_adenine_dinucleotide">NAD or NADP<sup>+</sup></a>, the acetyl group carried by <a title="Coenzyme A" href="http://en.wikipedia.org/wiki/Coenzyme_A">coenzyme A</a>, formyl, methenyl or methyl groups carried by <a title="Folic acid" href="http://en.wikipedia.org/wiki/Folic_acid">folic acid</a> and the methyl group carried by <a title="S-adenosylmethionine" href="http://en.wikipedia.org/wiki/S-adenosylmethionine">S-adenosylmethionine</a>.</p>
<p>Since coenzymes are chemically changed as a consequence of enzyme action, it is useful to consider coenzymes to be a special class of substrates, or second substrates, which are common to many different enzymes. For example, about 700 enzymes are known to use the coenzyme NADH.<sup class="reference" id="_ref-29"><a title="" href="http://en.wikipedia.org/wiki/Enzyme#_note-29">[31]</a></sup></p>
<p>Coenzymes are usually regenerated and their concentrations maintained at a steady level inside the cell: for example, NADPH is regenerated through the <a title="Pentose phosphate pathway" href="http://en.wikipedia.org/wiki/Pentose_phosphate_pathway">pentose phosphate pathway</a> and <em>S</em>-adenosylmethionine by methionine adenosyltransferase.</p>
<p><a id="Thermodynamics" name="Thermodynamics"></a></p>
<h2><span class="editsection">[<a title="Edit section: Thermodynamics" href="http://en.wikipedia.org/w/index.php?title=Enzyme&action=edit&section=12">edit</a>]</span> <span class="mw-headline">Thermodynamics</span></h2>
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<div class="noprint"><em>Main articles: <a title="Activation energy" href="http://en.wikipedia.org/wiki/Activation_energy">Activation energy</a>, <a title="Thermodynamic equilibrium" href="http://en.wikipedia.org/wiki/Thermodynamic_equilibrium">Thermodynamic equilibrium</a>, and <a title="Chemical equilibrium" href="http://en.wikipedia.org/wiki/Chemical_equilibrium">Chemical equilibrium</a></em></div>
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<div style="width: 302px;" class="thumbinner"><a title="Diagram of a catalytic reaction, showing the energy niveau at each stage of the reaction. The substrates usually need a large amount of energy to reach the transition state, which then decays into the end product. The enzyme stabilizes the transition state, reducing the energy needed to form this species and thus reducing the energy required to form products." class="internal" href="http://en.wikipedia.org/wiki/Image:Activation2_updated.svg"><img width="300" height="235" class="thumbimage" longdesc="/wiki/Image:Activation2_updated.svg" alt="Diagram of a catalytic reaction, showing the energy niveau at each stage of the reaction. The substrates usually need a large amount of energy to reach the transition state, which then decays into the end product. The enzyme stabilizes the transition state, reducing the energy needed to form this species and thus reducing the energy required to form products." src="http://upload.wikimedia.org/wikipedia/commons/thumb/e/e3/Activation2_updated.svg/300px-Activation2_updated.svg.png" /></a>
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<div style="float: right;" class="magnify"><a title="Enlarge" class="internal" href="http://en.wikipedia.org/wiki/Image:Activation2_updated.svg"><img width="15" height="11" alt="" src="http://en.wikipedia.org/skins-1.5/common/images/magnify-clip.png" /></a></div>
Diagram of a catalytic reaction, showing the energy <em>niveau</em> at each stage of the reaction. The substrates usually need a large amount of energy to reach the transition state, which then decays into the end product. The enzyme stabilizes the transition state, reducing the energy needed to form this species and thus reducing the energy required to form products.</div>
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<p>As all catalysts, enzymes do not alter the position of the chemical equilibrium of the reaction. Usually, in the presence of an enzyme, the reaction runs in the same direction as it would without the enzyme, just more quickly. However, in the absence of the enzyme, other possible uncatalyzed, "spontaneous" reactions might lead to different products, because in those conditions this different product is formed faster.</p>
<p>Furthermore, enzymes can couple two or more reactions, so that a thermodynamically favorable reaction can be used to "drive" a thermodynamically unfavorable one. For example, the hydrolysis of <a title="Adenosine triphosphate" href="http://en.wikipedia.org/wiki/Adenosine_triphosphate">ATP</a> is often used to drive other chemical reactions.</p>
<p>Enzymes catalyze the forward and backward reactions equally. They do not alter the equilibrium itself, but only the speed at which it is reached. For example, <a title="Carbonic anhydrase" href="http://en.wikipedia.org/wiki/Carbonic_anhydrase">carbonic anhydrase</a> catalyzes its reaction in either direction depending on the concentration of its reactants.</p>
<dl><dd><img alt="\mathrm{CO_2 + H_2O {}^\mathrm{\quad Carbonic\ anhydrase} \!\!\!\!\!\!\!\!\!\!\!\!\!\!\!\!\!\!\!\!\!\!\!\!\!\!\!\!\!\!\!\!\!\!\!\!\!\!\!\!\!\! \overrightarrow{\qquad\qquad\qquad\qquad} H_2CO_3}" src="http://upload.wikimedia.org/math/c/8/c/c8c255d26e3f46da6bbacb1606142e6f.png" class="tex" /> (in <a title="Biological tissue" href="http://en.wikipedia.org/wiki/Biological_tissue">tissues</a>; high CO<sub>2</sub> concentration)</dd><dd><img alt="\mathrm{H_2CO_3 {}^\mathrm{\quad Carbonic\ anhydrase} \!\!\!\!\!\!\!\!\!\!\!\!\!\!\!\!\!\!\!\!\!\!\!\!\!\!\!\!\!\!\!\!\!\!\!\!\!\!\!\!\!\! \overrightarrow{\qquad\qquad\qquad\qquad} CO_2 + H_2O}" src="http://upload.wikimedia.org/math/7/e/2/7e2012023ac3500d9e061834ffc2074e.png" class="tex" /> (in <a title="Lung" href="http://en.wikipedia.org/wiki/Lung">lungs</a>; low CO<sub>2</sub> concentration)</dd></dl>
<p>Nevertheless, if the equilibrium is greatly displaced in one direction, that is, in a very <a title="Exergonic" href="http://en.wikipedia.org/wiki/Exergonic">exergonic</a> reaction, the reaction is <em>effectively</em> irreversible. Under these conditions the enzyme will, in fact, only catalyze the reaction in the thermodynamically allowed direction.</p>
<p><a id="Kinetics" name="Kinetics"></a></p>
<h2><span class="editsection">[<a title="Edit section: Kinetics" href="http://en.wikipedia.org/w/index.php?title=Enzyme&action=edit&section=13">edit</a>]</span> <span class="mw-headline">Kinetics</span></h2>
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<div class="noprint"><em>Main article: <a title="Enzyme kinetics" href="http://en.wikipedia.org/wiki/Enzyme_kinetics">Enzyme kinetics</a></em></div>
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<div style="width: 302px;" class="thumbinner"><a title="Mechanism for a single substrate enzyme catalyzed reaction. The enzyme (E) binds a substrate (S) and produces a product (P)." class="internal" href="http://en.wikipedia.org/wiki/Image:Simple_mechanism.svg"><img width="300" height="117" class="thumbimage" longdesc="/wiki/Image:Simple_mechanism.svg" alt="Mechanism for a single substrate enzyme catalyzed reaction. The enzyme (E) binds a substrate (S) and produces a product (P)." src="http://upload.wikimedia.org/wikipedia/en/thumb/9/96/Simple_mechanism.svg/300px-Simple_mechanism.svg.png" /></a>
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<div style="float: right;" class="magnify"><a title="Enlarge" class="internal" href="http://en.wikipedia.org/wiki/Image:Simple_mechanism.svg"><img width="15" height="11" alt="" src="http://en.wikipedia.org/skins-1.5/common/images/magnify-clip.png" /></a></div>
Mechanism for a single substrate enzyme catalyzed reaction. The enzyme (E) binds a substrate (S) and produces a product (P).</div>
