Structural Biochemistry/Enzyme Catalytic Mechanism/Enzyme Classification/Oxidoreductases

Oxidoreductases are a class of enzymes that facilitates (catalyzes) the transfer of electrons from reductants (electron donors) to oxidants (electron acceptors). This type of reaction is also known as a oxidoreduction reaction. The reaction generally follows the following scheme: A– + B → A + B– where A is the reductant and B is the oxidant.

Definition
Oxidoreductases catalyze oxidation-reduction reactions. For example, an alcohol dehydrogenase catalyzes the oxidation reaction of ethanol to form an aldehyde. In the reaction, NAD+ is reduced. the two electrons in the C-H bond of ethanol are transferred to NAD+. Besides alcohol and aldehyde functional groups, dehydrogenases also act as electron donors on functional groups, such as -CH2-CH2-, -CH2-NH2, and -CH=NH, and on coenzymes NADH, NADPH, FADH, and FMNH.

Oxidoreductases specifically catalyze the transfer of electrons from one molecule (the oxidant) to another molecule (the reductant). Oxidoreductases catalyze reactions similar to this example: A– + B → A + B– where A is the oxidant and B is the reductant. Oxidorecuctases can be oxidases or dehydrogenases. Oxidases are enzymes involved when molecular oxygen acts as an acceptor of hydrogen or electrons. On the other hand, dehydrogenases are enzymes that oxidize a substrate by transferring hydrogen to an acceptor that is either NAD+/NADP+ or a flavin enzyme. Other oxidoreductases include peroxidases, hydroxylases, oxygenases, and reductases. Peroxidases are localized in peroxisomes, and they catalyze the reduction of hydrogen peroxide. Hydroxylases add hydroxyl groups to its substrates. Oxygenases incorporate oxygen from molecular oxygen into organic substrates. Reductases catalyze reductions, and in most cases reductases can act like an oxidases.

Oxidoreductase enzymes also play an important role in both aerobic and anaerobic metabolism. They can be found in glycolysis, TCA cycle, oxidative phosphorylation, and in amino acid metabolism. In glycolysis, the enzyme glyceraldehydes-3-phosphate dehydrogenase catalyzes the reduction of NAD+ to NADH. In order to maintain the re-dox state of the cell, this NADH must be re-oxidized to NAD+, which occurs in the oxidative phosphorylation pathway. The product of glycolysis, pyruvate enters the citric acid cycle in the form of acetyl-CoA. During anaerobic glycolysis, the oxidation of NADH occurs through the reduction of pyruvate to lactate. The lactate is then oxidized to pyruvate in muscle and liver cells, and the pyruvate is further oxidized in the citric acid cycle. All twenty of the amino acids, except leucine and lysine, can be degraded to citric acid cycle intermediates. This allows the carbon skeletons of the amino acids to be converted into oxaloacetate and subsequently into pyruvate.

A page with a list of oxidoreductases can be found here []







Subclasses
Oxidases transfer two electrons from the donor to oxygen, forming hydrogen peroxide.

Cytochrome oxidase produces water molecules rather than hydrogen peroxide by reducing the oxygen. Cytochrome P-450 catalyzes oxidation reactions in the liver and adrenal cortex, helping to detoxify some substances by adding hydroxyl groups that make the compounds more water-soluble and more susceptible to further reactions. Unfortunately, at times this process has the reverse effect because some relatively safe molecules are converted into potent carcinogens. A group of cytochromes serve as oxidation-reduction agents, converting the energy of the oxidation process into the synthesis of adenosine triphosphate (ATP), which makes the energy more available to other reactions.

Oxygenases oxidize a substrate by incorporating oxygen to it. Monooxygenases incorporate a single oxygen atom, and the other oxygen is reduced by electrons from substrate to water. Dioxygenases transfer two oxygen atoms in a molecule of O2 to the substrate.

Peroxidases and Catalases etoxify hydrogen peroxide, which acts as both electron donor and acceptor. Peroxidases and catalases are Fe (III)-heme compounds that decompose hydrogen peroxide and organic peroxides. The reactions seem to proceed through Fe(IV) compounds with another unpaired electron electron on the porphyrin, which becomes a radical cation. Similar intermediates are also known in simpler porphyrin molecules.