Editing Dehydrogenase

{{short description|Class of enzymes}}
A '''dehydrogenase''' is an [[enzyme]] belonging to the group of [[oxidoreductases]] that oxidizes a substrate by reducing an electron acceptor, usually [[Nicotinamide adenine dinucleotide|NAD<sup>+</sup>/NADP<sup>+</sup>]]<ref>An [[IUPAC]] panel on biochemical  thermodynamics convened by [[Robert A. Alberty|Robert Alberty]] pointed out that the oxidized form of NAD is negatively charged, and that NAD<sup>+</sup> is an inappropriate symbol for an anion [{{cite journal
 | last = Alberty
 | first = R.A.
 | title = Recommendations for Nomenclature and Tables in Biochemical Thermodynamics (IUPAC Recommendations 1994)
 | journal = Pure and Applied Chemistry
 | volume = 66
 | issue = 8
 | pages = 1641–1666
 | date = 1994
| doi = 10.1351/pac199466081641
 | s2cid = 96307963
 | doi-access = free
 }}] However, NAD<sup>+</sup> and, similarly, NADP<sup>+</sup> remain in almost universal use and alternatives such as NAD<sub>oxidized</sub> have been very little adopted.</ref> or a [[Flavin group|flavin]] [[coenzyme]] such as [[Flavin adenine dinucleotide|FAD]] or [[Flavin mononucleotide|FMN]]. Like all catalysts, they catalyze reverse as well as forward reactions, and in some cases this has physiological significance: for example, [[alcohol dehydrogenase]] catalyzes the oxidation of [[ethanol]] to [[acetaldehyde]] in animals, but in yeast it catalyzes the production of ethanol from acetaldehyde.

== IUBMB classification ==
Oxidoreductases, enzymes that catalyze oxidation-reduction reactions, constitute Class EC 1 of the IUBMB classification of enzyme-catalyzed reactions.<ref name = ":4" /> Any of these may be called dehydrogenases, especially those in which NAD<sup>+</sup> is the electron acceptor (oxidant), but [[reductase]] is also used when the physiological emphasis on reduction of the substrate, and [[oxidase]] is used ''only'' when O<sub>2</sub> is the electron acceptor.<ref name="rules">{{Cite web
|url = https://www.qmul.ac.uk/sbcs/iubmb/enzyme/rules.html
|title = Classification and Nomenclature of Enzymes by the Reactions they Catalyse
|access-date = 30 March 2021
}}</ref> The systematic name of an oxidoreductase is "donor:acceptor oxidoreductase", but, when possible, it is more conveniently named as "donor dehydrogenase".<!--Oxidation-reduction reactions are essential to growth and survival of organisms, as the oxidation of organic molecules produces energy.  Energy-producing reactions can drive forward the synthesis of important energy molecules, such as ATP in [[glycolysis]]. For this reason, dehydrogenases have pivotal roles in metabolism.-->

== Reactions catalyzed ==
[[File:Sulcatone reductase reaction.PNG|thumb|A reaction catalyzed by a reductase enzyme]]
Dehydrogenases oxidize a substrate by transferring hydrogen to an electron acceptor, common electron acceptors being [[NAD+|NAD<sup>+</sup>]] or [[Flavin adenine dinucleotide|FAD.]] This would be considered an oxidation of the substrate, in which the substrate either loses hydrogen atoms or gains an oxygen atom (from water).<ref name=":3">{{Cite web|url = http://www.chemguide.co.uk/inorganic/redox/definitions.html|title = Definitions of Oxidation and Reduction (Redox)|date = 2002|access-date = February 14, 2016|website = Chemguide|last = Clark|first = Jim}}</ref> The name "dehydrogenase" is based on the idea that it facilitates the removal (de-) of hydrogen (-hydrogen-) and is an enzyme (-ase). Dehydrogenase reactions come most commonly in two forms: the transfer of a hydride and release of a proton (often with water as a second reactant), and the transfer of two hydrogens.