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<p>Enzyme kinetics is the investigation of how enzymes bind substrates and turn them into products. The rate data used in kinetic analyses are obtained from <a title="Enzyme assay" href="http://en.wikipedia.org/wiki/Enzyme_assay">enzyme assays</a>. In 1913 <a title="Leonor Michaelis" href="http://en.wikipedia.org/wiki/Leonor_Michaelis">Leonor Michaelis</a> and <a title="Maud Menten" href="http://en.wikipedia.org/wiki/Maud_Menten">Maud Menten</a> proposed a quantitative theory of enzyme kinetics, which is referred to as <a title="Michaelis-Menten kinetics" href="http://en.wikipedia.org/wiki/Michaelis-Menten_kinetics">Michaelis-Menten kinetics</a>.<sup class="reference" id="_ref-30"><a title="" href="http://en.wikipedia.org/wiki/Enzyme#_note-30">[32]</a></sup> Their work was further developed by <a title="George Edward Briggs" href="http://en.wikipedia.org/wiki/George_Edward_Briggs">G. E. Briggs</a> and <a title="J. B. S. Haldane" href="http://en.wikipedia.org/wiki/J._B._S._Haldane">J. B. S. Haldane</a>, who derived kinetic equations that are still widely used today.<sup class="reference" id="_ref-31"><a title="" href="http://en.wikipedia.org/wiki/Enzyme#_note-31">[33]</a></sup></p>
<p>The major contribution of Michaelis and Menten was to think of enzyme reactions in two stages. In the first, the substrate binds reversibly to the enzyme, forming the enzyme-substrate complex. This is sometimes called the Michaelis-Menten complex in their honor. The enzyme then catalyzes the chemical step in the reaction and releases the product.</p>
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<div style="width: 302px;" class="thumbinner"><a title="Saturation curve for an enzyme reaction showing the relation between the substrate concentration (S) and rate (v)." class="internal" href="http://en.wikipedia.org/wiki/Image:MM_curve.png"><img width="300" height="229" class="thumbimage" longdesc="/wiki/Image:MM_curve.png" alt="Saturation curve for an enzyme reaction showing the relation between the substrate concentration (S) and rate (v)." src="http://upload.wikimedia.org/wikipedia/commons/thumb/2/26/MM_curve.png/300px-MM_curve.png" /></a>
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<div style="float: right;" class="magnify"><a title="Enlarge" class="internal" href="http://en.wikipedia.org/wiki/Image:MM_curve.png"><img width="15" height="11" alt="" src="http://en.wikipedia.org/skins-1.5/common/images/magnify-clip.png" /></a></div>
Saturation curve for an enzyme reaction showing the relation between the substrate concentration (S) and rate (<em>v</em>).</div>
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<p>Enzymes can catalyze up to several million reactions per second. For example, the reaction catalysed by <a title="Orotidine 5'-phosphate decarboxylase" href="http://en.wikipedia.org/wiki/Orotidine_5%27-phosphate_decarboxylase">orotidine 5'-phosphate decarboxylase</a> will consume half of its substrate in 78 million years if no enzyme is present. However, when the decarboxylase is added, the same process takes just 25 milliseconds.<sup class="reference" id="_ref-32"><a title="" href="http://en.wikipedia.org/wiki/Enzyme#_note-32">[34]</a></sup> Enzyme rates depend on solution conditions and substrate concentration. Conditions that denature the protein abolish enzyme activity, such as high temperatures, extremes of pH or high salt concentrations, while raising substrate concentration tends to increase activity. To find the maximum speed of an enzymatic reaction, the substrate concentration is increased until a constant rate of product formation is seen. This is shown in the saturation curve, shown on the right. Saturation happens because, as substrate concentration increases, more and more of the free enzyme is converted into the substrate-bound ES form. At the maximum velocity (<em>V</em><sub>max</sub>) of the enzyme, all enzyme active sites are saturated with substrate, and the amount of ES complex is the same as the total amount of enzyme.</p>
<p>However, <em>V</em><sub>max</sub> is only one kinetic constant of enzymes. The amount of substrate needed to achieve a given rate of reaction is also important. This is given by the <a title="Michaelis-Menten constant" href="http://en.wikipedia.org/wiki/Michaelis-Menten_constant">Michaelis-Menten constant</a> (<em>K</em><sub>m</sub>), which is the substrate concentration required for an enzyme to reach one-half its maximum velocity. Each enzyme has a characteristic <em>K</em><sub>m</sub> for a given substrate, and this can show how tight the binding of the substrate is to the enzyme. Another useful constant is <em>k</em><sub>cat</sub>, which is the number of substrate molecules handled by one active site per second.</p>
<p>The efficiency of an enzyme can be expressed in terms of <em>k</em><sub>cat</sub>/<em>K</em><sub>m</sub>. This is also called the specificity constant and incorporates the <a title="Rate constant" href="http://en.wikipedia.org/wiki/Rate_constant">rate constants</a> for all steps in the reaction. Because the specificity constant reflects both affinity and catalytic ability, it is useful for comparing different enzymes against each other, or the same enzyme with different substrates. The theoretical maximum for the specificity constant is called the diffusion limit and is about 10<sup>8</sup> to 10<sup>9</sup> (M<sup>-1</sup> s<sup>-1</sup>). At this point every collision of the enzyme with its substrate will result in catalysis, and the rate of product formation is not limited by the reaction rate but by the diffusion rate. Enzymes with this property are called <em><a title="Catalytically perfect enzyme" href="http://en.wikipedia.org/wiki/Catalytically_perfect_enzyme">catalytically perfect</a></em> or <em>kinetically perfect</em>. Example of such enzymes are <a title="Triosephosphateisomerase" href="http://en.wikipedia.org/wiki/Triosephosphateisomerase">triose-phosphate isomerase</a>, <a title="Carbonic anhydrase" href="http://en.wikipedia.org/wiki/Carbonic_anhydrase">carbonic anhydrase</a>, <a title="Acetylcholinesterase" href="http://en.wikipedia.org/wiki/Acetylcholinesterase">acetylcholinesterase</a>, <a title="Catalase" href="http://en.wikipedia.org/wiki/Catalase">catalase</a>, fumarase, ß-lactamase, and <a title="Superoxide dismutase" href="http://en.wikipedia.org/wiki/Superoxide_dismutase">superoxide dismutase</a>.</p>
<p>Some enzymes operate with kinetics which are faster than diffusion rates, which would seem to be impossible. Several mechanisms have been invoked to explain this phenomenon. Some proteins are believed to accelerate catalysis by drawing their substrate in and pre-orienting them by using dipolar electric fields. Other models invoke a quantum-mechanical <a title="Quantum tunneling" href="http://en.wikipedia.org/wiki/Quantum_tunneling">tunneling</a> explanation, whereby a proton or an electron can tunnel through activation barriers, although for proton tunneling this model remains somewhat controversial.<sup class="reference" id="_ref-33"><a title="" href="http://en.wikipedia.org/wiki/Enzyme#_note-33">[35]</a></sup><sup class="reference" id="_ref-34"><a title="" href="http://en.wikipedia.org/wiki/Enzyme#_note-34">[36]</a></sup> Quantum tunneling for protons has been observed in <a title="Tryptamine" href="http://en.wikipedia.org/wiki/Tryptamine">tryptamine</a>.<sup class="reference" id="_ref-35"><a title="" href="http://en.wikipedia.org/wiki/Enzyme#_note-35">[37]</a></sup> This suggests that enzyme catalysis may be more accurately characterized as "through the barrier" rather than the traditional model, which requires substrates to go "over" a lowered energy barrier.</p>
<p><a id="Inhibition" name="Inhibition"></a></p>
<h2><span class="editsection">[<a title="Edit section: Inhibition" href="http://en.wikipedia.org/w/index.php?title=Enzyme&action=edit&section=14">edit</a>]</span> <span class="mw-headline">Inhibition</span></h2>
