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Enzyme class[edit]

Dehydrogenases are a subclass of the class of enzymes called “oxidoreductases.” Oxidoreductases, in general, catalyze oxidation and reduction reactions. These enzymes fall into six categories: oxygenases, reductases, peroxidases, oxidases, hydroxylases, and dehydrogenases. Most oxidoreductase enzymes are dehydrogenases, although reductases are also common. The accepted nomenclature for dehydrogenases is "donor dehydrogenase," where the donor is the substrate that can be oxidized.[1]

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.[2]

Reactions catalyzed[edit]

A reaction catalyzed by a reductase enzyme

Dehydrogenases oxidize a substrate by transferring hydrogen to an electron acceptor, common electron acceptors being NAD+ or 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).[3] 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[edit]

Sometimes a dehydrogenase catalyzed reaction will look like this: AH + B+ ↔ A+ + BH when a hydride is transferred.



The result of a dehydrogenase catalyzed reaction is not always the acquiring 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.[5]

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[edit]


In the above case, the dehydrogenase has transferred a hydride while releasing a proton, H+, but dehydrogenases can also transfer two hydrogens, using FAD as an electron acceptor. This would be depicted as AH2 + B ↔ A + BH2. 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[edit]



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: AH2 + O2 ↔ A + H2O2.

Sometimes an oxidase reaction will look like this: 4A + 4H+ + O2 ↔ 4A+ + 2H2O. 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.[2] An example of an oxidase that functions like this is complex IV in the Electron Transport Chain (ETC).[6]

Note that oxidases typically transfer the equivalent of dihydrogen (H2), and the acceptor is a dioxygen. Similarly, a peroxidase (another subclass of oxidoreductases) will use a peroxide (H2O2) as the electron acceptor, rather than an oxygen.[5]

Electron acceptors[edit]

Nicotinamide Adenine Dinucleotide






Nicotinamide adenosine dinucleotide phosphate


NADP+ differs from NAD+ 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.[8][9]



Flavin Adenine Dinucleotide




Biological implications[edit]





More examples[edit]

TCA cycle examples:

