Editing Dehydrogenase (section)

== 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.