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<div style="width: 402px;" class="thumbinner"><a title="Competitive inhibitors bind reversibly to the enzyme, preventing the binding of substrate. On the other hand, binding of substrate prevents binding of the inhibitor. Substrate and inhibitor compete for the enzyme." class="internal" href="http://en.wikipedia.org/wiki/Image:Competitive_inhibition.svg"><img width="400" height="280" class="thumbimage" longdesc="/wiki/Image:Competitive_inhibition.svg" alt="Competitive inhibitors bind reversibly to the enzyme, preventing the binding of substrate. On the other hand, binding of substrate prevents binding of the inhibitor. Substrate and inhibitor compete for the enzyme." src="http://upload.wikimedia.org/wikipedia/commons/thumb/f/fe/Competitive_inhibition.svg/400px-Competitive_inhibition.svg.png" /></a>
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<div style="float: right;" class="magnify"><a title="Enlarge" class="internal" href="http://en.wikipedia.org/wiki/Image:Competitive_inhibition.svg"><img width="15" height="11" alt="" src="http://en.wikipedia.org/skins-1.5/common/images/magnify-clip.png" /></a></div>
Competitive inhibitors bind reversibly to the enzyme, preventing the binding of substrate. On the other hand, binding of substrate prevents binding of the inhibitor. Substrate and inhibitor compete for the enzyme.</div>
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<dl><dd>
<div class="noprint"><em>Main article: <a title="Enzyme inhibitor" href="http://en.wikipedia.org/wiki/Enzyme_inhibitor">Enzyme inhibitor</a></em></div>
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<p>Enzyme reaction rates can be decreased by various types of <a title="Enzyme inhibitor" href="http://en.wikipedia.org/wiki/Enzyme_inhibitor">enzyme inhibitors</a>.</p>
<p><a id="Reversible_inhibitors" name="Reversible_inhibitors"></a></p>
<h3><span class="editsection">[<a title="Edit section: Reversible inhibitors" href="http://en.wikipedia.org/w/index.php?title=Enzyme&action=edit&section=15">edit</a>]</span> <span class="mw-headline">Reversible inhibitors</span></h3>
<p><strong>Competitive inhibition</strong></p>
<p>In competitive inhibition the inhibitor binds to the substrate binding site (figure <em>right</em>, top, thus preventing substrate from binding (EI complex). Often competitive inhibitors strongly resemble the real substrate of the enzyme. For example, <a title="Methotrexate" href="http://en.wikipedia.org/wiki/Methotrexate">methotrexate</a> is a competitive inhibitor of the enzyme <a title="Dihydrofolate reductase" href="http://en.wikipedia.org/wiki/Dihydrofolate_reductase">dihydrofolate reductase</a>, which catalyzes the reduction of <a title="Folic acid" href="http://en.wikipedia.org/wiki/Folic_acid">dihydrofolate</a> to <a title="Folic acid" href="http://en.wikipedia.org/wiki/Folic_acid">tetrahydrofolate</a>. The similarity between the structures of folic acid and this drug are shown in the figure to the <em>right</em> bottom.</p>
<p><strong>Non-competitive inhibition</strong></p>
<p>Non-competitive inhibitors can bind either to the active site, or to other parts of the enzyme far away from the substrate-binding site. Moreover, non-competitive inhibitors bind to the enzyme-substrate (ES) complex and to the free enzyme. Their binding to this site changes the shape of the enzyme and stops the active site binding substrate(s). Consequently, since there is no direct competition between the substrate and inhibitor for the enzyme, the extent of inhibition depends only on the inhibitor concentration and will not be affected by the substrate concentration.</p>
<p><a id="Irreversible_inhibitors" name="Irreversible_inhibitors"></a></p>
<h3><span class="editsection">[<a title="Edit section: Irreversible inhibitors" href="http://en.wikipedia.org/w/index.php?title=Enzyme&action=edit&section=16">edit</a>]</span> <span class="mw-headline">Irreversible inhibitors</span></h3>
<p>Some enzyme inhibitors react with the enzyme and form a <a title="Covalent bond" href="http://en.wikipedia.org/wiki/Covalent_bond">covalent</a> adduct with the protein. The inactivation produced by this type of inhibitor cannot be reversed. A class of these compounds called <a title="Suicide inhibitor" href="http://en.wikipedia.org/wiki/Suicide_inhibitor">suicide inhibitors</a> includes <a title="Eflornithine" href="http://en.wikipedia.org/wiki/Eflornithine">eflornithine</a> a drug used to treat the parasitic disease <a title="Sleeping sickness" href="http://en.wikipedia.org/wiki/Sleeping_sickness">sleeping sickness</a>.</p>
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<div style="width: 402px;" class="thumbinner"><a title="The coenzyme folic acid (left) and the anti-cancer drug methotrexate (right) are very similar in structure. As a result, methotrexate is a competitive inhibitor of many enzymes that use folates." class="internal" href="http://en.wikipedia.org/wiki/Image:Methotrexate_and_folic_acid_compared.png"><img width="400" height="128" class="thumbimage" longdesc="/wiki/Image:Methotrexate_and_folic_acid_compared.png" alt="The coenzyme folic acid (left) and the anti-cancer drug methotrexate (right) are very similar in structure. As a result, methotrexate is a competitive inhibitor of many enzymes that use folates." src="http://upload.wikimedia.org/wikipedia/en/6/67/Methotrexate_and_folic_acid_compared.png" /></a>
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<div style="float: right;" class="magnify"><a title="Enlarge" class="internal" href="http://en.wikipedia.org/wiki/Image:Methotrexate_and_folic_acid_compared.png"><img width="15" height="11" alt="" src="http://en.wikipedia.org/skins-1.5/common/images/magnify-clip.png" /></a></div>
The coenzyme folic acid (left) and the anti-cancer drug methotrexate (right) are very similar in structure. As a result, methotrexate is a competitive inhibitor of many enzymes that use folates.</div>
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<p><a id="Uses_of_inhibitors" name="Uses_of_inhibitors"></a></p>
<h3><span class="editsection">[<a title="Edit section: Uses of inhibitors" href="http://en.wikipedia.org/w/index.php?title=Enzyme&action=edit&section=17">edit</a>]</span> <span class="mw-headline">Uses of inhibitors</span></h3>
<p>Inhibitors are often used as drugs, but they can also act as poisons. However, the difference between a drug and a poison is usually only a matter of amount, since most drugs are toxic at some level, as <a title="Paracelsus" href="http://en.wikipedia.org/wiki/Paracelsus">Paracelsus</a> wrote, "<em>In all things there is a poison, and there is nothing without a poison.</em>"<sup class="reference" id="_ref-36"><a title="" href="http://en.wikipedia.org/wiki/Enzyme#_note-36">[38]</a></sup> Equally, <a title="Antibiotics" href="http://en.wikipedia.org/wiki/Antibiotics">antibiotics</a> and other anti-infective drugs are just specific poisons that can kill a pathogen but not its host.</p>
<p>An example of an inhibitor being used as a drug is <a title="Aspirin" href="http://en.wikipedia.org/wiki/Aspirin">aspirin</a>, which inhibits the <a title="Cyclooxygenase" href="http://en.wikipedia.org/wiki/Cyclooxygenase">COX-1</a> and <a title="Cyclooxygenase" href="http://en.wikipedia.org/wiki/Cyclooxygenase">COX-2</a> enzymes that produce the <a title="Inflammation" href="http://en.wikipedia.org/wiki/Inflammation">inflammation</a> messenger <a title="Prostaglandin" href="http://en.wikipedia.org/wiki/Prostaglandin">prostaglandin</a>, thus suppressing pain and inflammation. The poison <a title="Cyanide" href="http://en.wikipedia.org/wiki/Cyanide">cyanide</a> is an irreversible enzyme inhibitor that combines with the copper and iron in the active site of the enzyme <a title="Cytochrome c oxidase" href="http://en.wikipedia.org/wiki/Cytochrome_c_oxidase">cytochrome c oxidase</a> and blocks <a title="Cellular respiration" href="http://en.wikipedia.org/wiki/Cellular_respiration">cellular respiration</a>.<sup class="reference" id="_ref-37"><a title="" href="http://en.wikipedia.org/wiki/Enzyme#_note-37">[39]</a></sup></p>
