Editing Enzyme (section)

===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>