=== Transferring a hydride and releasing a proton ===
Sometimes a dehydrogenase catalyzed reaction will look like this: AH + B<sup>+</sup> ↔ A<sup>+</sup> + BH when a [[hydride]] is transferred. [[File:Alcohol dehydrogenase.png|thumb|500px|Alcohol dehydrogenase oxidizes ethanol, with the help of the electron carrier NAD<sup>+</sup>, yielding acetaldehyde]]A represents the substrate that will be oxidized, while B is the hydride acceptor. Note how when the hydride is transferred from A to B, the A has taken on a positive charge; this is because the enzyme has taken two electrons from the substrate in order to reduce the acceptor to BH.

The result of a dehydrogenase catalyzed reaction is not always the acquisition of a positive charge. Sometimes the substrate loses a proton. This may leave free electrons on the substrate that move into a double bond. This happens frequently when an alcohol is the substrate; when the proton on the oxygen leaves, the free electrons on the oxygen will be used to create a double bond, as seen in the oxidation of ethanol to acetaldehyde carried out by alcohol dehydrogenase in the image on the right.<ref name=":4">{{Cite web
|url = https://qmul.ac.uk/sbcs/iubmb/enzyme/
|title = Enzyme Nomenclature: Recommendations of the Nomenclature Committee of the International Union of Biochemistry and Molecular Biology on the Nomenclature and Classification of Enzymes by the Reactions they Catalyse
|access-date = 29 March 2021
}}</ref>

Another possibility is that a water molecule will enter the reaction, contributing a [[hydroxide ion]] to the substrate and a proton to the environment. The net result on the substrate is the addition of one oxygen atom. This is seen for example in the oxidation of [[acetaldehyde]] to [[acetic acid]] by [[acetaldehyde dehydrogenase]], a step in the metabolism of ethanol and in the production of vinegar.

=== Transferring two hydrogens ===
[[File:Krebs_cycle_6_succinate_to_fumarate.svg|thumb|Reaction catalyzed by succinate dehydrogenase, note the double bond formed between the two central carbons when two hydrogens are removed]]
In the above case, the dehydrogenase has transferred a hydride while releasing a proton, H<sup>+</sup>, but dehydrogenases can also transfer two hydrogens, using FAD as an electron acceptor. This would be depicted as AH<sub>2</sub> + B ↔ A + BH<sub>2</sub>.
A double bond is normally formed in between the two atoms that the hydrogens were taken from, as in the case of [[succinate dehydrogenase]]. The two hydrogens have been transferred to the carrier or the other product, with their electrons.

=== Identifying a dehydrogenase reaction ===
The distinction between the subclasses of oxidoreductases that catalyze oxidation reactions lies in their electron acceptors.<ref name=":1">{{Cite book
|title = Fundamentals of Biochemistry: Life at the Molecular Level
|last1 = Voet |first1 = Donald
|last2 = Voet |first2 = Judith G.
|last3 = Pratt |first3 = Charlotte W.
|edition = 5th
|publisher = Wiley
|date = 2016
|location = New York
|isbn =  9781118918401
}}</ref>
[[File:Vanillyl-alcohol oxidase reaction.PNG|thumb|Reaction catalyzed by an oxidase, note the reduction of oxygen as the electron acceptor]]
Dehydrogenase and [[oxidase]] are easily distinguishable if one considers the electron acceptor.  An oxidase will remove electrons from a substrate as well, but only uses oxygen as its electron acceptor. One such reaction is: AH<sub>2</sub> + O<sub>2</sub> ↔ A + H<sub>2</sub>O<sub>2</sub>.

Sometimes an oxidase reaction will look like this: 4A + 4H<sup>+</sup> + O<sub>2</sub> ↔ 4A<sup>+</sup> + 2H<sub>2</sub>O. In this case, the enzyme is taking electrons from the substrate, and using free protons to reduce the oxygen, leaving the substrate with a positive charge. The product is water, instead of hydrogen peroxide as seen above. An example of an oxidase that functions like this is complex IV in the Electron Transport Chain ([[Electron transport chain|ETC]]).<ref>{{Cite journal|last1=Yoshikawa|first1=Shinya|last2=Shimada|first2=Atsuhiro|date=2015-01-20|title=Reaction Mechanism of Cytochrome c Oxidase|journal=Chemical Reviews|language=EN|volume=115|issue=4|pages=1936–1989|doi=10.1021/cr500266a|pmid=25603498}}</ref>