<p>In many organisms inhibitors may act as part of a <a title="Feedback" href="http://en.wikipedia.org/wiki/Feedback">feedback</a> mechanism. If an enzyme produces too much of one substance in the organism, that substance may act as an inhibitor for the enzyme that produces it, causing production of the substance to slow down or stop when there is sufficient amount. This is a form of <a title="Negative feedback" href="http://en.wikipedia.org/wiki/Negative_feedback">negative feedback</a>.</p>
<p><a id="Biological_function" name="Biological_function"></a></p>
<h2><span class="editsection">[<a title="Edit section: Biological function" href="http://en.wikipedia.org/w/index.php?title=Enzyme&action=edit&section=18">edit</a>]</span> <span class="mw-headline">Biological function</span></h2>
<p>Enzymes serve a wide variety of functions inside living organisms. They are indispensable for <a title="Signal transduction" href="http://en.wikipedia.org/wiki/Signal_transduction">signal transduction</a> and cell regulation, often via <a title="Kinase" href="http://en.wikipedia.org/wiki/Kinase">kinases</a> and <a title="Phosphatase" href="http://en.wikipedia.org/wiki/Phosphatase">phosphatases</a>.<sup class="reference" id="_ref-38"><a title="" href="http://en.wikipedia.org/wiki/Enzyme#_note-38">[40]</a></sup> They also generate movement, with <a title="Myosin" href="http://en.wikipedia.org/wiki/Myosin">myosin</a> hydrolysing ATP to generate <a title="Muscle contraction" href="http://en.wikipedia.org/wiki/Muscle_contraction">muscle contraction</a> and also moving cargo around the cell as part of the <a title="Cytoskeleton" href="http://en.wikipedia.org/wiki/Cytoskeleton">cytoskeleton</a>.<sup class="reference" id="_ref-39"><a title="" href="http://en.wikipedia.org/wiki/Enzyme#_note-39">[41]</a></sup> Other ATPases in the cell membrane are <a title="Ion pump (biology)" href="http://en.wikipedia.org/wiki/Ion_pump_%28biology%29">ion pumps</a> involved in <a title="Active transport" href="http://en.wikipedia.org/wiki/Active_transport">active transport</a>. Enzymes are also involved in more exotic functions, such as <a title="Luciferase" href="http://en.wikipedia.org/wiki/Luciferase">luciferase</a> generating light in <a title="Firefly" href="http://en.wikipedia.org/wiki/Firefly">fireflies</a>.<sup class="reference" id="_ref-40"><a title="" href="http://en.wikipedia.org/wiki/Enzyme#_note-40">[42]</a></sup></p>
<p><a title="Virus" href="http://en.wikipedia.org/wiki/Virus">Viruses</a> can contain enzymes for infecting cells, such as the <a title="HIV integrase" class="new" href="http://en.wikipedia.org/w/index.php?title=HIV_integrase&action=edit">HIV integrase</a> and <a title="Reverse transcriptase" href="http://en.wikipedia.org/wiki/Reverse_transcriptase">reverse transcriptase</a>, or for viral release from cells, like the <a title="Influenza" href="http://en.wikipedia.org/wiki/Influenza">influenza</a> virus <a title="Neuraminidase" href="http://en.wikipedia.org/wiki/Neuraminidase">neuraminidase</a>.</p>
<p>An important function of enzymes is in the <a title="Digestive systems" href="http://en.wikipedia.org/wiki/Digestive_systems">digestive systems</a> of animals. Enzymes such as <a title="Amylases" href="http://en.wikipedia.org/wiki/Amylases">amylases</a> and <a title="Proteases" href="http://en.wikipedia.org/wiki/Proteases">proteases</a> break down large molecules (<a title="Starch" href="http://en.wikipedia.org/wiki/Starch">starch</a> or <a title="Protein" href="http://en.wikipedia.org/wiki/Protein">proteins</a>, respectively) into smaller ones, so they can be absorbed by the intestines. Starch is inabsorbable in the intestine but enzymes hydrolyse the starch chains into smaller molecules such as <a title="Maltose" href="http://en.wikipedia.org/wiki/Maltose">maltose</a> and eventually <a title="Glucose" href="http://en.wikipedia.org/wiki/Glucose">glucose</a>, which can then be absorbed. Different enzymes digest different food substances. In <a title="Ruminants" href="http://en.wikipedia.org/wiki/Ruminants">ruminants</a> which have a <a title="Herbivorous" href="http://en.wikipedia.org/wiki/Herbivorous">herbivorous</a> diets, bacteria in the gut produce another enzyme, <a title="Cellulase" href="http://en.wikipedia.org/wiki/Cellulase">cellulase</a> to break down the cellulose cell walls of plant fiber.</p>
<p>Several enzymes can work together in a specific order, creating <a title="Metabolic pathway" href="http://en.wikipedia.org/wiki/Metabolic_pathway">metabolic pathways</a>. In a metabolic pathway, one enzyme takes the product of another enzyme as a substrate. After the catalytic reaction, the product is then passed on to another enzyme. Sometimes more than one enzyme can catalyse the same reaction in parallel, this can allow more complex regulation: with for example a low contant activity being provided by one enzyme but an inducible high activity from a second enzyme.</p>
<p>Enzymes determine what steps occur in these pathways. Without enzymes, metabolism would neither progress through the same steps, nor be fast enough to serve the needs of the cell. Indeed, a metabolic pathway such as <a title="Glycolysis" href="http://en.wikipedia.org/wiki/Glycolysis">glycolysis</a> could not exist independently of enzymes. Glucose, for example, can react directly with ATP to become <a title="Phosphorylation" href="http://en.wikipedia.org/wiki/Phosphorylation">phosphorylated</a> at one or more of its carbons. However, if <a title="Hexokinase" href="http://en.wikipedia.org/wiki/Hexokinase">hexokinase</a> is present, <a title="Glucose-6-phosphate" href="http://en.wikipedia.org/wiki/Glucose-6-phosphate">glucose-6-phosphate</a> is the only product, as this reaction will occur most swiftly. Consequently, the network of metabolic pathways within each cell depends on the set of functional enzymes that are present.</p>
<p><a id="Control_of_activity" name="Control_of_activity"></a></p>
<h2><span class="editsection">[<a title="Edit section: Control of activity" href="http://en.wikipedia.org/w/index.php?title=Enzyme&action=edit&section=19">edit</a>]</span> <span class="mw-headline">Control of activity</span></h2>
<p>There are five main ways that enzyme activity is controlled in the cell.</p>
<ol>
<li>Enzyme production (<a title="Transcription (genetics)" href="http://en.wikipedia.org/wiki/Transcription_%28genetics%29">transcription</a> and <a title="Translation (genetics)" href="http://en.wikipedia.org/wiki/Translation_%28genetics%29">translation</a> of enzyme genes) can be enhanced or diminished by a cell in response to changes in the cell's environment. This form of <a title="Regulation of gene expression" href="http://en.wikipedia.org/wiki/Regulation_of_gene_expression">gene regulation</a> is called <a title="Enzyme induction and inhibition" href="http://en.wikipedia.org/wiki/Enzyme_induction_and_inhibition">enzyme induction and inhibition</a>. For example, bacteria may become <a title="Antibiotic resistance" href="http://en.wikipedia.org/wiki/Antibiotic_resistance">resistant to antibiotics</a> such as <a title="Penicillin" href="http://en.wikipedia.org/wiki/Penicillin">penicillin</a> because enzymes called <a title="Beta-lactamase" href="http://en.wikipedia.org/wiki/Beta-lactamase">beta-lactamases</a> are induced that hydrolyse the crucial <a title="Beta-lactam" href="http://en.wikipedia.org/wiki/Beta-lactam">beta-lactam ring</a> within the penicillin molecule. Another example are enzymes in the <a title="Liver" href="http://en.wikipedia.org/wiki/Liver">liver</a> called <a title="Cytochrome P450 oxidase" href="http://en.wikipedia.org/wiki/Cytochrome_P450_oxidase">cytochrome P450 oxidases</a>, which are important in <a title="Drug metabolism" href="http://en.wikipedia.org/wiki/Drug_metabolism">drug metabolism</a>. Induction or inhibition of these enzymes can cause <a title="Drug interaction" href="http://en.wikipedia.org/wiki/Drug_interaction">drug interactions</a>.</li>
