Editing Enzyme (section)

==Cofactors==
[[File:Transketolase + TPP.png|thumb|400px|alt=Thiamine pyrophosphate displayed as an opaque globular surface with an open binding cleft where the substrate and cofactor both depicted as stick diagrams fit into.|Chemical structure for [[thiamine pyrophosphate]] and protein structure of [[transketolase]]. Thiamine pyrophosphate cofactor in yellow and [[xylulose 5-phosphate]] substrate in black. ({{PDB|4KXV}})]]

{{main|Cofactor (biochemistry)}}

Some enzymes do not need additional components to show full activity. Others require non-protein molecules called cofactors to be bound for activity.<ref>{{cite web | url = http://www.chem.qmul.ac.uk/iupac/bioinorg/CD.html#34 | title = Glossary of Terms Used in Bioinorganic Chemistry: Cofactor | access-date = 30 October 2007 | vauthors = de Bolster MW | year = 1997 | publisher = International Union of Pure and Applied Chemistry | archive-url = https://web.archive.org/web/20170121172848/http://www.chem.qmul.ac.uk/iupac/bioinorg/CD.html#34#34 | archive-date = 21 January 2017 | url-status = dead}}</ref> Cofactors can be either [[inorganic]] (e.g., metal [[ion]]s and [[iron–sulfur cluster]]s) or [[organic compound]]s (e.g., [[flavin group|flavin]] and [[heme]]). These cofactors serve many purposes; for instance, metal ions can help in stabilizing nucleophilic species within the active site.<ref>{{Cite book |title=Fundamentals of Biochemistry | vauthors = Voet D, Voet J, Pratt C |publisher=John Wiley & Sons, Inc. |year=2016 |isbn=978-1-118-91840-1 |location=Hoboken, New Jersey |pages=336}}</ref> Organic cofactors can be either [[coenzyme]]s, which are released from the enzyme's active site during the reaction, or [[prosthetic groups]], which are tightly bound to an enzyme. Organic prosthetic groups can be covalently bound (e.g., [[biotin]] in enzymes such as [[pyruvate carboxylase]]).<ref name="pmid10470036">{{cite journal | vauthors = Chapman-Smith A, Cronan JE | title = The enzymatic biotinylation of proteins: a post-translational modification of exceptional specificity | journal = Trends in Biochemical Sciences | volume = 24 | issue = 9 | pages = 359–363 | date = September 1999 | pmid = 10470036 | doi = 10.1016/s0968-0004(99)01438-3 }}</ref>

An example of an enzyme that contains a cofactor is [[carbonic anhydrase]], which uses a zinc cofactor bound as part of its active site.<ref>{{cite journal | vauthors = Fisher Z, Hernandez Prada JA, Tu C, Duda D, Yoshioka C, An H, Govindasamy L, Silverman DN, McKenna R | title = Structural and kinetic characterization of active-site histidine as a proton shuttle in catalysis by human carbonic anhydrase II | journal = Biochemistry | volume = 44 | issue = 4 | pages = 1097–1105 | date = February 2005 | pmid = 15667203 | doi = 10.1021/bi0480279 }}</ref> These tightly bound ions or molecules are usually found in the active site and are involved in catalysis.<ref name = "Stryer_2002"/>{{rp|8.1.1}} For example, flavin and heme cofactors are often involved in [[redox]] reactions.<ref name = "Stryer_2002"/>{{rp|17}}

Enzymes that require a cofactor but do not have one bound are called ''apoenzymes'' or ''apoproteins''. An enzyme together with the cofactor(s) required for activity is called a ''holoenzyme'' (or haloenzyme). The term ''holoenzyme'' can also be applied to enzymes that contain multiple protein subunits, such as the [[DNA polymerase]]s; here the holoenzyme is the complete complex containing all the subunits needed for activity.<ref name = "Stryer_2002"/>{{rp|8.1.1}}

===Coenzymes===

Coenzymes are small organic molecules that can be loosely or tightly bound to an enzyme. Coenzymes transport chemical groups from one enzyme to another.<ref name = "Wagner_1975">{{cite book | author = Wagner AL | title = Vitamins and Coenzymes | publisher = Krieger Pub Co | year = 1975 | isbn = 0-88275-258-8}}</ref> Examples include [[Nicotinamide adenine dinucleotide|NADH]], [[Nicotinamide adenine dinucleotide phosphate|NADPH]] and [[adenosine triphosphate]] (ATP). Some coenzymes, such as [[flavin mononucleotide]] (FMN), [[flavin adenine dinucleotide]] (FAD), [[thiamine pyrophosphate]] (TPP), and [[tetrahydrofolate]] (THF), are derived from [[vitamin]]s. These coenzymes cannot be synthesized by the body ''[[De novo synthesis|de novo]]'' and closely related compounds (vitamins) must be acquired from the diet. The chemical groups carried include: 
* the [[hydride]] ion (H<sup>−</sup>), carried by [[nicotinamide adenine dinucleotide|NAD or NADP<sup>+</sup>]] 
* the phosphate group, carried by [[adenosine triphosphate]] 
* the acetyl group, carried by [[coenzyme A]] 
* formyl, methenyl or methyl groups, carried by [[folic acid]] and 
* the methyl group, carried by [[S-adenosylmethionine]]<ref name = "Wagner_1975"/>
 
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 1000 enzymes are known to use the coenzyme NADH.<ref>{{cite web | url = http://www.brenda-enzymes.org | title = BRENDA The Comprehensive Enzyme Information System | publisher = Technische Universität Braunschweig | access-date = 23 February 2015 }}</ref>

Coenzymes are usually continuously regenerated and their concentrations maintained at a steady level inside the cell. For example, NADPH is regenerated through the [[pentose phosphate pathway]] and ''S''-adenosylmethionine by [[methionine adenosyltransferase]]. This continuous regeneration means that small amounts of coenzymes can be used very intensively. For example, the human body turns over its own weight in ATP each day.<ref>{{cite journal | vauthors = Törnroth-Horsefield S, Neutze R | title = Opening and closing the metabolite gate | journal = Proceedings of the National Academy of Sciences of the United States of America | volume = 105 | issue = 50 | pages = 19565–19566 | date = December 2008 | pmid = 19073922 | pmc = 2604989 | doi = 10.1073/pnas.0810654106 | doi-access = free | bibcode = 2008PNAS..10519565T }}</ref>