Note that oxidases typically transfer the equivalent of dihydrogen (H<sub>2</sub>), and the acceptor is a dioxygen. Similarly, a [[peroxidase]] (another subclass of oxidoreductases) will use a peroxide (H<sub>2</sub>O<sub>2</sub>) as the electron acceptor, rather than an oxygen.<ref name=":4" />

== Electron acceptors ==
[[File:NAD+ phys alt.svg|thumb|170x170px|Nicotinamide Adenine Dinucleotide]]
Dehydrogenase enzymes transfer electrons from the substrate to an electron carrier; what carrier is used depends on the reaction taking place.  Common electron acceptors used by this subclass are NAD<sup>+</sup>, FAD, and NADP<sup>+</sup>. Electron carriers are reduced in this process and considered oxidizers of the substrate. Electron carriers are [[Coenzymes and cofactors|coenzymes]] that are often referred to as "redox cofactors."<ref name=":1" />

=== NAD<sup>+</sup> ===
[[Nicotinamide adenine dinucleotide|NAD<sup>+</sup>]], or nicotinamide adenine dinucleotide, is a dinucleotide, containing two nucleotides.  One of the nucleotides it contains is an adenine group, while the other is nicotinamide.  In order to reduce this molecule, a hydrogen and two electrons must be added to the 6-carbon ring of nicotinamide; one electron is added to the carbon opposite the positively charged nitrogen, causing a rearrangement of bonds within the ring to give nitrogen more electrons; it will lose its positive charge as a result. The other electron is "stolen" from an additional hydrogen, leaving the hydrogen ion in solution.<ref name=":1" /><ref name=":0" /> <blockquote>Reduction of NAD<sup>+</sup>: NAD<sup>+</sup> + 2H<sup>+</sup> + 2e<sup>−</sup> ↔ NADH + H<sup>+</sup></blockquote>NAD<sup>+</sup> is mostly used in catabolic pathways, such as [[Glycolysis cycle|glycolysis]], that break down energy molecules to produce ATP. The ratio of NAD<sup>+</sup> to NADH is kept very high in the cell, keeping it readily available to act as an oxidizing agent.<ref name=":0">{{Cite book|title = Molecular Biology of the Cell|last1 = Alberts|first1 = B|publisher = Garland Science|year = 2002|isbn = 978-0-8153-3218-3|location = New York|last2 = Johnson|first2 = A|url = https://www.ncbi.nlm.nih.gov/books/NBK26838/#A273|display-authors=etal}}</ref><ref name=":5">{{Cite journal|last=Ying|first=Weihai|s2cid=42000527|date=2008-02-01|title=NAD+/NADH and NADP+/NADPH in cellular functions and cell death: regulation and biological consequences|journal=Antioxidants & Redox Signaling|volume=10|issue=2|pages=179–206|doi=10.1089/ars.2007.1672|issn=1523-0864|pmid=18020963}}</ref> 
[[File:NADP+ phys alt.svg|thumb|196x196px|Nicotinamide Adenine Dinucleotide Phosphate]]

=== NADP<sup>+</sup> ===
[[Nicotinamide adenine dinucleotide phosphate|NADP<sup>+</sup>]] differs from NAD<sup>+</sup> only in the addition of a phosphate group to the adenosine 5-membered carbon ring.  The addition of the phosphate does not alter the electron transport abilities of the carrier. The phosphate group creates enough contrast between the two groups that they bind to the active site of different enzymes, generally catalyzing different types of reactions.<ref name=":5" /><ref name=":6">{{Cite web|url=http://watcut.uwaterloo.ca/webnotes/Metabolism/hmsNadphRole.html|title=The physiological role of NADPH|website=watcut.uwaterloo.ca|access-date=2016-03-06|archive-date=2016-03-06|archive-url=https://web.archive.org/web/20160306193914/http://watcut.uwaterloo.ca/webnotes/Metabolism/hmsNadphRole.html|url-status=dead}}</ref>