<li>Enzymes can be compartmentalized, with different metabolic pathways occurring in different <a title="Cellular compartment" href="http://en.wikipedia.org/wiki/Cellular_compartment">cellular compartments</a>. For example, <a title="Fatty acids" href="http://en.wikipedia.org/wiki/Fatty_acids">fatty acids</a> are synthesized by one set of enzymes in the <a title="Cytosol" href="http://en.wikipedia.org/wiki/Cytosol">cytosol</a>, <a title="Endoplasmic reticulum" href="http://en.wikipedia.org/wiki/Endoplasmic_reticulum">endoplasmic reticulum</a> and the <a title="Golgi apparatus" href="http://en.wikipedia.org/wiki/Golgi_apparatus">Golgi apparatus</a> and used by a different set of enzymes as a source of energy in the <a title="Mitochondrion" href="http://en.wikipedia.org/wiki/Mitochondrion">mitochondrion</a>, through <a title="Β-oxidation" href="http://en.wikipedia.org/wiki/%CE%92-oxidation">β-oxidation</a>.<sup class="reference" id="_ref-41"><a title="" href="http://en.wikipedia.org/wiki/Enzyme#_note-41">[43]</a></sup></li>
<li>Enzymes can be regulated by <a title="Enzyme inhibitor" href="http://en.wikipedia.org/wiki/Enzyme_inhibitor">inhibitors</a> and activators. For example, the end product(s) of a metabolic pathway are often inhibitors for one of the first enzymes of the pathway (usually the first irreversible step, called <em>committed step</em>), thus regulating the amount of end product made by the pathways. Such a regulatory mechanism is called a <a title="Negative feedback" href="http://en.wikipedia.org/wiki/Negative_feedback">negative feedback mechanism</a>, because the amount of the end product produced is regulated by its own concentration. Negative feedback mechanism can effectively adjust the rate of synthesis of intermediate metabolites according to the demands of the cells. This helps allocate materials and energy economically, and prevents the manufacture of excess end products. Like other <a title="Homeostasis" href="http://en.wikipedia.org/wiki/Homeostasis">homeostatic devices</a>, the control of enzymatic action helps to maintain a stable internal environment in living organisms.</li>
<li>Enzymes can be regulated through <a title="Post-translational modification" href="http://en.wikipedia.org/wiki/Post-translational_modification">post-translational modification</a>. This can include <a title="Phosphorylation" href="http://en.wikipedia.org/wiki/Phosphorylation">phosphorylation</a>, <a title="Myristic acid" href="http://en.wikipedia.org/wiki/Myristic_acid">myristoylation</a> and <a title="Glycosylation" href="http://en.wikipedia.org/wiki/Glycosylation">glycosylation</a>. For example, in the response to <a title="Insulin" href="http://en.wikipedia.org/wiki/Insulin">insulin</a>, the <a title="Phosphorylation" href="http://en.wikipedia.org/wiki/Phosphorylation">phosphorylation</a> of multiple enzymes, including <a title="Glycogen synthase" href="http://en.wikipedia.org/wiki/Glycogen_synthase">glycogen synthase</a>, helps control the synthesis or degradation of <a title="Glycogen" href="http://en.wikipedia.org/wiki/Glycogen">glycogen</a> and allows the cell to respond to changes in <a title="Blood sugar" href="http://en.wikipedia.org/wiki/Blood_sugar">blood sugar</a>.<sup class="reference" id="_ref-42"><a title="" href="http://en.wikipedia.org/wiki/Enzyme#_note-42">[44]</a></sup> Another example of post-translational modification is the cleavage of the polypeptide chain. <a title="Chymotrypsin" href="http://en.wikipedia.org/wiki/Chymotrypsin">Chymotrypsin</a>, a digestive <a title="Protease" href="http://en.wikipedia.org/wiki/Protease">protease</a>, is produced in inactive form as <a title="Chymotrypsinogen" href="http://en.wikipedia.org/wiki/Chymotrypsinogen">chymotrypsinogen</a> in the <a title="Pancreas" href="http://en.wikipedia.org/wiki/Pancreas">pancreas</a> and transported in this form to the <a title="Stomach" href="http://en.wikipedia.org/wiki/Stomach">stomach</a> where it is activated. This stops the enzyme from digesting the pancreas or other tissues before it enters the gut. This type of inactive precursor to an enzyme is known as a <a title="Zymogen" href="http://en.wikipedia.org/wiki/Zymogen">zymogen</a>.</li>
<li>Some enzymes may become activated when localized to a different environment (eg. from a reducing (<a title="Cytoplasm" href="http://en.wikipedia.org/wiki/Cytoplasm">cytoplasm</a>) to an oxidising (<a title="Periplasm" href="http://en.wikipedia.org/wiki/Periplasm">periplasm</a>) environment, high pH to low pH etc). For example, <a title="Hemagglutinin" href="http://en.wikipedia.org/wiki/Hemagglutinin">hemagglutinin</a> of the <a title="Influenza" href="http://en.wikipedia.org/wiki/Influenza">influenza</a> virus undergoes a conformational change once it encounters the acidic environment of the host cell <a title="Vesicle (biology)" href="http://en.wikipedia.org/wiki/Vesicle_%28biology%29">vesicle</a> causing its activation. <sup class="reference" id="_ref-43"><a title="" href="http://en.wikipedia.org/wiki/Enzyme#_note-43">[45]</a></sup></li>
</ol>
<p><a id="Involvement_in_disease" name="Involvement_in_disease"></a></p>
<h2><span class="editsection">[<a title="Edit section: Involvement in disease" href="http://en.wikipedia.org/w/index.php?title=Enzyme&action=edit&section=20">edit</a>]</span> <span class="mw-headline">Involvement in disease</span></h2>
<div class="thumb tright">
<div style="width: 202px;" class="thumbinner"><a title="Phenylalanine hydroxylase. Created from PDB 1KW0 " class="internal" href="http://en.wikipedia.org/wiki/Image:Phenylalanine_hydroxylase_brighter.jpg"><img width="200" height="210" class="thumbimage" longdesc="/wiki/Image:Phenylalanine_hydroxylase_brighter.jpg" alt="Phenylalanine hydroxylase. Created from PDB 1KW0 " src="http://upload.wikimedia.org/wikipedia/commons/thumb/9/97/Phenylalanine_hydroxylase_brighter.jpg/200px-Phenylalanine_hydroxylase_brighter.jpg" /></a>
<div class="thumbcaption">
<div style="float: right;" class="magnify"><a title="Enlarge" class="internal" href="http://en.wikipedia.org/wiki/Image:Phenylalanine_hydroxylase_brighter.jpg"><img width="15" height="11" alt="" src="http://en.wikipedia.org/skins-1.5/common/images/magnify-clip.png" /></a></div>
<a title="Phenylalanine hydroxylase" href="http://en.wikipedia.org/wiki/Phenylalanine_hydroxylase">Phenylalanine hydroxylase</a>. Created from <a rel="nofollow" title="http://www.rcsb.org/pdb/explore.do?structureId=1KW0" class="external text" href="http://www.rcsb.org/pdb/explore.do?structureId=1KW0">PDB 1KW0</a></div>
</div>
</div>
<p>Since the tight control of enzyme activity is essential for <a title="Homeostasis" href="http://en.wikipedia.org/wiki/Homeostasis">homeostasis</a>, any malfunction (mutation, overproduction, underproduction or deletion) of a single critical enzyme can lead to a <a title="Genetic disease" href="http://en.wikipedia.org/wiki/Genetic_disease">genetic disease</a>. The importance of enzymes is shown by the fact that a lethal illness can be caused by the malfunction of just one type of enzyme out of the thousands of types present in our bodies.</p>
<p>One example is the most common type of <a title="Phenylketonuria" href="http://en.wikipedia.org/wiki/Phenylketonuria">phenylketonuria</a>. A mutation of a single amino acid in the enzyme <a title="Phenylalanine hydroxylase" href="http://en.wikipedia.org/wiki/Phenylalanine_hydroxylase">phenylalanine hydroxylase</a>, which catalyzes the first step in the degradation of <a title="Phenylalanine" href="http://en.wikipedia.org/wiki/Phenylalanine">phenylalanine</a>, results in build-up of phenylalanine and related products. This can lead to <a title="Mental retardation" href="http://en.wikipedia.org/wiki/Mental_retardation">mental retardation</a> if the disease is untreated.<sup class="reference" id="_ref-44"><a title="" href="http://en.wikipedia.org/wiki/Enzyme#_note-44">[46]</a></sup></p>
