Editing Dehydrogenase (section)

== 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" />