Oxidoreductases consist of a large class of enzymes catalyzing the transfer of electrons from an electron donor (reductant) to an electron acceptor (oxidant) molecule, generally taking nicotinamide adenine dinucleotide phosphate (NADP) or nicotinamide adenine dinucleotide (NAD) as cofactors. Since so many chemical and biochemical transformations comprise oxidation/reduction processes, it has long been an important goal in biotechnology to develop practical biocatalytic applications of oxidoreductases. During the past few years, significant breakthrough has been made in the development of oxidoreductase-based diagnostic tests and improved biosensors, and the design of innovative systems for the regeneration of essential coenzymes. Research on the construction of bioreactors for pollutants biodegradation and biomass processing, and the development of oxidoreductase-based approaches for synthesis of polymers and functionalized organic substrates have made great progress. Proper names of oxidoreductases are in a form of "donor:acceptor oxidoreductase"; while in most cases "donor dehydrogenase" is much more common. Common names also sometimes appeared as "acceptor reductase", such as NAD⁺ reductase. "Donor oxidase" is a special case when O₂ serves as the acceptor.

Classification

Oxidorecuctases can be either oxidases or dehydrogenases. Oxidases are generally involved when molecular oxygen functions as an acceptor of hydrogen or electrons. However, dehydrogenases work by oxidizing a substrate through transferring hydrogen to an acceptor that is either NAD/NADP or a flavin enzyme. Peroxidases, hydroxylases, oxygenases, and reductases also belong to oxidoreductases. Peroxidases are placed in peroxisomes, and could catalyze the reduction of hydrogen peroxide. Hydroxylases give hydroxyl groups to its substrates. Oxygenases could incorporate oxygen from molecular oxygen into organic substrates. In most cases, reductases can act like oxidases, but catalyzing reductions.

Oxidoreductases are sorted as EC 1 in the EC number classification of enzymes and can be further classified into 22 subclasses.

Reactions

The catalyzed reactions are similar to the following reaction in Figure 1, where A is the reductant and B is the oxidant. In biochemical reactions, the redox reactions are sometimes more difficult to observe, such as this reaction from glycolysis: Pi + glyceraldehyde-3-phosphate + NAD⁺ → NADH + H⁺ + 1,3-bisphosphoglycerate, where NAD⁺ is the oxidant (electron acceptor), and glyceraldehyde-3-phosphate functions as reductant (electron donor).

Figure 1. Redox reaction.

Functions

Oxidoreductase enzymes play significant roles in both aerobic and anaerobic metabolism. They can be found in biological procedures like glycolysis, TCA cycle, oxidative phosphorylation, and amino acid metabolism. In glycolysis, the enzyme glyceraldehydes-3-phosphate dehydrogenase accelerates the reduction of NAD⁺ to NADH. However, the re-oxidization of the generated NADH to NAD⁺ occurs in the oxidative phosphorylation pathway in order to maintain the redox state of the cell. Additional NADH molecules are produced in the TCA cycle. The glycolysis product pyruvate takes part in the TCA cycle in a form of acetyl-CoA. During anaerobic glycolysis, the oxidation of NADH is accomplished through the reduction of pyruvate to lactate, which is then oxidized to pyruvate in muscle and liver cells. Moreover, the pyruvate is further oxidized in the TCA cycle. All twenty of the amino acids, except for leucine and lysine, can be degraded to intermediates in TCA cycle, which allows the carbon skeletons of the amino acids to be converted into oxaloacetate and subsequently into pyruvate. The gluconeogenic pathway can then exploit the formed pyruvate.

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Oxidoreductases