<p>Another example is when <a title="Germline mutation" href="http://en.wikipedia.org/wiki/Germline_mutation">germline mutations</a> in genes coding for <a title="DNA repair" href="http://en.wikipedia.org/wiki/DNA_repair">DNA repair</a> enzymes cause hereditary cancer syndromes such as <a title="Xeroderma pigmentosum" href="http://en.wikipedia.org/wiki/Xeroderma_pigmentosum">xeroderma pigmentosum</a>. Defects in these enzymes cause cancer since the body is less able to repair mutations in the genome. This causes a slow accumulation of mutations and results in the development of many types of cancer in the sufferer.</p>
<p><a id="Naming_conventions" name="Naming_conventions"></a></p>
<h2><span class="editsection">[<a title="Edit section: Naming conventions" href="http://en.wikipedia.org/w/index.php?title=Enzyme&action=edit&section=21">edit</a>]</span> <span class="mw-headline">Naming conventions</span></h2>
<p>An enzyme's name is often derived from its substrate or the chemical reaction it catalyzes, with the word ending in <em>-ase</em>. Examples are <a title="Lactase" href="http://en.wikipedia.org/wiki/Lactase">lactase</a>, <a title="Alcohol dehydrogenase" href="http://en.wikipedia.org/wiki/Alcohol_dehydrogenase">alcohol dehydrogenase</a> and <a title="DNA polymerase" href="http://en.wikipedia.org/wiki/DNA_polymerase">DNA polymerase</a>. This may result in different enzymes, called isoenzymes, with the same function having the same basic name. Isoenzymes have a different amino acid sequence and might be distinguished by their optimal <a title="PH" href="http://en.wikipedia.org/wiki/PH">pH</a>, kinetic properties or immunologically. Furthermore, the normal physiological reaction an enzyme catalyzes may not be the same as under artifical conditions. This can result in the same enzyme being identified with two different names. <em>E.g.</em> <a title="Glucose isomerase" href="http://en.wikipedia.org/wiki/Glucose_isomerase">Glucose isomerase</a>, used industrially to convert <a title="Glucose" href="http://en.wikipedia.org/wiki/Glucose">glucose</a> into the sweetener <a title="Fructose" href="http://en.wikipedia.org/wiki/Fructose">fructose</a>, is a xylose isomerase <em>in vivo</em>.</p>
<p>The <a title="International Union of Biochemistry and Molecular Biology" href="http://en.wikipedia.org/wiki/International_Union_of_Biochemistry_and_Molecular_Biology">International Union of Biochemistry and Molecular Biology</a> have developed a <a title="Nomenclature" href="http://en.wikipedia.org/wiki/Nomenclature">nomenclature</a> for enzymes, the <a title="EC number" href="http://en.wikipedia.org/wiki/EC_number">EC numbers</a>; each enzyme is described by a sequence of four numbers preceded by "EC". The first number broadly classifies the enzyme based on its mechanism:</p>
<p>The top-level classification is</p>
<ul>
<li>EC 1 <em><a title="Oxidoreductase" href="http://en.wikipedia.org/wiki/Oxidoreductase">Oxidoreductases</a></em>: catalyze <a title="Oxidation" href="http://en.wikipedia.org/wiki/Oxidation">oxidation</a>/reduction reactions</li>
<li>EC 2 <em><a title="Transferase" href="http://en.wikipedia.org/wiki/Transferase">Transferases</a></em>: transfer a <a title="Functional group" href="http://en.wikipedia.org/wiki/Functional_group">functional group</a> (<em>e.g.</em> a methyl or phosphate group)</li>
<li>EC 3 <em><a title="Hydrolase" href="http://en.wikipedia.org/wiki/Hydrolase">Hydrolases</a></em>: catalyze the <a title="Hydrolysis" href="http://en.wikipedia.org/wiki/Hydrolysis">hydrolysis</a> of various bonds</li>
<li>EC 4 <em><a title="Lyase" href="http://en.wikipedia.org/wiki/Lyase">Lyases</a></em>: cleave various bonds by means other than hydrolysis and oxidation</li>
<li>EC 5 <em><a title="Isomerase" href="http://en.wikipedia.org/wiki/Isomerase">Isomerases</a></em>: catalyze <a title="Isomer" href="http://en.wikipedia.org/wiki/Isomer">isomerization</a> changes within a single molecule</li>
<li>EC 6 <em><a title="Ligase" href="http://en.wikipedia.org/wiki/Ligase">Ligases</a></em>: join two molecules with <a title="Covalent bond" href="http://en.wikipedia.org/wiki/Covalent_bond">covalent bonds</a></li>
</ul>
<p>The complete nomenclature can be browsed at <a rel="nofollow" title="http://www.chem.qmul.ac.uk/iubmb/enzyme/" class="external free" href="http://www.chem.qmul.ac.uk/iubmb/enzyme/">http://www.chem.qmul.ac.uk/iubmb/enzyme/</a>.</p>
<p><a id="Industrial_applications" name="Industrial_applications"></a></p>
<h2><span class="editsection">[<a title="Edit section: Industrial applications" href="http://en.wikipedia.org/w/index.php?title=Enzyme&action=edit&section=22">edit</a>]</span> <span class="mw-headline">Industrial applications</span></h2>
<p>Enzymes are used in the <a title="Chemical industry" href="http://en.wikipedia.org/wiki/Chemical_industry">chemical industry</a> and other industrial applications when extremely specific catalysts are required. However, enzymes in general are limited in the number of reactions they have evolved to catalyse and also by their lack of stability in <a title="Organic solvent" href="http://en.wikipedia.org/wiki/Organic_solvent">organic solvents</a> and at high temperatures. Consequently, <a title="Protein engineering" href="http://en.wikipedia.org/wiki/Protein_engineering">protein engineering</a> is an active area of research and involves attempts to create new enzymes with novel properties, either through rational design or <em>in vitro</em> evolution.<sup class="reference" id="_ref-45"><a title="" href="http://en.wikipedia.org/wiki/Enzyme#_note-45">[47]</a></sup><sup class="reference" id="_ref-46"><a title="" href="http://en.wikipedia.org/wiki/Enzyme#_note-46">[48]</a></sup></p>
<table class="wikitable">
<tbody>
<tr>
<td width="24%" align="center"><strong>Application</strong></td>
<td width="38%" align="center"><strong>Enzymes used</strong></td>
<td width="38%" align="center"><strong>Uses</strong></td>
</tr>
<tr>
<td rowspan="2" style="border-top: 3px solid rgb(170, 170, 170);"><strong><a title="Baking" href="http://en.wikipedia.org/wiki/Baking">Baking industry</a></strong>
<div class="center">
<div class="thumb tnone">
<div style="width: 182px;" class="thumbinner"><a title="alpha-amylase catalyzes the release of sugar monomers from starch" class="internal" href="http://en.wikipedia.org/wiki/Image:Amylose.svg"><img width="180" height="73" class="thumbimage" longdesc="/wiki/Image:Amylose.svg" alt="alpha-amylase catalyzes the release of sugar monomers from starch" src="http://upload.wikimedia.org/wikipedia/commons/thumb/4/45/Amylose.svg/180px-Amylose.svg.png" /></a>
<div class="thumbcaption">
<div style="float: right;" class="magnify"><a title="Enlarge" class="internal" href="http://en.wikipedia.org/wiki/Image:Amylose.svg"><img width="15" height="11" alt="" src="http://en.wikipedia.org/skins-1.5/common/images/magnify-clip.png" /></a></div>
alpha-amylase catalyzes the release of sugar monomers from starch</div>
</div>
</div>
</div>
</td>
<td style="border-top: 3px solid rgb(170, 170, 170);"><a title="Fungus" href="http://en.wikipedia.org/wiki/Fungus">Fungal</a> alpha-amylase enzymes are normally inactivated at about 50 degrees Celsius, but are destroyed during the baking process.</td>
<td style="border-top: 3px solid rgb(170, 170, 170);">Catalyze breakdown of starch in the <a title="Flour" href="http://en.wikipedia.org/wiki/Flour">flour</a> to sugar. Yeast action on sugar produces carbon dioxide. Used in production of white bread, buns, and rolls.</td>
</tr>
<tr>
<td>Proteases</td>
<td>Biscuit manufacturers use them to lower the protein level of flour.</td>
</tr>
<tr>
<td style="border-top: 3px solid rgb(170, 170, 170);"><strong><a title="Baby food" href="http://en.wikipedia.org/wiki/Baby_food">Baby foods</a></strong></td>