These two electron carriers are easily distinguished by enzymes and participate in very different reactions.  NADP<sup>+</sup> mainly functions with enzymes that catalyze anabolic, or biosynthetic, pathways.<ref name=":6" /> Specifically, NADPH will act as a reducing agent in these reactions, resulting in NADP<sup>+</sup>. These are pathways that convert substrates to more complicated products, using ATP. The reasoning behind having two separate electron carriers for anabolic and catabolic pathways relates to regulation of metabolism.<ref name=":0" />
The ratio of NADP<sup>+</sup> to NADPH in the cell is kept rather low, so that NADPH is readily available as a reducing agent; it is more commonly used as a reducing agent than NADP<sup>+</sup> is used as an oxidizing agent.<ref name=":5" />

=== FAD ===
[[File:FAD.svg|thumb|Flavin Adenine Dinucleotide]]
[[Flavin adenine dinucleotide|FAD]], or flavin adenine dinucleotide, is a prosthetic group (a non-polypeptide unit bound to a protein that is required for function) that consists of an adenine nucleotide and a flavin mononucleotide.<ref>{{Cite journal|title = Sequence-structure analysis of FAD-containing proteins|journal = Protein Science|date = 2001-09-01|issn = 1469-896X|pmc = 2253189|pmid = 11514662|pages = 1712–1728|volume = 10|issue = 9|doi = 10.1110/ps.12801|language = en|first1 = Orly|last1 = Dym|first2 = David|last2 = Eisenberg}}</ref>  FAD is a unique electron acceptor. Its fully reduced form is FADH<sub>2</sub> (known as the hydroquinone form), but FAD can also be partially oxidized as FADH by either reducing FAD or oxidizing FADH<sub>2</sub>.<ref>{{Cite journal|title = Riboflavin Metabolism|journal = New England Journal of Medicine|date = 1970-08-27|issn = 0028-4793|pmid = 4915004|pages = 463–472|volume = 283|issue = 9|doi = 10.1056/NEJM197008272830906|first = Richard S.|last = Rivlin}}</ref> Dehydrogenases typically fully reduce FAD to FADH<sub>2</sub>.  The production of FADH is rare.

The double-bonded nitrogen atoms in FAD make it a good acceptor in taking two hydrogen atoms from a substrate.  Because it takes two atoms rather than one, FAD is often involved when a double bond is formed in the newly oxidized substrate.<ref>{{Cite web|url=http://www.blobs.org/science/article.php?article=35#5|title=blobs.org - Metabolism|website=www.blobs.org|access-date=2016-03-01|archive-date=2016-02-01|archive-url=https://web.archive.org/web/20160201065212/http://www.blobs.org/science/article.php?article=35#5|url-status=dead}}</ref> FAD is unique because it is reduced by two electrons ''and'' two protons, as opposed to both NAD<sup>+</sup> and NADP, which only take one proton.

== Examples ==

=== Biological implications ===
[[File:Aldehyde dehydrogenase mechanism.png|thumb|535x535px|The mechanism of an aldehyde dehydrogenase, note the use of NAD<sup>+</sup> as an electron acceptor.]]
Aldehydes are the natural by-product of many physiological processes, as well as being the consequence of many industrial processes, put out into the environment in the form of smog and motor vehicle exhaust.  Build-up of aldehydes in the brain and pericardium can be detrimental to a person's health, as they can form adducts with important molecules and cause their inactivation.<ref name=":7" />