<td style="border-top: 3px solid rgb(170, 170, 170);"><a title="Trypsin" href="http://en.wikipedia.org/wiki/Trypsin">Trypsin</a></td>
<td style="border-top: 3px solid rgb(170, 170, 170);">To predigest baby foods.</td>
</tr>
<tr>
<td rowspan="6" style="border-top: 3px solid rgb(170, 170, 170);"><strong><a title="Brewing" href="http://en.wikipedia.org/wiki/Brewing">Brewing industry</a></strong>
<div class="center">
<div class="thumb tnone">
<div style="width: 182px;" class="thumbinner"><a title="Germinating barley used for malt." class="internal" href="http://en.wikipedia.org/wiki/Image:Sjb_whiskey_malt.jpg"><img width="180" height="135" class="thumbimage" longdesc="/wiki/Image:Sjb_whiskey_malt.jpg" alt="Germinating barley used for malt." src="http://upload.wikimedia.org/wikipedia/commons/thumb/3/32/Sjb_whiskey_malt.jpg/180px-Sjb_whiskey_malt.jpg" /></a>
<div class="thumbcaption">
<div style="float: right;" class="magnify"><a title="Enlarge" class="internal" href="http://en.wikipedia.org/wiki/Image:Sjb_whiskey_malt.jpg"><img width="15" height="11" alt="" src="http://en.wikipedia.org/skins-1.5/common/images/magnify-clip.png" /></a></div>
Germinating barley used for malt.</div>
</div>
</div>
</div>
</td>
<td style="border-top: 3px solid rgb(170, 170, 170);">Enzymes from barley are released during the mashing stage of beer production.</td>
<td style="border-top: 3px solid rgb(170, 170, 170);">They degrade starch and proteins to produce simple sugar, amino acids and peptides that are used by yeast for fermentation.</td>
</tr>
<tr>
<td>Industrially produced barley enzymes</td>
<td>Widely used in the brewing process to substitute for the natural enzymes found in barley.</td>
</tr>
<tr>
<td>Amylase, glucanases, proteases</td>
<td>Split polysaccharides and proteins in the <a title="Malt" href="http://en.wikipedia.org/wiki/Malt">malt</a>.</td>
</tr>
<tr>
<td>Betaglucosidase</td>
<td>Improve the filtration characteristics.</td>
</tr>
<tr>
<td>Amyloglucosidase</td>
<td>Low-calorie <a title="Beer" href="http://en.wikipedia.org/wiki/Beer">beer</a>.</td>
</tr>
<tr>
<td>Proteases</td>
<td>Remove cloudiness produced during storage of beers.</td>
</tr>
<tr>
<td style="border-top: 3px solid rgb(170, 170, 170);"><strong><a title="Juice" href="http://en.wikipedia.org/wiki/Juice">Fruit juices</a></strong></td>
<td style="border-top: 3px solid rgb(170, 170, 170);">Cellulases, pectinases</td>
<td style="border-top: 3px solid rgb(170, 170, 170);">Clarify fruit juices</td>
</tr>
<tr>
<td rowspan="4" style="border-top: 3px solid rgb(170, 170, 170);"><strong><a title="Dairy" href="http://en.wikipedia.org/wiki/Dairy">Dairy industry</a></strong>
<div class="center">
<div class="thumb tnone">
<div style="width: 182px;" class="thumbinner"><a title="Roquefort cheese" class="internal" href="http://en.wikipedia.org/wiki/Image:Roquefort_cheese.jpg"><img width="180" height="144" class="thumbimage" longdesc="/wiki/Image:Roquefort_cheese.jpg" alt="Roquefort cheese" src="http://upload.wikimedia.org/wikipedia/commons/thumb/9/92/Roquefort_cheese.jpg/180px-Roquefort_cheese.jpg" /></a>
<div class="thumbcaption">
<div style="float: right;" class="magnify"><a title="Enlarge" class="internal" href="http://en.wikipedia.org/wiki/Image:Roquefort_cheese.jpg"><img width="15" height="11" alt="" src="http://en.wikipedia.org/skins-1.5/common/images/magnify-clip.png" /></a></div>
Roquefort cheese</div>
</div>
</div>
</div>
</td>
<td style="border-top: 3px solid rgb(170, 170, 170);"><a title="Rennin" href="http://en.wikipedia.org/wiki/Rennin">Rennin</a>, derived from the stomachs of young <a title="Ruminant" href="http://en.wikipedia.org/wiki/Ruminant">ruminant animals</a> (like calves and lambs).</td>
<td style="border-top: 3px solid rgb(170, 170, 170);">Manufacture of cheese, used to <a title="Hydrolyze" href="http://en.wikipedia.org/wiki/Hydrolyze">hydrolyze</a> protein.</td>
</tr>
<tr>
<td>Microbially produced enzyme</td>
<td>Now finding increasing use in the dairy industry.</td>
</tr>
<tr>
<td><a title="Lipase" href="http://en.wikipedia.org/wiki/Lipase">Lipases</a></td>
<td>Is implemented during the production of <a title="Roquefort cheese" href="http://en.wikipedia.org/wiki/Roquefort_cheese">Roquefort cheese</a> to enhance the ripening of the <a title="Danish Blue cheese" href="http://en.wikipedia.org/wiki/Danish_Blue_cheese">blue-mould cheese</a>.</td>
</tr>
<tr>
<td>Lactases</td>
<td>Break down <a title="Lactose" href="http://en.wikipedia.org/wiki/Lactose">lactose</a> to <a title="Glucose" href="http://en.wikipedia.org/wiki/Glucose">glucose</a> and galactose.</td>
</tr>
<tr>
<td rowspan="2" style="border-top: 3px solid rgb(170, 170, 170);"><strong><a title="Starch" href="http://en.wikipedia.org/wiki/Starch">Starch industry</a></strong>
<div class="center">
<div class="thumb tnone">
<div style="width: 208px;">
<table cellspacing="0" style="border: 0pt none ; margin: 0pt; padding: 0pt; background: transparent none repeat scroll 0% 50%; -moz-background-clip: -moz-initial; -moz-background-origin: -moz-initial; -moz-background-inline-policy: -moz-initial;">
<tbody>
<tr>
<td style="border: 0pt none ; margin: 0pt; padding: 0pt;"><a title="Glucose" class="image" href="http://en.wikipedia.org/wiki/Image:Glucose_Haworth.png"><img width="100" height="79" longdesc="/wiki/Image:Glucose_Haworth.png" alt="Glucose" src="http://upload.wikimedia.org/wikipedia/commons/thumb/6/64/Glucose_Haworth.png/100px-Glucose_Haworth.png" /></a></td>
<td style="border: 0pt none ; margin: 0pt; padding: 0pt; width: 2px;"> </td>
<td style="border: 0pt none ; margin: 0pt; padding: 0pt;"><a title="Glucose" class="image" href="http://en.wikipedia.org/wiki/Image:Alpha-D-Fructose-structure-corrected.png"><img width="100" height="64" longdesc="/wiki/Image:Alpha-D-Fructose-structure-corrected.png" alt="Glucose" src="http://upload.wikimedia.org/wikipedia/commons/thumb/a/a8/Alpha-D-Fructose-structure-corrected.png/100px-Alpha-D-Fructose-structure-corrected.png" /></a></td>
</tr>
<tr>
<td style="border: 0pt none ; margin: 0pt; padding: 0pt;">
<div class="thumbcaption">Glucose</div>
</td>
<td style="border: 0pt none ; margin: 0pt; padding: 0pt;"> </td>
<td style="border: 0pt none ; margin: 0pt; padding: 0pt;">
<div class="thumbcaption">Fructose</div>
</td>
</tr>
</tbody>
</table>
</div>
</div>
</div>
<p><br />
</p>
</td>
<td style="border-top: 3px solid rgb(170, 170, 170);">Amylases, amyloglucosideases and glucoamylases</td>
<td style="border-top: 3px solid rgb(170, 170, 170);">Converts <a title="Starch" href="http://en.wikipedia.org/wiki/Starch">starch</a> into <a title="Glucose" href="http://en.wikipedia.org/wiki/Glucose">glucose</a> and various <a title="Inverted sugar syrup" href="http://en.wikipedia.org/wiki/Inverted_sugar_syrup">syrups</a>.</td>
</tr>
<tr>
<td>Glucose isomerase</td>
<td>Converts <a title="Glucose" href="http://en.wikipedia.org/wiki/Glucose">glucose</a> into fructose in production of (<a title="High-fructose corn syrup" href="http://en.wikipedia.org/wiki/High-fructose_corn_syrup">high fructose syrups</a> from starchy materials. These syrups have enhanced sweetening properties and lower <a title="Calorie" href="http://en.wikipedia.org/wiki/Calorie">calorific values</a>) than sucrose for the same level of sweetness.</td>
</tr>
<tr>
<td style="border-top: 3px solid rgb(170, 170, 170);"><strong><a title="Tenderizing" href="http://en.wikipedia.org/wiki/Tenderizing">Meat tenderizers</a></strong></td>
<td style="border-top: 3px solid rgb(170, 170, 170);"><a title="Papain" href="http://en.wikipedia.org/wiki/Papain">Papain</a></td>
<td style="border-top: 3px solid rgb(170, 170, 170);">To soften meat for cooking.</td>
</tr>
<tr>
<td rowspan="4" style="border-top: 3px solid rgb(170, 170, 170);"><strong><a title="Detergent" href="http://en.wikipedia.org/wiki/Detergent">Biological detergent</a></strong>
<div class="center">
<div class="thumb tnone">
<div style="width: 182px;" class="thumbinner"><a title="Laundry soap" class="internal" href="http://en.wikipedia.org/wiki/Image:Washingpowder.jpg"><img width="180" height="135" class="thumbimage" longdesc="/wiki/Image:Washingpowder.jpg" alt="Laundry soap" src="http://upload.wikimedia.org/wikipedia/commons/thumb/a/aa/Washingpowder.jpg/180px-Washingpowder.jpg" /></a>
<div class="thumbcaption">
<div style="float: right;" class="magnify"><a title="Enlarge" class="internal" href="http://en.wikipedia.org/wiki/Image:Washingpowder.jpg"><img width="15" height="11" alt="" src="http://en.wikipedia.org/skins-1.5/common/images/magnify-clip.png" /></a></div>
Laundry soap</div>
</div>
</div>
</div>
</td>
<td style="border-top: 3px solid rgb(170, 170, 170);">Primarily <a title="Protease" href="http://en.wikipedia.org/wiki/Protease">proteases</a>, produced in an <a title="Extracellular" href="http://en.wikipedia.org/wiki/Extracellular">extracellular</a> form from <a title="Bacteria" href="http://en.wikipedia.org/wiki/Bacteria">bacteria</a></td>
<td style="border-top: 3px solid rgb(170, 170, 170);">Used for presoak conditions and direct liquid applications helping with removal of protein stains from clothes.</td>
</tr>
<tr>
<td><a title="Amylase" href="http://en.wikipedia.org/wiki/Amylase">Amylases</a></td>
<td>Detergents for machine dish washing to remove resistant starch residues.</td>
</tr>
<tr>
<td><a title="Lipase" href="http://en.wikipedia.org/wiki/Lipase">Lipases</a></td>
<td>Used to assist in the removal of fatty and oily stains.</td>
</tr>
<tr>
<td><a title="Cellulase" href="http://en.wikipedia.org/wiki/Cellulase">Cellulases</a></td>
<td>Used in biological fabric conditioners.</td>
</tr>
<tr>
<td style="border-top: 3px solid rgb(170, 170, 170);"><strong><a title="Contact lens" href="http://en.wikipedia.org/wiki/Contact_lens">Contact lens cleaners</a></strong></td>
<td style="border-top: 3px solid rgb(170, 170, 170);"><a title="Proteases" href="http://en.wikipedia.org/wiki/Proteases">Proteases</a></td>
<td style="border-top: 3px solid rgb(170, 170, 170);">To remove <a title="Proteins" href="http://en.wikipedia.org/wiki/Proteins">proteins</a> on <a title="Contact lens" href="http://en.wikipedia.org/wiki/Contact_lens">contact lens</a> to prevent infections.</td>
</tr>
<tr>
<td style="border-top: 3px solid rgb(170, 170, 170);"><strong><a title="Rubber" href="http://en.wikipedia.org/wiki/Rubber">Rubber industry</a></strong></td>
<td style="border-top: 3px solid rgb(170, 170, 170);"><a title="Catalase" href="http://en.wikipedia.org/wiki/Catalase">Catalase</a></td>
<td style="border-top: 3px solid rgb(170, 170, 170);">To generate <a title="Oxygen" href="http://en.wikipedia.org/wiki/Oxygen">oxygen</a> from <a title="Peroxide" href="http://en.wikipedia.org/wiki/Peroxide">peroxide</a> to convert <a title="Latex" href="http://en.wikipedia.org/wiki/Latex">latex</a> into foam rubber.</td>
</tr>
<tr>
<td style="border-top: 3px solid rgb(170, 170, 170);"><strong><a title="Paper" href="http://en.wikipedia.org/wiki/Paper">Paper industry</a></strong>
<div class="center">
<div class="thumb tnone">
<div style="width: 162px;" class="thumbinner"><a title="A paper mill in South Carolina." class="internal" href="http://en.wikipedia.org/wiki/Image:InternationalPaper6413.jpg"><img width="160" height="120" class="thumbimage" longdesc="/wiki/Image:InternationalPaper6413.jpg" alt="A paper mill in South Carolina." src="http://upload.wikimedia.org/wikipedia/commons/thumb/c/c5/InternationalPaper6413.jpg/160px-InternationalPaper6413.jpg" /></a>
<div class="thumbcaption">
<div style="float: right;" class="magnify"><a title="Enlarge" class="internal" href="http://en.wikipedia.org/wiki/Image:InternationalPaper6413.jpg"><img width="15" height="11" alt="" src="http://en.wikipedia.org/skins-1.5/common/images/magnify-clip.png" /></a></div>
A paper mill in <a title="South Carolina" href="http://en.wikipedia.org/wiki/South_Carolina">South Carolina</a>.</div>
</div>
</div>
</div>
</td>
<td style="border-top: 3px solid rgb(170, 170, 170);"><a title="Amylase" href="http://en.wikipedia.org/wiki/Amylase">Amylases</a>, <a title="Xylanase" href="http://en.wikipedia.org/wiki/Xylanase">Xylanases</a>, <a title="Cellulase" href="http://en.wikipedia.org/wiki/Cellulase">Cellulases</a> and <a title="Lignin" href="http://en.wikipedia.org/wiki/Lignin">ligninases</a></td>
<td style="border-top: 3px solid rgb(170, 170, 170);">Degrade starch to lower <a title="Viscosity" href="http://en.wikipedia.org/wiki/Viscosity">viscosity</a>, aiding <a title="Sizing" href="http://en.wikipedia.org/wiki/Sizing">sizing</a> and coating paper. Xylanases reduce bleach required for decolorising; cellulases smooth fibers, enhance water drainage, and promote ink removal; lipases reduce pitch and lignin-degrading enzymes remove <a title="Lignin" href="http://en.wikipedia.org/wiki/Lignin">lignin</a> to soften paper.</td>
</tr>
<tr>
<td style="border-top: 3px solid rgb(170, 170, 170);"><strong><a title="Photography" href="http://en.wikipedia.org/wiki/Photography">Photographic industry</a></strong></td>
<td style="border-top: 3px solid rgb(170, 170, 170);">Protease (ficin)</td>
<td style="border-top: 3px solid rgb(170, 170, 170);">Dissolve <a title="Gelatin" href="http://en.wikipedia.org/wiki/Gelatin">gelatin</a> off scrap <a title="Photographic film" href="http://en.wikipedia.org/wiki/Photographic_film">film</a>, allowing recovery of its <a title="Silver" href="http://en.wikipedia.org/wiki/Silver">silver</a> content.</td>
</tr>
<tr>
<td style="border-top: 3px solid rgb(170, 170, 170);"><strong><a title="Molecular biology" href="http://en.wikipedia.org/wiki/Molecular_biology">Molecular biology</a></strong>
<div class="center">
<div class="thumb tnone">
<div style="width: 182px;" class="thumbinner"><a title="Part of the DNA double helix." class="internal" href="http://en.wikipedia.org/wiki/Image:DNA123_rotated.png"><img width="180" height="100" class="thumbimage" longdesc="/wiki/Image:DNA123_rotated.png" alt="Part of the DNA double helix." src="http://upload.wikimedia.org/wikipedia/commons/thumb/d/d9/DNA123_rotated.png/180px-DNA123_rotated.png" /></a>
<div class="thumbcaption">
<div style="float: right;" class="magnify"><a title="Enlarge" class="internal" href="http://en.wikipedia.org/wiki/Image:DNA123_rotated.png"><img width="15" height="11" alt="" src="http://en.wikipedia.org/skins-1.5/common/images/magnify-clip.png" /></a></div>
Part of the DNA <a title="Double helix" href="http://en.wikipedia.org/wiki/Double_helix">double helix</a>.</div>
</div>
</div>
</div>
</td>
<td style="border-top: 3px solid rgb(170, 170, 170);"><a title="Restriction enzyme" href="http://en.wikipedia.org/wiki/Restriction_enzyme">Restriction enzymes</a>, <a title="DNA ligase" href="http://en.wikipedia.org/wiki/DNA_ligase">DNA ligase</a> and <a title="Polymerases" href="http://en.wikipedia.org/wiki/Polymerases">polymerases</a></td>
<td style="border-top: 3px solid rgb(170, 170, 170);">Used to manipulate DNA in <a title="Genetic engineering" href="http://en.wikipedia.org/wiki/Genetic_engineering">genetic engineering</a>, important in <a title="Pharmacology" href="http://en.wikipedia.org/wiki/Pharmacology">pharmacology</a>, <a title="Agriculture" href="http://en.wikipedia.org/wiki/Agriculture">agriculture</a> and <a title="Medicine" href="http://en.wikipedia.org/wiki/Medicine">medicine</a>. Essential for <a title="Restriction enzyme" href="http://en.wikipedia.org/wiki/Restriction_enzyme">restriction digestion</a> and the <a title="Polymerase chain reaction" href="http://en.wikipedia.org/wiki/Polymerase_chain_reaction">polymerase chain reaction</a>. Molecular biology is also important in <a title="Forensic science" href="http://en.wikipedia.org/wiki/Forensic_science">forensic science</a>.</td>
</tr>
</tbody>
</table>

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