Considering how prevalent aldehydes are, there must be an enzyme to facilitate their oxidation to a less volatile compound.  [[Aldehyde dehydrogenase]]s (ALDH) are NAD<sup>+</sup>  dependent enzymes that function to remove toxic aldehydes from the body, functioning mostly in the mitochondria of cells. These enzymes are largely responsible for the detoxification of acetylaldehyde, which is an intermediate in the metabolism of ethanol.  It has been shown that a mutation in the ALDH2 gene (one of 19 aldehyde dehydrogenase genes) is what leads to the common occurrence in East Asian population of a flushed face after consuming alcohol, due to the build-up of acetaldehyde.<ref>{{Cite journal|title = Population genetic studies on aldehyde dehydrogenase isozyme deficiency and alcohol sensitivity|last1 = Goedde|first1 = HW|date = 1983|journal = Am J Hum Genet|pmid = 6881146|last2 = Agarwal|first2 = DP|pmc=1685745|volume=35|issue = 4|pages=769–72}}</ref> This build-up of acetaldehyde also causes headaches and vomiting ([[hangover]] symptoms) if not broken down quickly enough, another reason why those with acetaldehyde DH deficiencies have bad reactions to alcohol.<ref>{{Cite web|url=http://health.howstuffworks.com/wellness/drugs-alcohol/hangover4.htm|title=How Hangovers Work|website=HowStuffWorks|access-date=2016-03-06|date=2004-10-12}}</ref> Importantly, a lack of this enzyme has been linked to an increase in the risk of myocardial [[infarction]], while activation has shown the enzyme's ability to reduce damage caused by [[Ischemia|ischaemia]].<ref name=":7">{{Cite journal|title = Mitochondrial aldehyde dehydrogenase and cardiac diseases|journal = Cardiovascular Research|date = 2010-10-01|issn = 0008-6363|pmc = 2936126|pmid = 20558439|pages = 51–57|volume = 88|issue = 1|doi = 10.1093/cvr/cvq192|language = en|first1 = Che-Hong|last1 = Chen|first2 = Lihan|last2 = Sun|first3 = Daria|last3 = Mochly-Rosen}}</ref>

Deactivation of aldehyde dehydrogenases has been shown to be instrumental in the mechanisms of many cancers. ALDHs function in cell differentiation, proliferation, oxidation, and drug resistance.<ref>{{Cite journal|title = High aldehyde dehydrogenase activity identifies tumor-initiating and metastasis-initiating cells in human prostate cancer|journal = Cancer Research|date = 2010-06-15|issn = 1538-7445|pmid = 20516116|pages = 5163–5173|volume = 70|issue = 12|doi = 10.1158/0008-5472.CAN-09-3806|first1 = Christel|last1 = van den Hoogen|first2 = Geertje|last2 = van der Horst|first3 = Henry|last3 = Cheung|first4 = Jeroen T.|last4 = Buijs|first5 = Jenny M.|last5 = Lippitt|first6 = Natalia|last6 = Guzmán-Ramírez|first7 = Freddie C.|last7 = Hamdy|first8 = Colby L.|last8 = Eaton|first9 = George N.|last9 = Thalmann|doi-access = free}}</ref> These enzymes are only one example of the many different types of dehydrogenases in the human body; their wide array of functions, and the impact that their deactivation or mutations has upon crucial cell processes underscores the importance of all dehydrogenases in maintaining body homeostasis.

=== More examples ===
* [[acetaldehyde dehydrogenase]] 
* [[alcohol dehydrogenase]]
* [[Delta12-fatty acid dehydrogenase]]
* [[glutamate dehydrogenase]] (an enzyme that can convert [[glutamate]] to α-[[Ketoglutarate]] and vice versa).
* [[lactate dehydrogenase]] (used to convert NADH back to NAD<sup>+</sup> in anaerobic glycolysis, and in the back reaction to produce NADH)
* [[pyruvate dehydrogenase]] (A common enzyme that feeds the [[Citric acid cycle|TCA Cycle]] by converting [[pyruvate]] to [[acetyl CoA]], using NAD<sup>+</sup>. In this reaction, the substrate not only is oxidized but also loses a [[carbon dioxide]] molecule, and is attached to the CoA coenzyme.)
* [[glucose-6-phosphate dehydrogenase]] (involved in the [[pentose phosphate pathway]], producing NADPH)
* [[glyceraldehyde-3-phosphate dehydrogenase]] (involved in [[glycolysis]], uses NAD<sup>+</sup>)
* [[sorbitol dehydrogenase]]

[[TCA cycle]] examples:
* [[isocitrate dehydrogenase]] (uses NAD<sup>+</sup>, also has an [[isozyme]] that uses NADP) 
* [[alpha-ketoglutarate dehydrogenase]] (uses NAD<sup>+</sup>) 
* [[succinate dehydrogenase]] (uses FAD) 
* [[malate dehydrogenase]] (uses NAD<sup>+</sup>)

== References ==
<references />{{Enzymes}}
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[[Category:Oxidoreductases]]